A Water Resilience Plan for Xochimilco Report

A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC AND MILPA ALTA

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC AND MILPA ALTA

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ACKNOWLEDGEMENTS CDMX Resilience Agency

Dr. Arnoldo Matus Kramer CDMX chief Resilience Officer Lic. Pipola Gómez Sánchez Jessica Hernandez Alvaro Soldevila 100 Resilient Cities

PROJECT TEAM Deltares

Metropolitan Autonomous University UAM

Evaluación de Riesgos Naturales (ERN) Proyectos Keystone BARRAZA PREPARED BY FINANCIAL SUPPORT DATE DESIGN AND PHOTOGRAPHY

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Dr. Hans Gehrels Team Leader Corine ten Velden MSc. Didrik Meijer MSc. Marta Faneca Sánchez MSc. Laurene Bouaziz MSc. Tommer Vermaas MSc. Begoña Arellano Jaimerena MSc. Stefania Velanzuela Velazquez Dr. Eugenio Gómez Reyes Project manager UAM-Iztapalapa Dr. Alberto González Pozo UAM-Xochimilco Mtro. Roberto Constantino Toto UAM-Iztapalapa Dr. Felipe Omar Tapia Silva UAM-Iztapalapa Dr. César Arredondo Vélez Dr. Marco Antonio Torres Pérez Negrón Arturo Farias Juan Pablo Rico Mtra. Cecilia Barraza Mtra. Nora A. Morales UAM-Cuajimalpa DELTARES, UAM, ERN, Keystone and Barraza Rockefeller Philanthropy Advisors, Inc. JULY 2019 Nora A. Morales Zaragoza Cecilia Barraza

A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC AND MILPA ALTA Resilient Xochimilco

FINAL REPORT 2019

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A FWATER RESILIENCE PLAN FORPLAN THE FOR HERITAGE ZONE OFZONE XOCHIMILCO, TLAHUAC AND MILPA ALTA A WATER RESILIENCE THE HERITAGE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

TABLE OF CONTENTS Executive Summary

1 Introduction

1.1 A Water Resilience Plan for the Zona Patrimonial

1.1.1 Zona Patrimonial of XTMA

1.1.2 History of the chinampas

1.2 Scope and objectives

1.3 Approach

1.3.2Regional and local modelling of the water system

1.3.3 Stakeholder consultations

1.3.4 Participatory workshops

1.3.5 Soliciting and selecting Project proposals

2 Challenges and principles for a resilient water system

2.1 Introduction

2.2 Water challenges in the Zona Patrimonial

2.2.1 Dependency on trans-basin diversion and

groundwater extraction

2.2.2 Overexploitation of the aquifer

2.2.3 Leakage from the potable water system

2.2.4 Inundation of chinampas

2.2.5 Informal settlement and water quality

2.3 The concept of resilience

2.3.1 Urban resilience

2.4 Resilience principles for the Zona Patrimonial

2.5 Zonation

2.5.1 Urban area

2.5.2 Chinampas area

2.5.3 Flood-prone areas

2.5.4 Agricultural areas

2.5.5 Informal settlements

3 Incorporating Geological Risk in Water Resilience

3.1 Executive Summary

3.2 Introduction 3.2.1 Objectives 3.2.1.1 Geology and subsidence 3.2.1.2 Seismic hazard 3.2.1.3 Risk estimation 3.2.2 Location

3.3 Assessing Seismic Hazard 3.3.1 Introduction and Background 3.3.2 Overview of the Seismicity in the Study Area 3.3.3 Methodology of Probabilistic Seismic Hazard

Assessment (PSHA) 3.3.4 Seismic Hazard Results

1 5 5 6 7 10 11 11 12 12 13 17 17 17 18 18 18 19 19 20 20 23 26 28 29 30 30 31 35 35 37 37 37 37 38 38 39 39 40 40 45

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3.3.4.1 Rock

3.3.4.2 Soil

3.3.4.3 Comparison between rock and soil results

3.3.5 Geology and Subsidence

3.3.5.1 Methodology

3.3.5.2 Subsidence Results

3.3.5.3 Conclusions and Recommendations

3.3.6 Influence of Subsidence in the Seismic Hazard

Results

3.4 Vulnerability functions for water infrastructure 3.4.1 Introduction 3.4.2 Vulnerability Functions for Drinking Water Network 3.4.2.1 Seismic vulnerability curves for drinking

water network 3.4.2.2 Subsidence vulnerability curves for drinking

water network 3.4.3 Vulnerability Functions for Drainage Network

System 3.4.4 Vulnerability Functions for Wells 3.4.4.1 Seismic vulnerability curves for wells 3.4.4.2 Subsidence vulnerability curves for wells 3.4.5 Vulnerability Curves for Tanks 3.4.5.1 Seismic vulnerability curve for tanks 3.4.5.2 Subsidence vulnerability curve for tanks 3.4.6 Chlorination and Wastewater Treatment Plants

3.5 Risk Estimation 3.5.1 Introduction 3.5.2 Probabilistic Risk Analysis 3.5.3 Exposure Summary 3.5.4 Probable Maximum Loss 3.5.5 Annual Average Loss 3.5.6 Additional Results

3.6 Implications for future investment 3.6.1 Reconnection of the Rio Amecameca to the ZP 3.6.2 Linear wetlands at transition boundaries 3.6.3 Water Control System 3.6.4 Regeneration of PEX Xochimilco 3.6.5 Green Corridors

4 Regional and local modelling of the water system

4.1 Introduction

4.2 Regional modelling of the Basin of Mexico 4.2.1 Hydrological model Results 4.2.1.1 Discharge simulations 4.2.1.2 Groundwater recharge 4.2.1.3 Water balance 4.2.2 Groundwater Model Results 4.2.2.1 Hydraulic heads 4.2.2.2 Validation of model results

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45 45 45 49 50 51 56 53 55 55 57 58 59 60 61 61 62 62 62 64 64 70 70 71 72 72 73 77 77 78 79 80 81 81 83 83 83 83 83 84 88 90 93 96

A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

4.2.2.3 Water balance

4.3 Local modelling of the Zona Patrimonial 4.3.2 Hydrodynamic Model Results 4.3.2.1 Discharge 4.3.2.2 Water balance for the Zona Patrimonial

98 100 100 101 102

5 Turning technical results into community action

5.1 Community workshops and interviews

5.2 Analysis description

5.3 Methodology

5.4 Dissemination and socialization of the hydrological model

5.5 Results

105 105 106 107 112 114

6 Development of a Project Portfolio

6.1 Identified institutional stakeholders

6.2 Soliciting Project proposals

6.3 Project selection 6.3.1 Reconnection of the Rio Amecameca to the ZP 6.3.2 Linear wetlands 6.3.3 Water control system 6.3.4 Green corridors 6.3.5 Water Culture Centre

6.4 Potential financing sources and responsibilities

6.5 Conclusions

7 Conclusions

References

APPENDICES

A Biophysical area description A.1 Basin of Mexico A.1.1 Location A.1.2 Topography A.1.3 Climate A.1.4 Main Waterbodies A.1.5 Water management A.2 Mexico City A.2.1 Location A.2.2 Climate A.2.3 Population A.2.4 Water system A.3 Zona Patrimonial: Xochimilco, Milpa Alta and Tlรกhuac A.3.1 Location A.3.2 Topography A.3.3 Climate A.3.4 Population A.3.5 Water System A.3.6 Water Quality

121 121 122 124 125 128 133 133 136 138 142 145 151 159 159 159 159 160 160 161 161 161 162 162 162 163 167 167 168 169 169 171 172

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B Data acquisition

B.1 Digital Elevation Model

B.2 Laser Imaging Detection and Ranging (LIDAR)

B.3 Watershed

B.4 Drainage network

B.5 Water treatment network

B.6 Meteorological data

B.7 Hydrography

B.8 Chinampa channel network

B.9 Bathymetry of channel network

B.10 Water quality of channel network

B.11 Runoff coefficient

B.12 Hydrometry

B.13 Aquifers

B.14 Geology

B.15 Groundwater abstractions

B.16 Groundwater levels

182 182 182 183 184 184 184 185 185 185 186 187 188 188 189 189 189

C Hydrological model for the Basin of Mexico

C.1 Hydrological concept: Wflow

C.2 Model structure

C.3 Soil, land cover and model parameters

C.4 Rainfall

C.5 Potential evapotranspiration

C.6 Model validation data

D Groundwater model for the Basin of Mexico

D.1 Hydrogeological conceptual model

D.2 Groundwater abstractions

D.5 Model limitations

E Hydraulic model for the Zona Patrimonial

E.1 Model setup E.1.1 Network: channels and cross sections E.1.2 Boundary conditions E.1.3 Lateral inflows

E.2 Model limitations

F Probabilistic Seismic Hazard Assessment

F.1 Historical Earthquake Catalog

F.2 Characterization of the Seismic Sources

F.3 Seismicity Models F.3.1 Modified Gutenberg-Richter (G-R) F.3.2 Characteristic earthquake model

F.4 Ground Motion Prediction Equations F.4.1 Interface sources F.4.2 Intraslab sources F.4.3 Intraplate sources

F.5 PSHA Methodology

190 190 193 194 199 200 201

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204 204 210 213 215 215 216 218 219 220 221 221 222 226 226 226 227 228 228 229 230

A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

F.5.1 Computation tool 230 F.5.2 Methodological framework 231

F.6 Modification of the Seismic Hazard Caused by Site Effects 232

F.7 Seismic Hazard Maps for Rock 233 F.7.1 Tr=43 years F.7.2 Tr=250 years F.7.3 Tr=475 years F.7.4 Tr=2475 years

F.8 Seismic Hazard Maps for Soil 237 F.8.1 Tr=43 years F.8.2 Tr=250 years F.8.3 Tr=475 years F.8.4 Tr=2475 years

F.9 Seismic Hazard Results Influenced by Subsidence Problem 241 F.9.1 Subsidence for 2020 241 F.9.1.1 Tr=43 years F.9.1.2 Tr=250 years F.9.1.3 Tr=475 years 243 F.9.1.4 Tr=2475 years 244 F.9.2 Subsidence for 2030 245 F.9.2.1 Tr=43 years F.9.2.2 Tr=250 years 246 F.9.2.3 Tr=475 years 247 F.9.2.4 Tr=2475 years 248 F.9.3 Subsidence for 2040 249 F.9.3.1 Tr=43 years F.9.3.2 Tr=250 years 250 F.9.3.3 Tr=475 years F.9.3.4 Tr=2475 years 251 F.9.4 Subsidence for 2050 251 F.9.4.1 Tr=43 years F.9.4.2 Tr=250 years F.9.4.3 Tr=475 years F.9.4.4 Tr=2475 years 251 F.9.5 Subsidence for 2070 252 F.9.5.1 Tr=43 years F.9.5.2 Tr=250 years 253 F.9.5.3 Tr=475 years F.9.5.4 Tr=2475 years

G Building of Vulnerability Functions 253

G.1 Methodology used 253 G.1.1 Fragility to vulnerability G.1.2 Simplified estimation of loss state associated to a performance parameter

G.2 Vulnerability Functions for Drinking Water Network 255 G.2.1 Seismic vulnerability curves for drinking water network G.2.2 Subsidence vulnerability curves for drinking 261 water network

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G.3 Vulnerability Functions for Wells G.3.1 Seismic vulnerability curves for wells G.3.2 Subsidence vulnerability curves for wells

G.4 Vulnerability Curves for Tanks G.4.1 Seismic vulnerability curve for tanks G.4.2 Subsidence vulnerability curve for tanks

266 266 268 268 269 273

H Exposure Summary

H.1 Information given to ERN H.1.1 Hydraulic infrastructure H.1.2 Urban Infrastructure

H.2 Complementary Information H.2.1 Hydraulic Infrastructure H.2.2 Urban Infrastructure

H.3 Exposure per infrastructure type

276 276

278

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I Annual Average Loss Maps

277

J Risk Analysis for September 19th Events and Critical Scenario

J.1 September 19, 1985 earthquake

J.2 September 19, 2017 earthquake

J.3 Critical Scenario

287 287 295 304

K The Process of Portfolio Development

K.1 Systemic analysis

K.2 Mapping institutional stakeholders K.2.1 Integral stakeholder analysis Step 1- Identification of all potential actors Step 2- Stakeholder Categorization Step 3 - Integral mapping of stakeholders Step 4- Determination of the engagement strategy

K.3 Participatory workshops K.3.1 Workshop 1 K.3.2 Workshop 2 K.3.3 Workshop conclusions on proposed projects

313 313 322

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

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A NWATER RESILIENCE PLAN FORPLAN THE FOR HERITAGE ZONE OFZONE XOCHIMILCO, TLAHUAC AND MILPA ALTA A WATER RESILIENCE THE HERITAGE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

EXECUTIVE SUMMARY

This is the final report of the project called ‘A Water Resilience Plan for the Heritage Zone of Xochimilco, Tlahuac and Milpa Alta’. The project has been carried out by Deltares, Metropolitan Autonomous University (UAM), Evaluación de Riesgos Naturales (ERN), Keystone and Cecilia Barraza, in close collaboration with the Resilience Agency of Mexico City (ARCDMX).

The general objective of the Resilience Agency with the project was to improve the water resilience of the Heritage Zone (Zona Patrimonial or ZP), in order to enable the ZP to face the challenges of climate change, geological risks, and social, economic and environmental issues. The project aims to restore the water system of the area in a way that it can cope with the effects of climate change and the socio-economic challenges that are pressuring the water system.

The water system of the Heritage Zone faces several challenges including leakages of the piping system, water shortages, water quality issues related to wastewater drainage from informal settlements. High rates of groundwater extraction within the basin cause compaction and subsidence and lead to damage of the water network infrastructure. The recent 2017 earthquake lead to a dam fracture and subsequent flooding.

To overcome these challenges and the increased pressure of population growth and climate change on urban areas, cities must explore strategies to develop their capacities for resilience. Urban resilience, as set out by 100 Resilient Cities, is defined as the capacity of individuals, communities, institutions, businesses, and systems within a city to survive, adapt and grow no matter what kind of chronic stress and acute shocks they experience (100 Resilient Cities, 2015).

Within this project, a geological and hydrological assessment of the Heritage Zone in combination with a stakeholder analysis are performed to develop a project portfolio for a water resilient plan to improve and sustain the water system of the ZP.

EXECUTIVE SUMMARY

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The general objective of the Resilience Agency was to improve the water resilience of the Heritage Zone and enable the zone to face the challenges of climate change, geological risks, and social, economic and environmental issues.

The project aims to restore the water system of the area in a way that it can cope with the effects of climate change and the socio-economic challenges that are pressuring the water system.

The project focuses on [1] understanding the challenges and principles for a resilient water system, [2] estimating the geological risk of earthquakes and subsidence on water infrastructure in the ZP, [3] understanding the water system including surface water and groundwater, [4] stakeholder analysis, workshops and interviews with local communities and important actors to map shared interests or conflicts between stakeholders, [5] the development of a water resilience plan that includes specific interventions for improving the water system, and [6] an impact assessment of selected measures.

RESILIENT WATER SYSTEM An important principle of a resilient water system is to realize that water is part of the problem and the solution. Main qualities of a resilient system include the ability to learn (reflectiveness), ability to easily repurpose resources (resourcefulness), ability to limit the spread of failures (robustness), the presence of a backup capacity (redundant), alternative strategies (flexibility), broad consultation and communication (inclusiveness) and the integration of systems that are able to work together.

GEOLOGICAL RISK A probabilistic analysis was made to estimate the intensities associated to seismic hazard within the Heritage Zone of Xochimilco Tlahuac and Milpa Alta. A catalogue with more than 100 years of historical earthquake data was compiled and about 45 seismic sources were defined along the Mexican territory. One main advantage of a probabilistic analysis is that it accounts for uncertainty, which is an important aspect as seismic occurrence is uncertain and can, therefore, not be analyzed as a deterministic process. Additionally, site effects, caused by soft deep strata, may occur in the region, and were therefore included by means of transfer functions, which in general terms, relate the stiffness of the soil (soil dominant period) to the amplification factor. Analyses were carried out for four different return periods: 43, 250, 475 and 2475 years.

Finally, the influence of subsidence in seismic intensities was studied.

The largest seismic intensities occur in the middle of Tlรกhuac District, just at the Heritage Zone boundary. It was observed that there is a direct relation between intensity changes and fracturing alignments; especially for the 0.2s spectral ordinate. Seismic intensities range from 0.03 to 1.4g, depending on the spectral ordinate and return period evaluated. These intensities were expected since this zone is charac-

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

terized by soft strata soil and site effects which considerably amplify the seismic waves.

REGIONAL AND LOCAL MODELLING OF THE WATER SYSTEM Data collection, analysis and modelling of the water system was done to understand the water system of the Zona Patrimonial. Regional numerical hydrological models were developed for the Basin of Mexico that defines the boundary conditions for the water system of the Zona Patrimonial. A localized numerical hydraulic model was developed for the water system of the Zona Patrimonial. The average groundwater recharge was assessed between 1979 to 2014 and average recharge was estimated at 0.6 mm per day in the basin of Mexico.

A numerical groundwater model was built based on extensive collection of existing data from various public sources, government agencies and universities. Groundwater is the main source for drinking water supply, sustainable groundwater resources management is, therefore, of paramount importance. The dynamics of the hydrological system simulated with the hydrological Wflow model were used for the top boundary conditions of the model (groundwater recharge). The city of Mexico suffered a severe drought during 2009, however, groundwater extraction continued at the regular pace, causing water levels to drop considerably with around 10 m, mainly in the urban areas. On average, the water balance of the Basin of Mexico is estimated at -138 mm/yr.

Groundwater abstraction exceeds groundwater recharge with a factor of four, as groundwater recharge is estimated at 44 mm/yr, while groundwater abstraction account for -182 mm/yr.

Participatory stakeholder analysis Workshops and interviews were conducted during the project to promote the understanding of the water socio-ecological system in the heritage zone, understand the needs and strengths of the different sectors that inhabit the heritage zone and build capacities for an integral management water strategy. This resulted in a map showing interrelations and interconnections between stakeholders, to visualize interests or conflicts between stakeholders. Involving local communities enables to develop adequate solutions and create ownership.

Additionally, hydrological modelling trainings were provided to technical staff of UAM, UNAM and ARCDMX in Mexico City.

EXECUTIVE SUMMARY

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Development of a project portfolio A portfolio of project proposals was developed to improve the water challenges of water supply, water quality, water distribution and subsidence, with a positive impact on socio-economic, cultural and governance aspects of the area.

The project selection includes:

• A reconnection of the Rio Amecameca to the Heritage Zone • Creation of linear wetlands in the streets bordering the channels at the southern border of the Xochimilco chinampa zone as passive treatment systems for the wastewater of surrounding houses, in order to discharge clean water into the wetland’s canals.

• Development of a lock system to control water flows at the chinampa canals of Xochimilco to restore hydraulic connectivity and remediate subsidence • Green corridors to improve the water system of the ZP by restoring part of the inflow from the springs south of the ZP and to improve urban public space through a connection to attractive linear parks. An improved spatial connection may raise awareness of the urban community to the chinampa area.

• Building a Centre for Water Culture at the Xochimilco Ecological Park (PEX) to create awareness on the strategic importance of the Xochimilco wetland system, in terms of its ecological, historical, economical and water production value.

The government of Mexico City and the Federal Government were identified as potential sources to finance these measures, either by own resources or by resources managed with international development banks, such as the World Bank, IDB or CAF. In addition to this, Impact Investment of private funds that seek financial returns while regenerating the environment may represent a co-financing possibility. Public-private partnerships (PPPs) initiatives involving public and private actors should be considered.

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

INTRODUCTION

A WATER RESILIENCE PLAN FOR THE ZONA PATRIMONIAL Mexico City (CDMX), one of the most populated areas of the world, is facing water-related challenges due to population growth and the associated urban expansion. Most relevant water-related shocks and stresses that impact the development of the city of Mexico are: urban flooding, regular water shortages and droughts, degradation of the water quality of both surface waters and groundwater, depletion of groundwater resources in the regional aquifers, subsidence associated with the high groundwater abstraction rates, high costs related to long-distance transportation of potable water and wastewater, inefficiency due to substantial leakage from the water supply system, insufficient wastewater treatment capacity and lack of infrastructure for water reuse, and limited storage of rain-water.

As a member of the 100 Resilient Cities Initiative powered by the Rockefeller Foundation, the Resilience Agency of Mexico City (CDMX) has prepared a Resilience Strategy to face the many challenges of the city, in which resilience of the urban water system is a key component.

One of the main focal points of de CDMX Resilience Strategy is the borough of Xochimilco because of its important cultural heritage values.

Over the past decades, Xochimilco suffered from high environmental degradation and pressure due to the expansion of informal settlements. This situation has resulted in the provision of inadequate urban services, which as a consequence has affected the conservation of the natural protected area. Despite its rich historical, cultural and environmental value, Xochimilco faces high dynamic vulnerability and fragility, and it is under severe existential threat.

Several actions have been implemented to encourage the protection of the region. In 1987, part of Xochimilco was declared a World Heritage Site (Zona Patrimonial) by the United Nations Educational, Scientific, and Cultural Organization (UNESCO). During the same year, the regional government declared 80% of the boroughs as an ecological zone for

INTRODUCTION

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biological preservation aiming to control urban development. Furthermore, in 2004 the World Heritage Site was listed under the RAMSAR Convention on Wetlands (Figueroa et al., 2014). Nevertheless, ongoing urbanisation has continued to put pressure on the natural ecosystems of the boroughs.

As part of the CDMX Resilience Strategy, a consortium of Deltares, Metropolitan Autonomous University (UAM), Evaluación de Riesgos Naturales (ERN), Keystone and Barraza is working on a project called “A Water Resilience Plan for the Heritage Zone of Xochimilco, Tlahuac and Milpa Alta”. This project aims to restore the water system of the area in a way that it can cope with the effects of climate change and the socio-economic challenges that are pressuring the water system.

The project focuses on [1] understanding of the water system, and [2] the development of a water resilience plan that includes specific interventions for improving the water system.

The first phase of the project has focused on understanding the water system of the Zona Patrimonial through data collection, analysis and modelling of the water system. Regional numerical hydrological models were developed for the Basin of Mexico that defines the boundary conditions for the water system of the Zona Patrimonial. A localized numerical hydraulic model was developed for the water system of the Zona Patrimonial. The second phase is about developing a portfolio of project proposals that improve the water challenges of water supply, water quality, water distribution and subsidence, with a positive impact on socio-economic, cultural and governance aspects of the area.

Zona Patrimonial of XTMA The Zona Patrimonial of Xochimilco, Tlahuac and Milpa Alta (XTMA) is located in the south-east region of Mexico City. The Zona Patrimonial has a high ecological and cultural value to Mexico City. It is a World Cultural and Environmental Heritage Site, home to the remainder of the former lake of the Valley of Mexico, a place of water channels and traditional agricultural plots from Aztec times, called chinampas. Chinampas are agricultural plots enclosed by interwoven poles of reeds and surrounded by canals (Cifuentes, Hurtado & Juarez, 1998).

Tláhuac is a borough with an extension of 89.5 km2 and has an average altitude of 2220 meters above sea level (m a.m.s.l). With a population of 361,593 inhabitants, it is one of the least populated boroughs of Mexico City (INEGI, 2015b).

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

Milpa Alta, with a territorial extension of 268 km2 and an average altitude of 2413 m a.m.s.l., is the least populated borough of Mexico City.

According to INEGI (2015b), Milpa Alta has a population of 137,927 inhabitants.

The borough of Xochimilco has a population of 415,007, being the ninthmost populous borough of Mexico City (INEGI, 2015b). It has a territorial extension of 125 km2 and is located at 2,275 m a.m.s.l. Furthermore, Xochimilco provides about 8% of the water demand of Mexico City, and it is an asset of major green and blue infrastructure for the city. Urban sprawl and current water management practices affect the quality and quantity of water and environmental services provided by the region (e.g. agriculture and tourism). Figure 1.1 shows an overview of the main issues that put the Zona Patrimonial under pressure.

History of the chinampas Xochimilco is the remaining part of the Texcoco Lake complex consisting of the five lakes present in the Mexican Valley before Tenochtitlan (ancient Mexico City) was founded in 1325 AD. This part of the city has been

Figure 1.1: A map of the Zona Patrimonial with the water related challenges schematically indicated.

INTRODUCTION

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declared a UNESCO World Cultural Heritage site.

The Aztecs settled along the shores of Texcoco Lake and founded the city of Tenochtitlan in 1325 AD. During the rule of the emperor Izcรณatl the Aztecs became the dominant society in the valley of Mexico. This lake was one of the five lakes (Zumpango, Xaltoca, Xochimilco, Chalco and Texcoco) within the Valley of Mexico. The five lakes were connected at the time. Lake Texcoco, being the lowest, received most of the discharge from the surrounding hills and lakes. The lake had a surface area of around 600 km2 at a mean altitude of 2400 m asl.

Figure 1.2: Lake System in the Valley of Mexico (Source: Consejo nacional de Fomento Educativo).

Water was an important issue for the Aztecs. The wet and dry seasons generated fluctuating water levels that needed to be managed in order to prevent floods in the city. Water management was also required to enable agriculture needed to feed the growing empire. The first impe-

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

rial engineers designed and built a series of “albarradones” (dikes) to prevent brackish water in the north from mixing with freshwater in the south. Another agricultural development in the lacustrine environment was the construction of small islands called chinampas.

Chinampas were common in the Mexican valley even before the Aztecs established themselves there (Parsons, et al.). In varying forms, this concept was used by different Andean cultures throughout Mesoamerica and what we see nowadays in Xochimilco is one of the last remaining samples of this concept. This technique was very attractive for societies because it enabled to reclaim land for agricultural activities. This “engineered landscape” offered the possibility over the centuries of a continuous production of crops within a growing urban environment.

Originally, the chinampa system was an engineered landscape with primary, secondary and tertiary canals. The primary canals were the central source of freshwater flowing into the system. One important characteristic of the main canals was the influence that they exerted over the water quality because some of them were discharging fresh water into brackish water, regulating the salt concentration in that way (Frederick et al., 2005). Secondary canals were directing water from the main canals to the centre of the system. The secondary canals were smaller and denser in order to penetrate into the central part of the water system. Tertiary canals were the end part of the system (Frederick et al., 2005). This system of connected canals was fully integrated with the urban landscape and regional economy.

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Figure 1.3 Chinampa system structure (Source: Mexicolor)

SCOPE AND OBJECTIVES Objectives 1. Improve the understanding of the ZP as part of the hydrological system of Mexico City, in relation with main stresses including droughts, floods, water quality and geological risks.

2. Estimate the geological risk in the ZP, i.e. the risk associated with earthquakes and subsidence, specifically those risks associated with water infrastructure.

3. Develop hydrological models (regional and localized) in support of decision making.

4. Provide training to technical staff of UAM, UNAM and ARCDMX in Mexico City on hydrological modeling.

5. Prepare a Water Resilience Plan to improve and sustain the water system of the ZP, as part of Mexico City’s Resilience Strategy.

6. As part of the Water Resilience Plan, develop a project portfolio that contributes to improvement of the water resilience.

Scope Hydrological Models (regional and localized of the ZP) including simulation scenarios, to be used for assessment and prioritization of actions and projects in relation to socio-economical and risk variables.

Geological risk report including methodological assumption for hazards, vulnerabilities and quantification of losses in the hydrological in-

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

frastructure, resulting from risk of geological hazards (subsidence and seismic hazard).

Stakeholder engagement strategy including decision making, project categories, key actors identification and participatory process with key actors.

Hydrological model socialization considering social engagement of key actors in the ZP, as well as its relationship for socialization and joint design of main actions and projects.

Hydrological resilience plan including the specific view, principles and objectives for managing the water resources, while considering geological risk.

Project Portfolio featuring the identification, classification and prioritization of projects and actions, as well as key actor analysis, their organizational context, economic feasibility and social and environmental impacts.

APPROACH Regional and local modelling of the water system Regional modelling A hydrological model was built to calculate the water balance for the entire Mexico Basin (Cuenca de MĂŠxico). The hydrological model contains data and information on rainfall and evaporation, ground elevation, catchment delineation, land use and soil characteristics in order to determine the response of the catchment to time series of rainfall events. The model calculates recharge to the groundwater system and runoff to the surface water system. These components have been used to compute the forcing (i.e. the boundary conditions) for the hydrodynamic and groundwater models.

A regional groundwater model was constructed of the entire Basin of Mexico. The model consists of four layers in which the physical properties of the aquitards and aquifers are represented. Vertical infiltration calculated with the hydrological model was used to represent the groundwater recharge through the unsaturated zone. The thickness of the layers and the hydrological conditions vary along the surface of the basin. The boundaries of the regional model are delineated by the natural conditions of the regions.

INTRODUCTION

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Local modelling A hydraulic model was constructed of the water system of the Zona Patrimonial. A 1D model of the ZP includes the cross sections of the main rivers and channels of the area. Ground elevation is based on LIDAR data. The model thus contains the main drainage network and detailed topographic model of the ZP. Stakeholder consultations In order to understand the complexity of the Xochimilco system, a series of interviews with experts in areas of development banking, water and public policy were carried out on the following topics: methodologies for the evaluation of water infrastructure projects, technologies for water treatment, current state of the Xochimilco Ecological Park (PEX), risk assessment and seismic vulnerability methodologies, investment prospects in infrastructure projects and current state of the SACMEX Cerro de la Estrella treatment plant. The results of these interviews are the systemic maps of Appendix K.1 Participatory workshops Keystone conducted 2 collaborative research and design workshops with the experts network of the Mexico City Resilience Agency (ARCDMX).

The first workshop focused on achieving a common understanding of what are the most important criteria for choosing a portfolio of projects that have the greatest impact on the solution to the water problem in Xochimilco.

The conversation was generated around the following questions: What is the relevant geographic area to develop projects that solve Xochimilco water system and why?

What results should the projects pursue and what are the metrics that determine their viability and impact?

Who are the decision makers and what process must be developed with them for the solutions to take place?

Based on the conclusions of this conversation, an open call for projects was made.

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The second participatory workshop had two objectives: that the project proposing teams presented their project to the Mexico City Resilience Agency team, and that these projects received feedback from all participants.

Selecting Project proposals Based on the conclusions of workshop 1 with experts, on the main criteria for proposing projects and the feedback provided to all the projects presented in workshop 2, Keystone, UAM and Deltares members had several work sessions to identify and select 5 viable projects with the greatest impact on each of the 5 areas previously defined as priorities: water quantity, water quality, surface water distribution, subsidence and water culture change. This portfolio of 5 projects can be considered as a single meta-project that must be implemented together for the system to function properly.

INTRODUCTION

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

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A 16 WATER RESILIENCE PLAN FORPLAN THE FOR HERITAGE ZONE OFZONE XOCHIMILCO, TLAHUAC AND MILPA ALTA A WATER RESILIENCE THE HERITAGE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

CHALLENGES AND PRINCIPLES FOR A RESILIENT WATER SYSTEM

INTRODUCTION The rehabilitation of the ZP of Xochimilco, Tlรกhuac and Milpa Alta requires a long-term water strategy that targets various issues, such as floods, droughts, land subsidence and water quality. Water management in the ZP requires underlying principles to make it coherent, understandable and effective. For example, a typical Dutch water management principle is formulated as to retain and store, delay and re-use; drain only when necessary. Another way of formulating this principle is that we have to learn to live with water rather than fighting against water.

In the Netherlands, it took many years, though, before this way of living with water was fully embraced.

This chapter first presents a general overview of the challenges pressuring the water system of the ZP. Because of the focus on a resilient water system, this chapter also explores the general concept of resilience. Specific objectives are listed depending on the various types of land use, addressing particular water issues that pressure the water system of the ZP. Finally, a long-term scenario analysis is given addressing socio-economic development and climate change.

WATER CHALLENGES IN THE ZONA PATRIMONIAL Figure 2.1 summarises the current water management in the ZP and the issues that this management practice has caused. It can be observed that the current water management practice is linear rather than circular, a paradigm that does not promote or allow for water reuse, recycling and storage. The sub-sections below describe the main challenges in more detail.

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Figure 2.1 Water management in Mexico City and overview of water issues in the Zona Patrimonial of Xochimilco, Tlรกhuac and Milpa Alta. (Source: adapted from https://waterresilience.wordpress.com.)

Dependency on trans-basin diversion and groundwater extraction The current water management practice in the City of Mexico has resulted in a high reliance on potable water supply over large distances from outside the basin (trans-basin diversion) and high rates of groundwater extraction within the basin. This unsustainable system of water supply comes with associated high costs, low efficiency, little flexibility and robustness, and high environmental impact.

Overexploitation of the aquifer The continued exploitation of the aquifer has led to the dehydration of the confining upper clay layer (aquitard) underlying Mexico City.

The collateral damage is continued compaction and differential subsidence, resulting in substantial damage to (linear) infrastructure and unexpected changes in the original slope of the drainage system. The latter made it necessary to implement a system of pumping stations throughout Mexico City to continue the drainage of wastewater and stormwater.

Leakage from the potable water system Leakages from the potable water system are the result of the continu-

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ous differential settlement taking place in Mexico City causing damage to the system. The water losses are equivalent to the amount of water that is imported from the Cutzamala-Lerma system (19.8m3/s). Moreover, localized water leaks have caused cases of liquefaction of the subsoil emerging as sinkholes, which may sometimes lead to human and economic consequences.

Inundation of chinampas Land subsidence in Mexico City not only affects the operation of the sewage system, but it has also created two new water bodies: Lake Tláhuac-Xico and San Gregorio Lagoon. The extent of both lakes is still growing, because as long as the imbalance between water extraction and water recharge of the aquifer continues, the subsidence process will also continue.

Several actions have been carried out to delineate the extension of the San Gregorio Lagoon, although the chinampas in the area of Atlapulco and Tlaxialtemalco are still prone to increased flooding. The recent earthquakes of 2017 have fractured the constructed dams of San Gregorio Lagoon at its southern part, resulting in renewed inundation of the neighbouring chinampas. It should be mentioned that the inundated chinampas are not only fed by runoff. Treated wastewater from the WWTP called Cerro de la Estrella is being discharged in the chinampa area, exacerbating flooding near the southern parts of the San Gregorio Lagoon.

The inundated areas of the chinampas have created a social conflict among the northern and southern inhabitants of the area. On the one hand, the agricultural and tourism sector required an increase in the water levels and rehabilitation of the secondary channels (known as “apantles”). On the other hand, the residents of the north of Atlapulco and Tlaxialtemalco do not want more water since their livelihoods have been lost. Hence, strategies for the recovery of the chinampas will need to address this conflict.

Informal settlement and water quality During the participatory workshops the sheer uncontrollable process of informal settlement was considered as one of the most significant social problems of the ZP. It has resulted in the continuous discharge of illegal wastewater throughout the chinampa area and the infiltration of wastewater to the groundwater system. This, in turn, leads to the

CHALLENGES AND PRINCIPLES FOR A RESILIENT WATER SYSTEM

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associated increase in human health risks, environmental degradation and a substantial reduction of agricultural productivity on the chinampas.

THE CONCEPT OF RESILIENCE The concept of resilience opens the doors for (semantic) discussion.

While there probably will never be a correct interpretation of resilience, one aspect that most definitions agree upon is that resilience has a positive connotation. Classified under the overarching title Sustainable cities and communities, resilience is explicitly mentioned in the United Nations (2015) Agenda for Sustainable Development: Goal 11 encompasses making cities and human settlements inclusive, safe, resilient and sustainable.

There has been a significant amount of scientific output regarding resilience and its relationship with other (abstract) concepts like vulnerability, sustainability, robustness, adaptability and recovery. In this section an exploration of the most common and prevalent interpretations in scientific literature is made.

Holling (2000) explains another concept within resilience: thresholds.

The assumption is made that a socio-ecological system adjusts itself to its environment (and changes therein); the system is considered stable as long as it does not reach the threshold. When a system is ‘pushed over the edge’ of a threshold by an external force, the system becomes unstable or changes into another system. The threats to a system primarily originate disruptions and instability, which is why the system will try to counter the disruptions.

Folke (2006) proposed a three-way definition of resilience in socio-ecological systems; (i) the maximum disruption a system can cope with, (ii) the extent to which the system can adapt and/or reorganise itself, and (iii) the degree to which the system can increase the capacity specified under point (ii). However, disruptions cannot be predicted and can devastate a system. Increasing resilience can help to reduce the impact of disruptions.

Urban resilience Cities are becoming increasingly more important and complex webs of institutions, infrastructure and social platforms. However, when densely crowded cities are not resilient, they are vulnerable to shocks and stresses, which can cause social breakdown, physical and economic

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collapse. The challenges at hand have evolved through time, from resource shortages, natural hazards, war and conflicts to climate change, disease pandemics, economic fluctuations and terrorism. Urban resilience, as set out by 100 Resilient Cities, is defined as the capacity of individuals, communities, institutions, businesses, and systems within a city to survive, adapt and grow no matter what kinds of chronic stresses and acute shocks they experience (100 Resilient Cities, 2015).

Resilience applies to cities because they are complex systems that are always adapting to changing conditions and circumstances. Conceptually, it becomes relevant when physical and social systems are threatened: a city’s ability to maintain essential functions can be threatened by acute shocks and chronic stresses. In turn, these shocks and increasing pressures may result in economic and physical damages, or even societal collapse (Arup, 2015). Examples of acute shocks are earthquakes, floods, terrorism and extreme cold or heat; examples of chronic stresses are water scarcity, lack of social cohesion, poor air quality and poverty. Resilient systems need certain conditions that enable them to withstand, respond and adapt in suitable ways. Arup (2015) identifies the following attributes (Figure 2.2):

Figure 2.2 Qualities of a resilient city (Arup, 2015)

• Reflective; reflective systems accept the increasing uncertainty and change in the world and have mechanisms to evolve. They will modify standards and norms based on evidence rather than seeking a ‘permanent’ solution. Institutions systematically analyse past experiences to improve their future decision-making.

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• Resourceful; people and institutions are capable to achieve goals and meet needs during shocks or when under stress. This can include setting priorities, and mobilising and coordinating human, financial and physical resources. Resourcefulness is of key importance in restoring functionality of critical systems.

• Robust; robust systems include well-constructed and managed physical assets, that can withstand the impacts of hazards without significant damage or loss of function. Robust design anticipates possible failures. Over-reliance on a single asset, cascading failure, and thresholds that might lead to catastrophic events when exceeded must be avoided. A robust water system also implies: being not very sensitive to unanticipated changes in pressures, reducing system fragility, allowing dynamics in water levels.

• Inclusive; inclusiveness emphasises the need for broad consultation and engagement of communicates, including vulnerable groups. Addressing shocks and stresses per sector, location or community in isolation is detrimental to inclusiveness. A sense of ‘shared ownership’ is needed to build urban resilience. An ‘inclusive water system’ could also involve a water system that includes recreation, agriculture, housing, and is connected to other urban areas regarding mobility and navigation.

• Integrated; integration and alignment between systems promote consistency and makes sure that all investments support a common goal or outcome. Exchange of information between systems enables a collective functioning and allows rapid response through shorter feedback loops in the city.

• Flexible; this implies that systems can adapt and change as a response to different circumstances. It can be achieved by introducing new knowledge and technologies. Furthermore, it also means using traditional knowledge and practices in new ways. Decentralised are usually favoured. A flexible water system would also mean: not a-priori excluding alternative strategies and preventing lock-in • Redundant; redundancy is spare capacity within created on purpose systems so that they can accommodate disruption, extreme pressures or surges in demand. Diversity is needed: the presence of multiple different methods to achieve a need or fulfil a certain goal. Redundancies must be intentional, cost-effective and prioritised at city-scale.

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RESILIENCE PRINCIPLES FOR THE ZONA PATRIMONIAL As part of the outcomes of the participatory workshops, several principles have been established for the development of future scenarios and a long-term water strategy for the Zona Patrimonial of Xochimilco, Tláhuac and Milpa Alta. These are as follows:

• Water is part of the problem and the solution.

• The water crisis is the result of the current water management.

• Main water challenges of the ZP are flooding, water management and water quality• Without channels there are no chinampas.

• Water control for water distribution.

• Water recovery for water reuse.

• Smaller measures are easier to implement.

• Flexible measures instead of strong measures.

• Sustainable housing a solution for irregular human. settlements.

Although these principles may have a broad value for the area of Zona Patrimonial, their definition enables the identification of challenges pressuring the water system.

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If we translate this into an overall principle, this could be formulated as a strategic water principle as follows:

Recycling and recovery, control and monitoring Recycling as a tool for treated water management and flow control in the channels, is of paramount importance to close cycles or water routes in the chinampera area. Treated water mainly for the WWTP Cerro de la Estrella will be discharged into the channels to increase the water levels in specific areas. The surplus of treated water will be used to meet the agricultural water demand in the recovered chinampas and the existing agricultural areas.

It is projected that by implementing control measures, the flow can be directed uniformly towards the seven existing WWTPs for further treatment. By diverting the water flow of the channels, and the integration of the WWTP into a single network, the hydraulic system of the ZP will not be affected in case of emergency or failure.

Sanitation is strongly linked to recycling. Within the ZP sanitation will be used in a broad term. The recovery of the areas of Xochimilco, Tlรกhuac and Milpa Alta does not refer only to improvements in water quality in the chinampera area. It also integrates the development of a sanitation network in the urban areas, especially in the regions with irregular settlements. To solve the illegal discharges of wastewater, it would be necessary to ensure the provision of sewage in this region, either by a centralised system or a decentralised system.

It is recommended to verify the efficiency of the treatment process of the current infrastructure since the concentration of pollutants, and the volumes reaching the WWTP are expected to increase.

Flow control in the distribution of treated water feeding the channels is of utmost relevance for the restoration of flooded chinampas. It is estimated that 90.1 ha of flooded chinampas and 1,099.0 ha of salvageable chinampas can be recovered with the implementation of control systems. These systems will benefit the development of the region since changes in

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the water levels can promote several economic activities such as tourism, recreation and agriculture. Also issues such as flood risk and connectivity of the channels will be addressed.

The main infrastructure required for the implementation of a hydraulic control system are gates, navigation locks and weirs. To formulate a better water strategy, it is recommended to validate and calibrate the surface water model, sewage model and groundwater model.

To avoid the overflow of the channels due to increase in runoff, it is recommended to store the water and promote groundwater recharge. Additional research is needed to evaluate the feasibility of absorption wells in the area. It is estimated that the adsorption wells will drain the surplus runoff in flood-prone areas. For this intervention, it is important to comply with regulations regarding water quality.

Furthermore, control systems are being integrated with monitoring systems to provide a better understanding of the urban water cycle. Although the knowledge of the control systems can be obtained through the application of numerical models, the acquisition of real data will provide a better understanding of the behaviour and situation of the channels. To ensure the proper operation of the channels of the chinampas, monitoring land subsidence becomes crucial. The monitoring of changes in water levels can be done either by simple methods, e.g. onsite measurements or more advanced methods, e.g. pressure sensors and LIDAR mapping.

Other aspects that require constant monitoring are water quality and treated water discharge. The implementation of monitoring systems in the WWTP will verify its proper operation, both in its capacity and in its treatment processes.

Discharge water quality should comply with the limits established by Mexican Standards (NOM). It is advisable to make frequently measures of the following parameters: temperature, pH, conductivity, coliforms and turbidity. In the event of discrepancies between NOM parameters and measurements, a complete chemical analysis should be carried out for the identification and correction of possible failures in the treatment.

CHALLENGES AND PRINCIPLES FOR A RESILIENT WATER SYSTEM

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ZONATION Once the general principles for water resilience in ZP have been formulated, and the challenges and vulnerabilities of the water system have been identified, specific objectives to promote water resilience needs to be set-up. Due to mixtures of land uses in the ZP, each of the areas will require a different water management model. The combination of residential areas, agricultural areas, cultural heritage, nature, recreational areas and tourism makes the ZP a unique system. Therefore, it is vital to optimise water management in such a way that can cope with the individual functions as well as with the system as a whole.

Figure 2.6 presents the land uses of the Zona Patrimonial of Xochimilco, Tlรกhuac and Milpa Alta. It can be observed that most of the agricultural areas are located within Xochimilco; in contrast, the agricultural areas with a surface of 2,176.59 ha are can be found mostly in Milpa Alta. Urban areas are distributed along the ZP polygon.

Figure 2.6 Land use map of the Zona Patrimonial of Xochimilco, Tlรกhuac and Milpa Alta.

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

A preliminary zoning map was developed as a guideline for the formulation of specific objectives (Figure 2.7 ).The zones for the implementation of the objectives can be divided as follows:

• Urban areas: water consumption is mainly for domestic purposes. The target is to have 100% coverage of the water network and drainage, as well as to improve welfare.

• Chinampas area: it includes the chinampas of Xochimilco, San Gregorio Atlapulco, San Luis Tlaxialtemalco, Tláhuac and Mixquic.

The management of the chinampas should promote an increase in treated wastewater discharge for the promotion of activities related to tourism, recreation and economic development, e.g. navigation of channels and agriculture.

• Flood-prone areas: include the flooded chinampas near San Gregorio Lagoon. Water management needs to address the risk of flooding and the recovery of chinampas by decrease treated wastewater discharge.

• Agricultural areas: Water reuse is exclusively for irrigation of the agricultural regions known as “tablas”. It includes areas of Xochimilco, Tláhuac and Milpa Alta. Water management should balance the used of treated water for irrigation and water used to feed the channels of the chinampera zone. Flood-prone areas located in the northwest corner of the ZP (Tláhuac) do not represent a problem since flooding helps to moisten the land.

• Informal settlements. This area is characterised by a strong social conflict since illegal settlements are not officially recognised by the government, and therefore the provision of public services such as drinking water and sanitation are not possible.

CHALLENGES AND PRINCIPLES FOR A RESILIENT WATER SYSTEM

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Figure 2.7 Zoning map and identification of water challenges in the ZP.

Urban area The implementation of measures for this area should target water security, according to the acceptable levels established by the OECD. Measurement needs to be flexible enough to cope with changes in the water flow due to the effects of climate change in the hydrological cycle.

Changes in water management for the urban areas should aim to reduce drinking water consumption on a household level. It is estimated that approximately 40% of the demanded water is used for showering (see Figure 2.8 ). By reusing this water stream for flushing toilets and irrigation, it will be possible to reduce water demand total water consumption. Other measures to reduce water consumption includes the implementation of water saving devices such as eco-shower heads, vacuum toilets and water-saving washing machines. For the application of these measures, minor changes are required such as the implementation of a second pipeline, a water tank for the collection of grey water, and the implementation of a water treatment system for greywater to be suitable for irrigation.

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Figure 2.8: Water consumption on a household level in Mexico City. (Adapted from UAM, 2017)

Rainwater harvesting has been proven as a suitable solution for the reduction of water demand. At the same time, this measure reduces the risk of flooding since the water flow that reaches the combined drainage of Mexico City will decrease. Experts affirm that rainwater harvesting on a household level allows water supply for 5 to 8 months.

However, changes in rainfall patterns may affect this estimation.

Since 2009, the non-profit organisation “Isla Urbana � has installed more than 4,870 systems in Mexico City. Their collections systems are capable of harvesting and filtering 5,000 litres of water. After the filtration, the water is considered potable. Minimum requirements for the installation of rainwater harvesting are a roof surface of 80 m2. However, the required investment for the implementation of this system is approximately 5,000 USD of which 20% it is covered by the residents and the rest by government subsidies and private donations.

2.5.2 Chinampas area Measures for the recovery of chinampas should target the different economic interests in the area. By implementing a regulation control, it will be possible to regulate the water levels in such a way that the risk of flooding in the chinampas near San Gregorio Lagoon decreases. Additionally, the implementation of this measure should allow the navi-

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gation of the channels as well as irrigation of the agricultural areas. In this case, the principle of “Smaller measures are easier to implement“ is applied.

Although an exhaustive analysis of the water quality of the region is part of phase II of the project ”A Resilient Water System for the Zona Patrimonial of Xochimilco, Milpa Alta and Tláhuac”; outputs of the participatory workshops indicate the viability of improvements in water quality by using the existing infrastructure. Currently, seven WWTPs are located in the region: El Llano, La Lupita, San Andres Mixquic, San Lorenzo, San Luis Tlaxialtemalco, San Nicolás Tetelco and Cerro de la Estrella. Actions to direct the water from the channels to these WWTPs are needed; likewise, adjustments in the treatment process are recommended.

The implementation of hydraulic control systems represents an added value for the recovery of flooded chinampas and the potential chinampas. By implementing a control system, it will be possible to distribute the water flow uniformly among the current channels as well as within the dry channels. Hence, flood-risk in the critical areas might decrease.

It is within the recovery of the chinampas that the fundamental principle “without channels there are not chinampas” is fulfilled.

In addition to water regulation in the channels and improvements in water quality, measurement and recovery of data regarding land subsidence become indispensable. A monitoring system allows the assessment of changes in the topography of the ZP, which in turn represent the imbalance between groundwater recharge and extraction.

In addition to water regulation in the channels and improvements in water quality, measurement and recovery of data regarding land subsidence become indispensable. A monitoring system allows the assessment of changes in the topography of the ZP, which in turn represent the imbalance between groundwater recharge and extraction. The reported information will serve as a baseline for the development of a resilient plan for the area of Xochimilco, Tláhuac and Milpa Alta.

2.5.3 Flood-prone areas Regulation of the water level of channels is a key component of flood risk reduction. By achieving an optimal distribution of discharges in the area, the probability of flood risk reduction in the areas of Atlapulco and Tlaxialtemalco increases. Strategies for flood-risk management should address changes in the water levels due to increase of treated discharges volume and available runoff.

2.5.4 Agricultural areas The reuse of treated water will target issues such as water scarcity, water security water availability and water quality, in the short and long-

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term. Projections estimate that the water flow from the WWTP Cerro de la Estrella might cover the increase in water demand for irrigation of the ZP.

Figure 2.9 Example of the principle “without channels there are no chinampas�.

(Source: UAM, 2017)

2.5.5 Informal settlements The implementation of measures in these areas is challenging since as previously mentioned, irregular settlements are not recognised by the government. Strategies for this area should target illegal discharges by promoting alternative sanitation system such as biodigesters for wastewater treatment. By improvements in the sanitation of the area, it is possible to achieve an increase in water quality and to decrease health risk.

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A WATER RESILIENCE PLAN FOR THE HERITAGE ZONE OF XOCHIMILCO, TLAHUAC AND MILPA ALTA

INCORPORATING GEOLOGICAL RISK IN WATER RESILIENCE

EXECUTIVE SUMMARY The Heritage Zone of Xochimilco Tlahuac and Milpa Alto is located on lacustrine deposits composed of 70 meters thick layers of soft silt and clay soils. The southern and northern parts of the study area cover a transition zone between lake soils and mountain ranges that surround the studied Heritage Zone. This transition zone is composed of alluvial clay material coming from higher topographic areas.

Consolidation of lacustrine soft soils is caused by declining water tables due to aquifer overexploitation and causes fracturing and differential subsidence of subsoil resulting in damages of civil infrastructure.

According to the information collected, we mapped already existing fractures and identified fracturing trends, which may lead to damages at the surface if aquifer overexploitation continues.

An analysis of indicators of ground control points was made in order to monitor consolidation speed of the superficial soft strata. This was useful to estimate changes in the fundamental period of the soil (until 2070) and to identify zones with the highest changes in the superficial thickness.

A probabilistic analysis was made to estimate the intensities associated to seismic hazard within the Heritage Zone of Xochimilco Tlahuac and Milpa Alta. A catalogue with more than 100 years of historical earthquake data was compiled and about 45 seismic sources were defined along the Mexican territory. One main advantage of a probabilistic analysis is that it accounts for uncertainty, which is an important aspect as seismic occurrence is uncertain and can, therefore, not be analyzed as a deterministic process. Additionally, site effects, caused by soft deep strata, may occur in the region, and were therefore included by means of transfer functions, which in general terms, relate the stiffness of the soil (soil dominant period) to the amplification factor. Analyses were carried out for four different return periods: 43, 250, 475 and 2475 years.

Finally, the influence of subsidence in seismic intensities was studied.

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Consolidation of lacustrine soft soils is caused by declining water tables due to aquifer overexploitation and causes fracturing and differential subsidence of subsoil resulting in damages of civil infrastructure.

The largest seismic intensities occur in the middle of Tlรกhuac District, just at the Heritage Zone boundary. Besides, it was observed that there is a direct relation between intensity changes and fracturing alignments; especially for the 0.2s spectral ordinate. Seismic intensities range from 0.03 to 1.4g, depending on the spectral ordinate and return period evaluated. These intensities were expected since this zone is characterized by soft strata soil and site effects which considerably amplify the seismic waves.

Risk can be seen as the possibility to suffer significant losses or negative results due to the occurrence of natural hazards. In this study, we focused on hazards related to earthquakes. Risk is related to both the probability of occurrence of an adverse event and its potential to cause damage to infrastructure. It is also closely linked to human perception, as it is clearly perceived when everyday activities are interrupted because of a natural disaster, e.g. water shortage due to damaged pipelines of drinking water due to an earthquake. It is, therefore, important to understand possible damages related to earthquake hazard in order to take actions to minimize the impacts.

Several aspects of risk can be considered, including, physical damages, direct and indirect economic losses, social impact, public health, interrupted public services, the impact on incomes or opportunities that are lost by communities or society during a contingency.

In this study, annual average loss, probable maximum loss and loss of a critical scenario are presented. The maximum modeled loss was found for the critical scenario. Additionally, the September 1985 and 2017 historical earthquakes losses were estimated and compared.

The results shown in this document can provide guidance to develop actions focusing on the knowledge, reduction, and risk transfer (financial protection) for an adequate emergency response plan, rehabilitation and reconstruction.

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INTRODUCTION Derived from studies for the project Heritage Zone (HZ), this report shows the results of a research on structural-geological documentation of the HZ area within the municipalities of Tláhuac-Xochimilco-Milpa Alta. On the other hand, it was analyzed the evolution of the fundamental period of the subsoil derived of the effects of consolidation of compressible lake deposits.

We used available superficial geological information from previous studies as a reference. To analyze the evolution of the fundamental period of the soil, we used work by the System of Evolution of Subsidence and Seismic Design Spectra, SEHEDIS (by its acronym in Spanish, Sistema de Evolución del Hundimiento y Espectros de Diseño Sísmico). This study documented subsidence in the Valley of Mexico, from monitored level banks of the Water System of Mexico City, SACMEX (by its acronym in Spanish, Sistema de Aguas de la ciudad de México), which allowed us to predict changes up to 2070. This information was collected in a Geographical Information System for subsequent risk analysis.

Objectives Geology and subsidence

• Compilation of available information of local geology and mapping subsoil fracturing in the Heritage Zone.

• Evaluation of fundamental period of the soil in the Heritage Zone for the years 2020, 2030, 2040 and 2070.

Seismic hazard

• Correct estimation of the Seismic Hazard at Heritage Zone of Xochimilco, Tláhuac and Milpa Alta.

• Modification of seismic hazard intensities due to possible site effects.

• Estimation of the impact of the subsidence problem for the seismic hazard intensities.

Risk estimation This study encompasses database structuration and loss estimation for seismic and subsidence hazards. In order to achieve these goals, the following infrastructure characteristics were taken into account:

• Geographic location

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37

• Structural system types • Building characteristics and details.

Location The Heritage Zone, located in the southeast of Mexico City, has a high ecological and cultural value and covers part of Tláhuac, Milpa Alta and Xochimilco Municipalities (Figure 3.1). The polygon of the Heritage Zone was originally based on the area of Historic Monuments in Xochimilco, Tláhuac and Milpa Alta, according to the presidential decree emitted in 1986.

The polygon has an area of 89km2, however, recent investigations (2005 and 2014) by Metropolitan Autonomous University, UAM (by its acronym in Spanish, Universidad Autónoma Metropolitana) and Xochimilco Municipality, show that the surface is 69km2 for which clarifications were requested in 2013 by the World Heritage Centre. These clarifications resulted in a modification of the original polygon, which includes, since 2014, three additional zones. The first one includes the canoeing track called Cuemanco and the chinampa zone Amalacachico-Toltenco; the second includes the archeological zone Cuahilama and the third includes the lake between Tláhuac and the limits with the State of Mexico. In total, the new polygon surface is 75 km2 (González-Pozos, 2016).

Figure 3.1. Location of heritage zone of Xochimilco, Tláhuac and Milpa Alta

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ASSESSING SEISMIC HAZARD Introduction and Background This section includes the details of the Probabilistic Seismic Hazard Analysis (PSHA) carried out in the heritage zone of Xochimilco Tlรกhuac and Milpa Alta. It describes and summarizes the methodology and input data used to develop the seismic hazard model, including details of the approaches and assumptions adopted for the data preparation.

It also describes the tool used for the PSHA computation and presents the results in terms of ground motion intensity distribution of peak ground acceleration (PGA) and spectral acceleration for different return periods.

The main objective of any PSHA is to provide a long-term relationship between the ground motion intensities and exceedance rates at any given site. In the light of this, the specific aim of this study is to obtain these relationships for the site of interest in terms of the above-mentioned metrics. The obtained results are, therefore, only valid for the location of the heritage zone of Xochimilco Tlรกhuac and Milpa Alta (see Figure 3.1).

According to the methodology proposal by Cornell (1968) and Esteva (1970), it is required that all earthquake sources affecting the site(s) of interest are identified and characterized to develop a seismic hazard model. In case individual faults are unknown, earthquake sources are identified by subdividing the study area into different regions where the seismicity occurrence can be assumed uniform. , We then computed quantitative measures of hazard intensities, such as ground motion levels in terms of spectral acceleration, based on the earthquake catalogues and the ground motion prediction equations (GMPE).

Due to uncertainties in both the occurrence of future earthquakes (i.e., size, location, frequency) and generated ground motion intensities, a probabilistic approach for the estimation of seismic hazard is required.

For a robust estimation of the seismic hazard, these uncertainties need to be identified, quantified and propagated during the analysis using state-of-the-art methodologies, such as the one implemented in the R-CRISIS software (Ordaz et al., 2019), which was used in this study.

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Overview of the Seismicity in the Study Area The scope of this study is to estimate seismic hazard at the heritage zone of Xochimilco, Tlรกhuac and Milpa Alta. To define the area of influence for seismic hazard analysis purposes, a comprehensive review of historical hazard footprints and ground motion recordings for Mexican earthquakes was performed, complemented with the verification of the ranges of distances used by the GMPEs considered in the model.

The seismic activity in Mexico and Central America is high, as evidenced by the large number of destructive earthquakes that have occurred in the recent past. The seismicity in the region is generated by two main tectonic systems: the first one corresponds to the onshore volcanic arc, characterized mostly by shallow seismicity, and the second one is associated to the subduction zone (offshore and inland). Among the most notorious and destructive earthquakes that have occurred in the region are those of Michoacรกn in September 1985 and more recently, in Puebla-Morelos in September 2017.

According to CFE (2015), Mexico City is categorized as a high seismic region (Zone C). It is influenced by several seismic sources and important earthquakes have been recorded in the past. Recorded earthquakes in these areas have had magnitudes above MW6.0. Besides, Mexico City, unlike other cities of Mexico, presents a particular characteristic, which joint to surrounding seismicity, may generate greater intensities in the presence of an earthquake. Such particular feature is known as site effects, which will be discussed further.

Methodology of Probabilistic Seismic Hazard Assessment (PSHA) R-CRISIS, as other software to assess the seismic hazard, follows the well-known Esteva-Cornell methodology to compute seismic hazard.

In general terms, this can be summed up in six steps depicted in Figure 3.2. A brief description of these six steps is presented here and a more detailed explanation is provided in Appendix F.

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Figure 3.2. Methodology of Probabilistic Seismic Hazard Assessment (PSHA)

1. Historical Earthquake Catalog: in this step, a complete and updated historical earthquake catalog is defined from different information sources such as ISC-GEM global instrumental catalogue (Storchak et al., 2013) and the USGS-NEIC. Figure 3.3 shows the epicentral locations of the earthquakes included in the working catalogue, classified by magnitude ranges.

2. Identification of seismic sources: once an earthquake catalog is defined, the geometry definition of seismic sources is performed by reviewing previous seismo-tectonic zonations and identifying regions where seismicity can be assumed uniform and with similar characteristics. Figure 3.4 shows the geometries of the seismic sources included in R-CRISIS for performing the PSHA. 3. Computation of the parameters of the seismicity models: In this study, the seismicity of all sources is assumed to follow a Poissonian process, which briefly assumes independency in time and space between events (reason why a de-clustering process of the historical catalogue was needed when developing the working catalogue). Two Poissonian seismicity models are commonly used

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in the development of a PSHA: • Modified Gutenberg-Richter (Cornell and VanMarcke, 1969) and, • Characteristic earthquake model (Youngs and Coppersmith, 1985).

The parameters that describe these seismicity models are determined by means of statistical procedures, which are mentioned in Appendix F. 4. Selection of ground motion models: these models provide an intensity measure (e.g. acceleration or velocity) due to a hypothetical earthquake that takes place at R distance from my site of study with some specific parameters (magnitude, type of faulting, rupture size, etc.). Selection of such models may be difficult because of the multiple options to select from, in these cases a logic-tree calculation is suggested. On the other hand, there are cases in which the offer of attenuations models is scarce, hence, models from regions with similar characteristics have to be applied.

5. Inclusion of site effects and digital elevation model (DEM) – optional: in cases where site effects are significant, the influence on seismic intensities has to be quantified in order to modify the rock intensities. R-CRISIS amplifies rock intensities by means of transfer functions, which contain amplification factors dependent of spectral period and rock intensity.

On the other hand, it is possible to include a Digital Elevation Model (DEM) to improve the real distances calculations. The DEM describes the real topography of the study region and depths/distances of all possible earthquakes will be more accurate.

6. Graphical representation of the results: once the seismic hazard calculation has finished, the results may be represented by means of hazard maps or curves; in Figure 3.5 and Figure 3.6, respectively, examples are presented.

F

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igure 3.3. Magnitude of the events included in the working catalogue

Figure 3.4. Geometries of the seismic sources considered in the model

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Figure 3.5. Example of hazard map for a certain spectral and return periods

Figure 3.6. Example of hazard curve for a certain site and return period

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Seismic Hazard Results Seismic hazard was calculated for four return periods and two spectral ordinates. These were defined based on our experience and the type of infrastructure present in the Heritage zone: Table 3.1. Return periods and spectral ordinates defined for the PSHA

Return period Tr (years)

Spectral ordinate T (s)

43 250 475 2475

0.0 (PGA) 0.2

Since our results are in function of acceleration, we followed the methodology proposed by Worden, Gerstenberger, Rhoades, & Wald (2012) to correlate seismic acceleration and Mercalli Intensities. It should be noted that these correlations are only valid for PGA, PGV, 0.3s, 1.0s and 3.0s spectral periods. Correlations between seismic accelerations and Mercalli Intensities for PGA are presented in Table 3.2.

3.3.4.1 Rock Seismic hazard maps for return periods and spectral ordinates as defined in Table 3.1 are presented in Appendix F. A summary of the results is shown in Table 3.3.

3.3.4.2 Soil Seismic hazard maps for return periods and spectral ordinates as defined in Table 3.1 are presented in Appendix F. A summary of the results is shown in Table 3.4.

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Table 3.2. Correlation between Seismic accelerations and Mercalli Intensities (USGS, n.d.; Worden et al., 2012)

Table 3.3. Summary of seismic hazard results on rock Spectral ordinate, T (s)

0.0 (PGA) Return period, Tr (years)

0.2

Acceleration (cm/s2) Min: __ Max: __

43

46 54

75 87

250

97 112

159 188

475

125 143

207 241

2475

224 258

376 442

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Table 3.4. Summary of seismic hazard results on soil Spectral ordinate, T (s)

0.0 (PGA) Return period, Tr (years)

0.2

Acceleration (cm/s2) Min: __ Max: __

43

33 211

52 272

250

67 446

112 581

475

87 569

144 755

2475

157 1031

264 1373

Comparison between rock and soil results As expected, the inclusion of the site effects could modify the rock results, not uniformly, but depending of the Response Spectra Ratios (RSR) defined for each point of the grid. In order to quantify how much the rock intensities were amplified due to the site effects of the zone, comparison between rock and soil results for PGA and T=0.2s are presented in Figure 3.7 and Figure 3.8, respectively.

The biggest amplifications of the peak ground acceleration (about 3%) are located in the middle of Tlรกhuac district, just at the border of the Heritage Zone. Another region with important amplifications (about 2%) is at the west of the Heritage zone, just at the border between Xochimilco and Tlalpan districts. The amplification outside of the Heritage Zone is minor, almost 0%, which means that site effects in such regions are not influencing the rock intensities. On the other hand, amplifications for T=0.2s are less than those for PGA, the maximum amplification is close to 2.3% and occurs in the same region as PGA. In general terms, the amplification ratios of T=0.2s follow the same trend as PGA.

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(a) (b)

(c) (d) Figure 3.7. Comparison between soil and rock results (PGA): a) Tr=43 years, b) Tr=250 years, c) Tr=475 years, d) Tr=2475 years

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(a) (b)

(c) (d) Figure 3.8. Comparison between soil and rock results (T=0.2s): a) Tr=43 years, b) Tr=250 years, c) Tr=475 years, d) Tr=2475 years

Geology And Subsidence Geologically, the Heritage Zone is located on a lacustrine plain (Figure 3.9), which originated from gradual silting in the basin as a result of the closure of the valley at the southern portion of the old valley, due to the appearance of the Chichinautzin Sierra (Mooser et al, 1995-2015).

The silting of volcanic products and transport of fluvial material in the area, led to new conditions that favored the generation of large lakes within the basin, the current lacustrine plain used to be the bottom of such lake.

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This plain is composed of proluvial (limes and clays) and lacustrine (clays and limes) materials, as well as intercalations of sands with volcanic ashes, which erode to highly compressible clays.

Figure 3.9. Geology of the Heritage Zone (Mooser et al, 1995-2015)

As a result of the erosion of hillsides and the fluvial carrying of igneous bodies, volcanic materials deposits were formed, which buried the irregular lacustrine relief. This led to a variable thickness of the deposits lacustrine. A thickness of lacustrine deposits of at least 70 meters from the surface to the basement constituted by igneous material is derived from documented stratigraphy of the deep of San Lorenzo Tezonco (Arce et al, 2015) located within the Heritage Zone.

In the north of Heritage Zone, there is a transitional zone composed of alluvial material. As you move uphill, the thickness of the alluvial material stratum is less and the igneous material begins to appear. In the south, at the border of ZP there is a transitional strip of alluvial material which does not get in contact with the igneous stratum. The monogenetics cones (Sierra Chichinautzin, south of the HZ), originating from volcanic events, are constituted by andesitic rock (INEGI, 2015).

Structurally, Xicomulto fault and Santa Catarina fault are mapped (GarcĂ­a Palomo et al, 2008), which together for Santa Catarina or TlĂĄhuac-Tuly-

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ehuaco dig (Arce et al, 2013a), whose origin is related to the basin formation since there is no evidence that it is active or any manifestation in surface.

On the other hand, available information of fracturing in the borders of volcanic structures and in the lacustrine plain is documented (MejĂ­a, 2012; CENAPRED, 2018). This fracturing phenomenon is associated to the limits of the old lake and the contact between the buried volcanic deposits with the lacustrine sediments (clays and limes). In this zones it is shown that most of the fractures are associated to fracturing patterns with geometries in the perimeter of the Sierra of Santa Catarina and in the Chichinautzin Sierra with preferential direction WE, which is associated to the border of volcanic structures and not to the presence of active faulting.

These contacts, originate vertical differential displacement with geometries of stepped and lineal fracturing which are mainly due to different fronts of volcanic deposits that were cover by argillaceous sequences.

3.3.5.1 Methodology Taking into account the available information of the thickness of compressible lake deposits and the dominant period of the soil, techniques, spatial interpolation were implemented to establish in each level bank the evolution of the dynamic parameters of the subsoil, specifically the fundamental period in which the dynamic response of soil can be represented.

Information of SEHEDIS was gathered, data developed by Libertad Pino (2018), where it was registered the subsidence in different level banks located in geotechnics zones of transition and lake.

After knowing the subsidence rate of the studied level banks, the data were adjusted to mathematical functions to estimate the future prediction of the evolution of the change in the fundamental period of the soil, as well as the thickness of compressible strata. The following expression relates the period, shear wave velocity and the thickness of the compressible strata.

T_s=4H/V_s

(3.1)

As initial parameters, the current values of compressible thickness and dominant period of soil vibration of the Seismic Design Construction Technical Standards (Gobierno del Distrito Federal, 2004) were taken.

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To obtain the values of compressible thickness for each level bank, the curves of the iso-thickness map were digitized, adding the location of the level banks with the projected subsidence rate.

On the other hand, the shear wave velocity were estimated by the year of 2004 and it was considered as constant for the projected estimations.

Figure 3.10. Location of level banks (SEHEDIS)

3.3.5.2 Subsidence Results From the established prediction of future subsidence in the analyzed level banks and its relation with the reduction of deposit thickness, the change in the fundamental period of soil vibration for different years were mapped. The different year maps are 2020, 2030, 2040, 2050 and 2070.

It is emphasized that compressible soil deposits with greater thickness are susceptible to suffer most significant changes in the seismic response over time (Figure 3.16).

The reduction of compressible thickness directly involves a change in the fundamental period of soil vibration and in the shear wave velocity.

The consolidation process affects the compressible thickness.

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Figure 3.11. Isoperiods map for 2020

Figure 3.12. Isoperiods map for 2030

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Figure 3.13. Isoperiod map for 2040

Figure 3.14. Isoperiod map for 2050

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Figure 3.15. Isoperiod map for 2070

Figure 3.16. Fundamental isoperiods rate 2070 vs 2020

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3.3.5.3 Conclusions and Recommendations It is identified that zones with high geological degree are concentrated in geological transition zone (parallel to contour lines), between the lake and alluvial units, direct consequence of regional consolidation presented in the Valley of Mexico. It manifests in the transition zones between the contacts of firm soils with soft soils and are characterized by presenting a step towards the area of greatest settlement.

The area where major modification of fundamental period occurs is in the central part of the basin of the Heritage Zone (red zone in Figure 3.16) due to regional consolidation of compressible stratum because of the extraction of water by pumping in deep layers.

It is recommended that zones where fracturing has already been identified (Figure 3.17), a monitoring campaign be carried out to characterize its evolution and new zones where fracturing is identified. It is recommended to perform lifting by interferometry along the transition zones.

It is recommended to make a detailed geological cartography in transition zone to refine and identify the limits of the lake and alluvial units, since it is an indicator of the areas that would present future geological fracturing.

Figure 3.17. Susceptibility to geological fracturing

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3.3.6 Influence Of Subsidence In The Seismic Hazard Results As mentioned before, subsidence is a problem caused by over-pumping water from deeper strata soil, which has increased over the years. Since the city is in continuous growth, Mexico City residents must figure out how to deal with it day by day. The reduced thickness of deeper strata and the stiffer soils, as a consequence of subsidence, directly affect the ground motion during an earthquake and affect the dynamic response against the seismic waves. The fact that the soil becomes stiffer is not always beneficial, sometimes this could mean that the amplitude of the response may be greater than before, in other cases, the response may be lighter; and in other cases, the response could be the same.

To better explain this problem, an illustration of how seismic hazard intensities may be affected (in a favorable and unfavorable way) by the subsidence problem is presented in Table 3.5. Evidently, for each site of analysis, there is a different soil dominant period and a different Response Spectral Ratio (RSR) function, however, all scenarios must be considered because they are equally probable to happen.

The seismic hazard model was rerun for each subsidence scenario defined in Section 3.3.4 and the seismic hazard maps are presented in Appendix F. The ground motion intensities influenced by subsidence were the same as those obtained in Section 3.3.4.2. This does not mean the seismic hazard model is incorrect, this means that soil dominant periods of Heritage Zone are located at either the beginning or at the end of the Response Spectra Ration (RSR) function (see “Same scenario� in Table 3.5). The reduction of soil period does not represent any changes in the ground motion because for both cases, before and after subsidence, the amplification factor is the same. For this region, the peak of the RSR function might be located in a different periods range.

It is important to clarify that results presented here are only evidence that the subsidence problem is not affecting the ground motion intensities, however, this does not mean that subsidence does not influence other aspects, such as differential foundation settling and surface fracturing. To quantify these other problems, another study with a different scope is needed.

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Table 3.5. How subsidence problem affects the seismic hazard intensities Favorable scenario

Unavorable scenario

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Same scenario

VULNERABILITY FUNCTIONS FOR WATER INFRASTRUCTURE Introduction Vulnerability is defined as the predisposition of an asset or group of assets to the damaging effects of a hazard. In risk context, vulnerability is expressed in terms of vulnerability functions, graphically represented in vulnerability curves, these curves describe the evolution of average damage or economic loss that the structure can have due to the increase of the hazard intensity. Figure 3.18 represents an example of a vulnerability curve.

Figure 3.18. Vulnerability curve

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In the framework of the seismic and subsidence risk assessment of hydraulic infrastructure of the heritage zones of Xochimilco, Milpa Alta and Tláhuac, vulnerability functions for the mentioned hazards and the following assets were developed (Figure 3.19):

• • • • • •

Drinking water network Sewer network Wells Tanks Wastewater treatment plants Chlorination plants

Figure 3.19. Vulnerability functions for the hydraulic infrastructure

From Section 3.4.2 to 3.4.6, vulnerability functions for drinking water network, sewer network, wells, tanks, and wastewater and chlorination plants are presented. The methodology used to obtain those vulnerability curves are explained in Appendix G.

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3.4.2 Vulnerability Functions for Drinking Water Network Drinking water network infrastructure is designed to carry water from affluence sources to final users. For the calculation of seismic and subsidence vulnerability functions, the vulnerability curves associated to each single structure considered were obtained following the methodology described below.

Drinking water network consists of a collection of structures (canals), underground pipelines, water treatment plants, regulation tanks and pumping stations. For pumping stations, the vulnerability functions are based on masonry buildings of medium and low height and associated electrical and mechanical equipment.

In this work, we evaluated water treatment plants and tanks separately.

Collection system structures consist of different types of aqueducts depending on the topography. In addition, there are open canals, which can be covered or not. Generally, the damages in canals are due to soil subsidence, liquefaction or landslides. Lining canals are less susceptible to damage, however, cracks can cause leaks. For this study, it is considered that collection canals are lined canals that collect water from rivers and nearby tributaries.

A water distribution system consists of a system of water pipelines with different diameter, with different functions along the distribution network. This piping system carries water from the water treatment plant to the final users. The materials used are steel, cast iron, fiber cement and PVC. The distribution lines are generally underground. Most of the observed faults are related to cutting efforts and deformations of the soil, mainly affecting the connections. Also, there are damages as a result of axial deformations caused by the relative deformations between the soil strata.

3.4.2.1 Seismic vulnerability curves for drinking water network Similar to Figure 3.18, in the following figures the vulnerability curves developed for this study are presented. In each one of them, the x axis represents the intensity analyzed: it can be PGA, which means Peak Ground Acceleration, Sa which means spectral acceleration, or Δ which is the relative settlement due to the subsidence. On the other hand, the y axis represents the expected value of the damage given the intensity

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of the x axis. For instance, in Figure 3.20 can be observed that for a PGA of 1000 gal, a damage of 20% is expected.

Figure 3.20. Seismic vulnerability curve associated to canals

Figure 3.21. Seismic vulnerability curve associated to underground pipelines

Figure 3.22. Seismic vulnerability function associated to pumping station

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3.4.2.2 Subsidence vulnerability curves for drinking water network

Figure 3.23. Subsidence vulnerability curve for sections canals

Figure 3.24. Subsidence vulnerability curve associated to underground pipelines

Figure 3.25. Subsidence vulnerability curve for electrical and mechanical equipment

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Figure 3.26. Subsidence vulnerability curves for medium or low height masonry structures

Figure 3.27. Subsidence vulnerability curves for pumping stations

3.4.3 Vulnerability Functions for Drainage Network System The waste water or drainage system recollects waste water in underground conduits in a safe and quick way up to the place of final disposal.

Sewer networks are hydraulic structures that operate at atmospheric pressure, by gravity. Sporadically, short sections of the pipe network function under pressure or vacuum. Sewer networks are usually composed of channels or circular, oval or compound pipes, mostly buried under public roads.

The elements of the sewer system for the calculation of vulnerability functions are shown in the following table.

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Table 3.6. Considered elements to derive vulnerability in the system SYSTEM COMPONENTS

ELEMENTS THAT DEFINE VULNERABILITY

Pipes

Underground pipes

Pumping stations

30% Buildings 20% Electrical equipment 50% Mechanical equipment

As can be seen from the table above, the functions corresponding to the analyzed assets have already been calculated previously, so, to know the process to obtain them, previous sections and Appendix G shall be consulted.

3.4.4 Vulnerability Functions for Wells Wells are buried structures which are used to extract water from the subsoil. These wells are generally constructed by excavation with shovels or using backhoes and are then lined with masonry. Wells are underground structures, and their performance is, therefore, linked to the soil surrounding them. Wells can suffer failures by compression and by shear related to soil deformation, such as soil failures.

Seismic vulnerability curves for wells

Figure 3.28. Seismic vulnerability curves for wells, showing expected value of loss (continuous line) and standard deviation (discontinuous line)

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Subsidence vulnerability curves for wells

Figure 3.29. Subsidence vulnerability curve for wells, showing expected value of loss (continuous line) and standard deviation (discontinuous line)

3.4.5 Vulnerability Curves for Tanks Tanks are used for regulation and storage of transported water and can be underground, superficial or elevated. Surface or underground tanks are constructed with reinforced concrete and have regular geometry.

Their foundation are made of sand or concrete bases. Elevated tanks are steel tanks, which are supported by steel columns.

Damage in underground tanks includes damage of walls and roof support systems. Surface tanks at ground level may also suffer from different kinds of damages. In case of steel tanks, damage can occur at the junction of the base and the walls. Additionally, wall bulging, rupture of the rigidly connected pipes, implosion due to sudden loss of content, differential subsidence, failure in the anchoring system, roof system failure and total collapse may also occur. For concrete tanks, the most common failure is roof system failure, cracking and sliding in construction joints. Elevated tanks generally fail because of inadequate support systems; if the damage condition exceeds a tipping point or leads to a series of damage, this generally results in total damage.

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3.4.5.1 Seismic vulnerability curve for tanks

Figure 3.30. Seismic vulnerability curves for surface tanks

Figure 3.31. Seismic vulnerability curves for underground tanks

Figure 3.32. Seismic vulnerability curve for elevated steel tanks

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3.4.5.2 Subsidence vulnerability curve for tanks

Figure 3.33. Subsidence vulnerability curve for underground and surface tanks

Figure 3.34. Subsidence vulnerability curve for elevated tanks

3.4.6 Chlorination and Wastewater Treatment Plants Wastewater treatment consists of several physical, chemical and biological processes meant to eliminate physical, chemical and biological contaminants present in wastewater. The objective is to produce clean water (or treated effluent) or reusable in the environment and also to produce solid residue or mud (also called biosolid or mud) for disposal or reuse.

The process to convert wastewater to drinking water is called purification. Purification processes vary from simple chlorine disinfection to eliminate pathogens to irradiation by ultraviolet rays, ozone application, etc.

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Treatment and purification plants consist of different devices and structures with each a specific objective and location to optimize process operations and costs.

The components considered in the estimation of the vulnerability curves for these types of plants are shown in the following Table.

Table 3.7. List and distribution of the components of a treatment plant Components

Distribution

Medium or low height masonry buildings

20%

Mechanical equipment

40%

Electrical equipment

15%

Superficial tanks

10%

Pipes

15%

Vulnerability functions of each asset have already been calculated individually.

The vulnerability function corresponding to treatment and purification plants is formed by the linear combination of the previously mentioned components. The following figures show these functions.

Figure 3.35. Seismic vulnerability curves for treatment and purification plants

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Figure 3.36. Subsidence vulnerability curve for treatment and purification plants

The vulnerability functions presented correspond to hydraulic infrastructure and should not be used for other types of infrastructure.

RISK ESTIMATION Introduction The risk assessment consisted in characterizing seismic and subsidence hazards in the study area, data preparation and database structuring for evaluation of exposed structures in ERN’s systems. Identification and assignment of structural vulnerability were done, as well as risk assessment through formal risk calculation.

Risk estimation obtained in this study corresponds to three types of traditional losses in portfolios studies for insurance purposes: Probable maximum loss (PML): is the loss associated with a predefined return period, usually set by the regulatory authorities of the insurance activity or, where appropriate, by reinsurance companies to determine the risk level. It is about the loss, for the whole set of structures analyzed, that will be exceeded on average once every T years, where T is the return period.

Pure risk premium: It is defined as the expected annual loss that a property or structure has or the totality of them. In view of this, we will have the possibility to analyze the results by property characteristics.

The loss per scenario: It is defined as the expected value of the

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current loss that would occur, for each structure or for the whole, for the critical scenario.

Results shown in this study are probabilistic in nature and consist of indicators of possible results due to the occurrence of future events, considering the current state of knowledge ; therefore, the results reported in this study do not represent, under any circumstance, future predictions of catastrophic events.

Probabilistic Risk Analysis The objective of probabilistic risk assessments is to determine probability distributions of losses due to natural hazards, considering uncertainties of each part of the process in a rational way.

The process of probabilistic risk analysis estimates the loss of every exposed asset for each hazard scenario and integrates it probabilistically using weight factors for the frequency or recurrence interval of each scenario. Therefore, probabilistic risk analysis involves uncertainties that cannot be underestimated and have to be considered in each step of the process.

Below, the basis of the general calculation is described: 1. Exposure definition: each exposed asset has to be defined in terms of exposure value, geographic location, structural system type, building characteristics and details.

2. Hazard evaluation: for each hazard, several scenarios are defined with their respective frequency. Each scenario is described by its hypocenter (longitude, latitude and depth) together with its intensity footprint, which is a grid describing the intensities, spatially distributed, generated by the occurrence of this event.

In some cases, this grid is also described by statistical parameters which, besides the intensity grid, defines a probability distribution function. All those events are gathered into one file with extension *.AME. Each scenario has the spatial distribution of parameters that allows to build the probability distribution of intensities produced by its occurrence.

3. Vulnerability of exposure: each structural type and natural hazard requires a vulnerability function. This function characterize the behavior of the construction during a hazard. Vulnerability functions define the expected loss as a function of the hazard intensity and are represented by curves that relate the expected value of damage to the hazard intensity.

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3.5.3 Exposure Summary Information provided by the client and obtained from open databases was updated to adequately represent the geometry. This information was divided into two groups: urban infrastructure and hydraulic infrastructure as shown in Figure 3.37.

Figure 3.37. Exposure summary at The Heritage Zone

Table 3.8 shows the total length of roads, sewage, reclaimed and drinking water network, considered within the Heritage zone. Table 3.9 shows items per infrastructure type at The Heritage Zone. The hydraulic and urban infrastructure distribution is presented in Appendix H. Table 3.8. Length of water supply, sewage, reclaimed water and road networks in the Heritage Zone Infrastructure Type

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Total Length (KM)

Water Supply Network

1,426

Sewage Network

1,323

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Infrastructure Type

Total Length (KM)

Reclaimed Water Network

45

Roads

4,981

Table 3.9. Number of items per infrastructure type Infrastructure Type

Total

Houses

462,202

Schools

3,393

Churches

475

Drinking Water Wells

259

Healthcare Buildings

205

Markets

170

Town Squares

135

Water Storage Tanks

57

Pumping Stations

41

Government Buildings

39

Cemeteries

33

Sport Centers

20

Sewage Pumping Station

17

Public Transport Stations

16

Water Pollution Control Treatment Plants

6

Drinking Water Plants

2

For modelling and evaluation in the ERN’s system, records were modelled as independent insurance policy with a coverage of 100% and no coinsurance and deductible.

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3.5.4 Probable Maximum Loss Table 3.10 details seismic probable maximum loss of hydraulic and urban infrastructure associated with different return periods and the probabilities (ruin probability) that these losses will be exceeded in the next 5, 10, 25, 50 and 100 years (exposure period) for the 2020 subsidence scenario. Moreover, seismic PML values for the 2020 subsidence scenario are presented in Figure 3.38.

Subsidence scenarios at 2030, 2040, 2050 and 2070 were omitted due to the small or null variations observed.

Table 3.10. Seismic Probable Maximum Loss for The 2020 subsidence scenario Tr

PML Hydraulic Infrastructure

PML Urban Infrastructure

PML Total

25

0.17%

0.26%

0.25%

18.13% 32.97% 63.21%

86.47% 98.17%

50

0.42%

1.00%

0.94%

9.52%

18.13%

39.35%

63.21%

75

0.81%

2.02%

1.90%

6.45%

12.48%

28.35% 48.66% 73.64%

100

1.31%

3.10%

2.91%

4.88% 9.52%

22.12%

39.35%

63.21%

200

3.69%

6.99%

6.63%

2.47%

4.88%

11.75%

22.12%

39.35%

500

9.30%

14.22%

13.65%

1.00%

1.98%

4.88%

9.52%

18.13%

1000

14.57%

20.23%

19.57%

0.50%

1.00%

2.47%

4.88%

9.52%

1500

17.82%

23.60%

22.93%

0.33%

0.66%

1.65%

3.28%

6.45%

2000

20.14%

25.86%

25.20%

0.25%

0.50%

1.24%

2.47%

4.88%

2500

21.94%

27.54%

26.89%

0.20%

0.40%

1.00%

1.98%

3.92%

(years)

74

Ruin Probability Exposure period (Te, years) 5

10

25

50

100

86.47%

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Figure 3.38. Seismic Probable Maximum Loss for the 2020 subsidence scenario

3.5.5 Annual Average Loss The overall seismic annual average loss (AAL) is estimated at 0.14% of the total exposure for the 2020 subsidence scenario. The AAL was 0.12% and 0.14% of hydraulic and urban infrastructure, respectively.

Seismic AAL of hydraulic infrastructure is detailed in Table 3.11. Sewage Pumping Stations have the highest seismic AAL with 0.70%. Additionally, seismic AAL distribution per hydraulic infrastructure type is presented in Figure 3.39.

Table 3.11. Seismic Annual average loss to hydraulic infrastructure Infrastructure Name

AAL - 2020

Sewage Pumping Stations

0.70%

Pumping Stations

0.39%

Reclaimed Water Network

0.14%

Sewage Network

0.12%

Water Supply Network

0.12%

Water Pollution Control Treatment Plants

0.08%

Drinking Water Plants

0.06%

Drinking Water Wells

0.04%

Water Storage Tanks

0.02%

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Figure 3.39. Annual average loss to hydraulic infrastructure

Seismic AAL of urban infrastructure is detailed in Table 3.12. Markets have the highest seismic AAL with 0.84%. Additionally, seismic AAL distribution per urban infrastructure type is presented in Figure 3.40.

Subsidence scenarios at 2030, 2040, 2050 and 2070 were omitted due to the small or null variations observed.

Risk maps with AAL distributions are presented in Appendix I. Table 3.12. Annual average loss to urban infrastructure Infrastructure Name Markets

AAL - 2020 0.84%

Healthcare Buildings

0.26%

Public Transport Station

0.20%

Government Buildings

0.15%

Houses

0.14%

Roads

0.14%

Sport Centers

0.10%

Schools

0.02%

Churches

0.02%

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Infrastructure Name

AAL - 2020

Town Squares

0.00%

Cemeteries

0.00%

Figure 3.40. Annual average loss to urban infrastructure

3.5.6 Additional Results Additionally, we performed a risk analysis of the earthquakes of September 19th of 1985 and 2017. These events were simulated in our seismic hazard model and the losses are presented in Appendix J. The most critical scenario with highest expected loss is also presented.

IMPLICATIONS FOR FUTURE INVESTMENT Some of the projects announced in Section 6.3 are more vulnerable than others to geological fracturing and subsidence problem. Based on the seismic study results, a couple of technical recommendations are given to consider in their future planning and construction.

Reconnection of the Rio Amecameca to the ZP The project aims at reconnecting the Rio Amecameca to the ZP to increase the volume of water available for water management in the ZP. This can be done by allowing (part of) the river water to flow in the southeast corner of the ZP instead of along the ZP to the north. This project will include channels and water treatment plants.

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The southeast corner of the ZP is not exposed to a high level of geological fracturing (see Figure 3.41), a few of them have, however, been identified and should be taken into account when the project is being planned. Channels are vulnerable to fracturing and mitigation measures are required to avoid leakages.

The subsidence analysis shows a probable change of soil period between -0.45 and -0.075s (see Figure 3.16) in the southeast corner. This implies a decrease of the soil strata thickness and an increase in stiffness. It is important to remember that channels are rigid structures, therefore, the seismic intensities experienced by these structures will be close to the Peak Ground Acceleration (PGA). We recommend to quantify the change of soil period and implications at the design stage, to ensure that future intensities will not more damaging than those initially estimated.

The same considerations apply for the design and construction of the water treatment plants. These kind of structures are not high-rise building, therefore, the structures will rigid.

Figure 3.41. Reconnection of the Rio Amecameca and geological fracturing (red lines)

3.6.2 Linear wetlands at transition boundaries The linear wetlands are underground structures and are, therefore, vulnerable to geological fracturing. The center part of the linear wetland

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is close to a geological fracturing system. Based on the pictures presented in Section 6.3.2, it can be inferred that it is important to avoid fracturing in the vicinity of the structures envisioned in this project, to prevent channel contamination due to leakage.

Figure 3.42. Linear wetlands (blue line) and geological fracturing (red lines)

We consider that structures in the project will not be affected by subsidence because they are simple, underground and shallow (lightly depth) structures. A change in soil period should not be relevant for the operation of these structures.

3.6.3 Water Control System Section 6.3.3 describes the proposed Water control system. It is envisioned that the project consists of the following elements:

• • • •

Hydrodynamics of channel systems Hydraulic structures Segmentation of the chinampa system 5 locks and 15 dams distributed in the chinampa´s wetlands of Xochimilco.

At this stage, however, the exact location for these structures is not yet decided. No further conclusions are therefore drawn here.

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3.6.4 Regeneration of PEX Xochimilco A cultural center could be (re)built to raise awareness about the integral regeneration of water. PEX Xochimilco is located at the northwest corner of the ZP, in a region where geological fracturing does not occur.

The subsidence analysis shows an expected change of soil period of approximately -0.075s. The change is minimum, however, the effect on seismic intensities should be evaluated to ensure that future intensities will not exceed the current ones.

3.6.5 Green Corridors The Green Corridors will be located at the southern limits of ZP. The corridor associated to San Luis Tlactaltemalco is the only one exposed to geological fracturing.

The subsidence problem is not as relevant as fracturing, as the expected change of soil period is about -0.075s, which corresponds to a minimum change of soil stiffness.

The Green Corridors should consist of long sidewalks and parks, without high-rise, making them invulnerable to the change of soil period. Geological fracturing should, however, be considered and strict prevention measures have to be made to avoid cracking.

Figure 3.43. Green corridors (green lines) and geological fracturing (red lines).

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81

A 82 WATER RESILIENCE PLAN FORPLAN THE FOR HERITAGE ZONE OFZONE XOCHIMILCO, TLAHUAC AND MILPA ALTA A WATER RESILIENCE THE HERITAGE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

REGIONAL AND LOCAL MODELLING OF THE WATER SYSTEM

INTRODUCTION The water system of the Zona Patrimonial is dominated by the water system of the Basin of Mexico. Adjustments in the water system of the Basin of Mexico will affect the water system of the ZP and vice versa.

To understand this relationship, regional models of the hydrological and groundwater systems of the Basin of Mexico were developed. This regional model describes and quantifies the water system above and below the ground and it serves to define and quantify the boundary conditions for the local model of Xochimilco and Zona Patrimonial.

The regional surface water model was set up with the distributed hydrological wflow modelling framework (see Appendix C.1), which describes the natural hydrometeorological situation of the Mexico Basin.

This means that with the surface water model the boundary conditions for the other models are calculated, based on the natural hydrometeorological wat balance of the surface water system. In the regional groundwater model, set up in iMOD, the artificial abstractions such as water intakes and groundwater abstractions are included.

REGIONAL MODELLING OF THE BASIN OF MEXICO Hydrological model Results Discharge simulations One of the outputs of the hydrological model is simulated hydrographs, for the full model period and for any location within the model area.

These hydrographs serve as input for the hydrodynamic model as discussed in Section 4.3.

A rough model performance check was done for the hydrological model by comparing the measured discharge from the gauging stations with the simulated discharge from the model. However, in light of the data quality issues of the discharge measurements as discussed in Appendix C.6, these comparisons serve merely as an illustration. At this point no

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performance statistics were calculated based on these locations; this would not make sense because of these quality issues. As an illustration, Figure 4.1, Figure 4.2 and Figure 4.3 show the measured and the simulated hydrographs at three gauging station locations, Molino Blanco, La Grande and Santa Teresa stations.

Figure 4.1 Measured and simulated hydrographs at Molino Blanco station – recorded discharge (Q.obs, blue line) and simulated discharge (Q.sim, green line) Figure 4.2 Measured and simulated hydrographs at La Grande station – recorded discharge (Q.obs, blue line) and simulated discharge (Q.sim, green line) Figure 4.3 Measured and simulated hydrographs at Santa Teresa station – recorded discharge (Q.obs, blue line) and simulated discharge (Q.sim, green line)

Groundwater recharge

In the Wflow model the soil is considered as a single bucket with a cer-

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tain depth, divided into a saturated store and an unsaturated store. In the natural situation, water infiltrates in the soil, and water that cannot infiltrate is added to the kinematic wave routine module for surface water routing. Part of the water in the unsaturated store is evaporated by vegetation or from the soil, and part of this water percolates to the saturated store. Once water is in the saturated store, vegetation can extract this water for transpiration, the water can flow underground to the next grid cell, or water can exfiltrate. The natural recharge from the unsaturated to the saturated part of the soil is thus calculated by finding the transfer from the unsaturated to the saturated store, minus the evaporation from the saturated store. The water in the saturated store that does not evaporate, flows laterally from one grid cell to another, based on the difference of water level of both pixels. When the water level of the saturated store is at the surface, water may exfiltrate and will be added to the kinematic wave for surface water routing. In the natural situation, all water that recharges to the groundwater (after discounting for evaporation) will eventually exfiltrate from the soil at some point and be added to the kinematic wave. The only exception would be water flowing underground to a different watershed, which is not included for the case of the Cuenca de MĂŠxico. A schematization of the soil routine in the Wflow-SBM model is shown in Figure C.1.

The recharge that is thus calculated in Wflow serves as input for the groundwater model (see section 4.2.2). A first discussion of the groundwater recharge simulations is presented here, based on the full model simulation of 36 years for the period 1979-2014. Outputs are given in mm per day. Results show that groundwater recharge varies significantly throughout the year. It was observed that during the dry months (mostly from November to April) there is limited infiltration.

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85

In contrast, during the rainy months (May to October) groundwater recharge occurs mainly in the Sierra de las Cruces and the Sierra Nevada, the southern regions of the Basin. In this region infiltration varies from 0 to 7 mm per day, reaching 13 mm per day during extremely wet months. In the rest of the basin recharge ranges from 0 mm to 1 mm per day. Simulations show that groundwater recharge in the State of Hidalgo is limited even during the wet months.

An overview of the total groundwater recharge during the months of February and June is presented in Figure 4.4. White areas represent cells with values near or almost zero.

a) February-1982

b) June-1982

c) February-1995

d) June-1995

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e) February-2012

f) June-2012

Figure 4.4 Simulated groundwater recharge throughout the simulated period (19792014).

Figure 4.5 presents the average groundwater recharge for 1979 to 2014. It is observed that infiltration ranges from 3.4 mm to -2.2 mm per day. White areas represent cells with values near or almost zero. In general it can be concluded that the basin of Mexico has an average recharge of 0.6 mm per day.

Figure 4.5 Average groundwater recharge during 1979-2014 (mm)

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Figure 4.6 shows the evolution of the groundwater recharge from 1979 to 2014. On average recharge during the dry season accounts for about 562,000 m3/d, whereas during the rainy season recharge accounts for about 3,620,000 m3/d. On average, annual recharge is estimated at ca.

2,093,000 m3/d.

Figure 4.6a shows that recharge varies significantly over the years. The time series also show that during the year 2009 the basin faced a severe drought (27% less than average recharge).

In the case of Xochimilco and the Zona Patrimonial, average recharge is estimated at 54,500m3/d. During the rainy season, infiltration to the vadose zone is calculated at 101,700 m3/d, while during the dry season infiltration accounts for 7,200 m3/d.

Basin of Mexico

Zona Patrimonial

Figure 4.6 Evolution of groundwater recharge from 1979 to 2014 (m3/s) showing a comparison between dry and wet months is presented. Values are given in m3 per day. (a) for the Basin of Mexico; (b) for the Zona Patrimonial.

4.2.1.3 Water balance The hydrometeorological water balance for the Mexico City Basin was calculated with the Wflow model over the period 1979-2014, as follows: P - ET - dS/dt = Q in which: P = Precipitation ET = Evapotranspiration dS/dt = Change in soil water storage Q = Surface runoff

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The basin is assumed to be a closed system. The rainfall is the only source of natural water into the system. All water that enters the system as rainfall leaves the system either via evapotranspiration or via surface runoff. The change in soil water storage can be assumed to be zero over a long period of time (36 years). Surface runoff is made up of quick runoff (infiltration excess water and direct runoff in the top part of the soil) and slow runoff - the water that percolates through the unsaturated zone, where it becomes recharge as it enters the saturated zone, and eventually will flow out again as surface water in the rivers and streams.

Figure 4.7 Maps showing the long term average annual values (mm/y) for the water balance components precipitation (top), evapotranspiration (mid) and recharge (bottom), calculated with Wflow over the period of 1979-2014.

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Table 4.1 presents the hydrometeorological water balance for the Basin of Mexico averaged over the 36 years of analysis (1979-2014). Values are given as annual averages.

Table 4.1 Water balance for the Basin of Mexico, average annual values in mm over the period 1979-2014.

Annual average 1979-2014 [mm/yr] Precipitation (P)

678

Evapotranspiration (ET)

501

Recharge (Rch)

117

Change in storage (ΔS)

0

Direct surface runoff (Qquick) (P - ET - dS/dt)

60

4.2.2 Groundwater Model Results In the hydrogeological setting of the Basin of Mexico, where groundwater is the main source for drinking water supply, sustainable groundwater resources management is of paramount importance. Groundwater models, both numerical and conceptual, are very useful tools to support groundwater management and decision making. Groundwater models facilitate understanding of the hydrogeological processes by visualising and quantifying the 3D hydrogeological system and the dynamics of groundwater flow.

A numerical groundwater model was built with the Deltares software package IMOD (Interactive Modelling version 4.0). The constructed numerical model has been based on extensive collection of existing data from various public sources, government agencies and universities. In addition, the dynamics of the hydrological system simulated with Wflow were used for the top boundary conditions of the model (groundwater recharge).

The regional IMOD model was set up in WGS1984 with a pixel size of 200x200 m. Simulations were carried out with monthly time steps.

Yearly time steps were used for validation purposes. The model con-

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tains 698 columns, 629 rows and four layers that represent the aquitards and aquifers of the basin. Figure 4.8 presents the hydrogeological units built in IMOD.

Figure 4.8 Hydrogeological units of the Basin of Mexico built in IMOD.

The hydrogeological setting of the Basin of Mexico is described by Gonzalez-Moran et al. (1999) and consists of an upper aquitard formed by lacustrine deposits from the Quaternary and a partly confined aquifer formed by alluvial and volcanic deposits from the Quaternary. Older, Tertiary, deposits form a lower aquitard below the aquifer, with a second aquifer from Cretaceous/Tertiary age below. The general characteristics of the system are described by Veláquez (2017): Upper Aquitard It has an average thickness of 60 m, and it is represented by lacustrine sediments (clays) with good porosity, and high storage capacity but with low permeability; and volcanic rock, sandy-clayey deposits at a depth of 10 m. It is divided by a thin layer of sand with high permeability at 33 m. This layer receives the name of “capa dura” and has an average thickness of 3 m.

Upper Aquifer This unit corresponds to the central unit of water extraction. With an average of 600 m reaching 1,000 m in some areas, it is formed by alluvial deposits, basaltic rocks, volcanic clays with small layers of sand, alluvial and pyroclastic deposits from the Quaternary. It is a free aquifer in the mountain areas and alluvial plains, whereas from Zumpango to Chalco, and from Texcoco to Cerro de Guadalupe the aquifer is confined. The aquifer is semi-confined in the central area of the Ba-

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91

sin, a layer of lacustrine clays is present. Moreover, most of the water extracted in the Basin of Mexico comes from this layer at an average depth of 300 m.

Lower Aquitard With a mean thickness of 1.500 m, this aquitard consists of sedimentary clasts, pyroclastic materials, volcanic clays, fractured volcanic rocks at a depth of 3,000 m from the Pliocene; andesitic volcanic rocks from the plio-quaternary; basaltic rock and andesitic of the Upper Pliocene; Stratified series, volcanic rock, lacustrine deposits of the lower Pliocene; volcanic rocks from the Tertiary; lacustrine deposits from the Upper Tertiary; acidic igneous rock from the Miocene; intermediate igneous rock of the Oligocene; and conglomerates from the Eocene.

Lower Aquifer It is constituted by volcanic rocks, type andesitic and dacitic, from the mid-Tertiary to upper Tertiary; carbonate rocks from the Cretaceous. It has an average thickness of 500 m.

The groundwater model was set up using this knowledge from the literature. Permeabilities were coupled to units distinguished on the geological map. Just before finishing the reporting new boreholes became available. Due to time constraints, these could not be used in updating the groundwater model. The new boreholes were compared to the used model to assess possible effects and define recommendations for future use of the model. The main differences are:

• The upper aquitard is less extensive than the schematization in the model. The recharge of the aquifer will therefore be higher than currently calculated by the model; • The thickness of the aquifer is lower in the south, this is expected to have minor effect on the model results; • The top of the lower aquitard is possible more permeable than in the model, this will lead to recharge to this part of the aquitard from the aquifer. This is expected to have minor effect on the model results.

For future use of the model it is recommended to: 1 study the information from the boreholes in more detail and acquire original information (currently only simplified boreholes were available), 2 adjust the extension of the upper aquitard, 3 base the boundaries of the aquifers

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and aquitards on the information of the boreholes and 4 finetune the permeabilities, now based on main geological units.

In Appendix D.1 overview figures and some cross-sections are shown, were both the schematized model layers are visible and the results from the boreholes.

4.2.2.1 Hydraulic heads This section provides examples of calculated model output in the form of maps and time series. Results are presented annual time steps Figure 4.9 shows the simulated hydraulic heads for the year 2010. Values vary from 2000 m a.m.s.l. (State of Mexico) to 3700 m a.m.s.l. (Sierra de RĂ­o FrĂ­o). In general, the direction of groundwater flow is from the mountainous edges of the basin towards the central valley area, where most of the groundwater abstraction takes place. The direction of groundwater water flow in Mexico City is from the south to the centre of the basin, and tends to concentrate in the areas of Xochimilco, Texcoco, and Chalco.

Figure 4.9 Hydraulic heads of the granular aquifer simulated for the year 2010. Green areas are the lowest water tables in the valley floor where most of the abstraction takes place.

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Hydraulic heads in the area of Texcoco and Chalco vary from 2190 to 2250 m a.m.s.l. while in the area of Tizayuca, Hidalgo, water levels change from 2250 to 2300 m a.m.s.l. Hydraulic heads in the north of CDMX near the airport are around 2150 m a.m.s.l.

In the borough of Xochimilco, simulated hydraulic heads in the mountain areas (in the south) are around 2450 m a.m.s.l., while hydraulic heads in the urban areas located in the north of the borough are around 2180 m a.m.s.l.

Heads in the areas of Tlรกhuac and Milpa Alta are computed around 2180 to 2300 m a.m.s.l. Figure 4.10 Hydraulic heads of the granular aquifer simulated for the year 2010 in the area of the Zona Patrimonial.

Figure 4.10 Hydraulic heads of the granular aquifer simulated for the year 2010 in the area of the Zona Patrimonial.

Results show that water levels are shallow in the mountains areas, whereas in the urban areas hydraulic heads tend to be more profound.

Moreover, in the basin groundwater flows from south to centre and from west to east.

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According to reports of Conagua (2002, 2013), the Soltepec aquifer is considered in equilibrium (meaning that groundwater recharge is equal to abstraction), while water depletion in the Apan aquifer is considered as site-specific, and in general, the aquifer reports a recovery on the water levels. Figure 4.11 shows time series of hydraulic heads in the Soltepec and Apan aquifers. Even though water levels are very variable through the years, these simulations seem to confirm that groundwater levels in these aquifers are recovering.

Figure 4.11 Time series of hydraulic head during 1987-2014 in the Soltepec (top; fluctuations between 2578 m a.s.l – 2581 m a.m.s.l.) and Apan (bottom; fluctuations between 2492 m a.s.l – 2504 m a.m.s.l.) aquifers, suggesting a gradual recovery of groundwater levels.

Figure 4.12 shows time series of hydraulic heads from 1987 to 2014 in the borough of Xochimilco. Even though during some years the water levels have increased, this has not stopped the degradation of the groundwater sources.

According to reports of the Monitor de Sequía en México, the city of Mexico suffered a severe drought during 2009, classified as D2 (severe drought). In the State of Mexico, the drought was classified per municipality and varies from D0 (abnormally dry) to D2. Ground water extraction continued at the regular pace though, causing water levels to

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95

drop considerably with around 10 m, mainly in the urban areas (Figure 4.12 bottom).

Figure 4.12 Evolution of the hydraulic heads in the area of Xochimilco, and regions of Tlรกhuac and Milpa Alta belonging to the ZP; Top: centre of Xochimilco; Bottom: Sierra del Ajusco (south of Mexico City).

4.2.2.2 Validation of model results Very little information is available on the monitoring of groundwater levels in the Basin of Mexico. In fact, the only sources that are available in the present study are maps reported in the literature. These literature sources were used to perform a qualitative validation of the model.

The simulated outputs of the model with annual time steps were compared with the hydraulic heads reported by Lesser-Illeades (2005). Figure 4.13 presents a qualitative comparison for the years of 1985, 1990, 1995, 2000 and 2003. Even though the maps are not easily compared, the simulated spatial patterns seem to match with the maps reported

by Lesser-Illeades. The depression cones in the areas of Xochimilco and Chalco show similar patterns. In the municipality of Texcoco, cones of lesser magnitude are simulated.

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1985

1990

1995

2000

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97

2003 Figure 4.13 Comparison between simulated hydraulic heads (left column) and hydraulic heads reported by Lesser-Illades (2005) (right column) for the years 1985, 1990, 1995, 2000 and 2003.

Recommendations to improve the model The model could be improved by adding the surface water network composed of natural rivers and drainage channels. For such purpose, information on the resistance of the river and channel bed should be known, particularly if the river and channel section have been impermeabilized of not.

Other information that could be added to the model are the losses of the water supply pipes network. This could increase recharge to the aquifer system.

Another important factor is the inflow and outflow through the aquifer boundaries. In this model, it is assumed that there is a connection with the surrounding aquifers by assigning a fixed head at the boundary of the model. The effect of such boundary on the heads of the area of interest is relatively limited, but it does have a significant contribution in the water balance as it can be seen in the next section.

4.2.2.3 Water balance A water balance for the Mexico City Basin was calculated with the groundwater model over the period 1979-2014 as follows: Recharge-Discharge=change in water storage In a more detailed sense, the groundwater equation used is expressed as follows: (Rch+Bndin)-(Abs+Ofl+Bndout)=ΔS

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Where: Rch is the diffuse recharge from rainfall (input from Wflow) Bndin and Bndout is the flux entering or leaving the aquifer

through its borders Abs is the water abstraction pumped by wells Ofl is the water leaving the system through overland flow ΔS is the change in water storage The water balance is calculated with the IMOD model with monthly time steps over the period 1979-2014. Average values over the entire period of calculation are given in Table 4.2.

Table 4.2 Water balance for the Mexico City Basin, average annual values in mm over the period 1979-2014 according to model results.

Annual average 1979-2014 [mm/yr] Recharge (Rch)

60

Aquifer boundaries (Bnd net)

1088

Groundwater abstraction (Abs)

-148

Drainage (Olf)

-155

Change in storage (ΔS)

-844

The water balance shows that the inflow from the boundaries and the water supplied by the storage need to compensate for the high abstractions and the low recharge. The overland flow shown happens in some wet months mostly close to the basin borders; this is probably caused by the high heads assigned in the boundary of the aquifer system, which together with high recharge and no abstraction in these areas, causes the overland flow.

Figure 4.15 presents the water balance of the borough of Xochimilco and the regions of Tláhuac and Milpa Alta belonging to the Zona Patrimonial (152.8 km2). On average a volume of 54,500 m3/d (13.3 mm/yr) enters to the system as infiltration, while a volume of 614,000m3/d (1466 mm/ yr) leaves the systems as water abstractions. Water abstractions can be due to evapotranspiration or water abstractions. The water balance is

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calculated at ca.

-559,000 m3/d (–1336.2 mm/yr) On average water inputs during the simulated period accounts for 7,200 m3/d (8.5 mm/yr) and 101,700 m3/d (121.8 mm/yr) during the dry and rainy season respectively. Water abstractions are simulated at 601,000 m3/d (-711.8 mm/yr) during the dry season and 627,000 m3/d (-754.7 mm/yr) during the rainy season.

Figure 4.15 Water balance of the borough of Xochimilco and the Zona Patrimonial during 1979 to 2014). For purposes of representation, the water balance and water outputs are given as absolute values.

4.3

LOCAL MODELLING OF THE ZONA PATRIMONIAL

A detailed hydraulic model has been constructed of the water system in Xochimilco, TlĂĄhuac and Milpa Alta region of the Zona Patrimonial (ZP).

This model represents the essential hydraulic features in the area.

The Deltares SOBEK modelling suite was used, which is an integrated model package with modules for rainfall-runoff, 1D river and sewer flow, 1D2D inundation modelling, real-time control (RTC) and water quality. The 1D modelling requires cross-sectional data of open water channels and the 2D modelling requires a digital terrain model with elevation data. The model requires upstream and downstream boundary conditions. Typically these are upstream discharge hydrographs and downstream water levels. Also, characteristics of hydraulic structures like reservoirs, weirs, gates and their operation need to be specified in the hydrodynamic module.

For the Zona Patrimonial, the surface water system was modelled as

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a 1D model. Based on the available information, the regional surface water system was refined with the secondary network of the surface water and WWTP inlet systems. A detailed description of the model is given in Appendix E. 4.3.2 Hydrodynamic Model Results With the constructed hydraulic model of the Zona Patrimonial the current situation was simulated. The runs were performed for the years 2013-2014, with an hourly model time step.

In the current water system, practically all springs that used to naturally discharge the runoff from the upstream sub-catchments have run dry, since all of these spring locations are now being pumped for potable water production. It is therefore assumed that no water enters the ZP from these upstream sub-catchments. In addition, it is assumed that no water from the Amecameca River enters the water system of the ZP, because the river is permanently blocked from entering the ZP. The results are given below in terms of a discharge regime at the outlet of the ZP model area and as a water balance for the Zona Patrimonial.

4.3.2.1 Discharge The output location where the outflowing discharge is modelled as shown in Figure E.5. Calculations results are shown in Figure 4.16. It can be seen in this figure that there is a high seasonality. The annual average outflow is 1.4 m3/s.

Figure 4.16 Modelled discharge for the current situation at the outlet of the ZP area (Canal Nacional, see exact location in Figure 4.20).

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4.3.2.2 Water balance for the Zona Patrimonial This section presents the water balance for the Zona Patrimonial simulated for the current situation (Table 4.3).

Table 4.3 Water balance for the Zona Patrimonial simulated for the current situation (period of 2013-2014)

Current Situation Annual [m3/s]

average

Annual sum [m3]

-

-

-

-

Runoff within ZP (generated by rainfall)

0.15

4,867,320

Inflow from wastewater treatment plants

1.25

39,262,320

Total in

1.40

44,129,640

Outflow at Canal Nacional

1.40

44,264,183

Total out

1.40

44,264,183

0.00

-134,543

In: Inflow from sub-catchments/ springs

surrounding

Inflow from Río Amecameca

Out:

Difference: Total difference

The water balance clearly shows that, in the absence of water recharging the ZP from the surrounding sub-catchments, the water system is completely dominated by the inflow from the WWTPs. Rainfall-generated runoff is only 11% of the total water budget.

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A 104 WATER RESILIENCE PLAN FORPLAN THE FOR HERITAGE ZONE OFZONE XOCHIMILCO, TLAHUAC AND MILPA ALTA A WATER RESILIENCE THE HERITAGE OF XOCHIMILCO, TLAHUAC Y MILPA ALTA

TURNING TECHNICAL RESULTS INTO COMMUNITY ACTION

COMMUNITY WORKSHOPS AND INTERVIEWS The diagnosis shown here refers to the work done with the community of the heritage zone in Xochimilco, Tláhuac and Milpa Alta aimed to uncover the perceptions the habitants have with the water in their daily practices, and socialize the hydrological model as some of the project proposals.

The implementation of any intervention in the territory has to take into account people’s lifestyles and practices in relation to the lake, which have been slowly transformed through the years by effect of urban expansion and changes in their economic and labour practices. Another cause for this transformation is due to the gradual abandonment of the agricultural practices in the chinampa’s fields.

We argue that by studying these activities and practices as well as some of the ongoing projects generated by the community could be the basic driveforce for a sustainable development in the future of the area.

Collective organized actions from the community are very good sources for project implementation, monitoring and follow up.

The information gained during the workshops provides insight in the implementation of the hydrologic model, the perceptions and actions of local groups to solve the water problem and the undertaken local initiatives.

The analysis allows us to understand the perception of the inhabitants in relation with projects developed by outside actors. For many years Xochimilco’s community has witnessed the presence and interventions from a variety of sectors: academic, government, ONG’s and other social and civil organizations which have proposed many programs and initiatives that either have not been implemented. Residents are tired and skeptical about the presence of researchers, politicians and other external sectors that seek some benefit from the area.

We perceived a deep discomfort from the groups we interviewed because they did not feel involved, or only for brief moments in order to

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Inclusion and representativeness must always be the axis and vector of the actions and development in Xochimilco’s region. Local knowledge and experience are fundamental to manage and articulate the projects or initiatives.

meet quota required by institutionalized participation demands. It is advisable not to over promise the scope of solutions and to be transparent and share the objectives and implications of new initiatives with the population.

We conclude that inclusion and representativeness must always be the axis and vector of the actions and development in Xochimilco’s region. Local knowledge and experience are fundamental to manage and articulate the projects or initiatives. By listening to their beliefs and recommendations, we might foster urban resilience and favour appropriation of proposed solutions for the socio-ecological water system challenges in the region. The appropriation of the projects and integration of multiple stakeholders will result in an adequate application and follow-up strategy. Local sectors must be actively involved in the solutions, otherwise we will end up with projects are not adopted by the local population or are blocked or minimized by political interests.

5.2 ANALYSIS DESCRIPTION The lacustrine area of Xochimilco and TlĂĄhuac have been the object of diagnostics, studies, research, programs and public policies, however, the amount of information, resources and projects developed over the years has not been reciprocal with the actual conditions of improvement and wellbeing of their inhabitants. This fact shows that, at some point in the process of information, resources generation or political measures had been stuck or applied discretionally, originating a diffuse attention or diverted resources to other organizations or sectors.

Due to this fact, the project of socializing the hydrologic model is organized through the frame of a participatory methodology and the emergent practice of Transition Design (Irwin, T. 2019) which frames the water problem for the community (families, inhabitants and especially productive tourist and agriculture sectors) in order to engage them in a dialog and encourage them to express their concerns and relationship with the lake area from a daily life perspective: productive, domestic and leisure activities considering that by visualizing this relations we can identify conflicting agendas between other actors and amplify their voice for a diverse portfolio of solutions.

After we recognize these relations we inquire about the projections that different local actors have about the future in the region. It was vital to know their expectations and the capacities of action to put them to work and sustain projects collectively.

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5.2.1 General Objective Facilitate and enrich to the implementation of the Water Resilience Plan through the incorporation of local knowledge and experiences. Inhabitants are probably the first to benefit from the plan, however, they are often not involved enough in the design process.

5.2.2 Specific Objective

• Promote the understanding of the water socio-ecological system in the heritage zone.

• Understand the needs and strengths of the different sectors that inhabit the heritage zone.

• Build capacities for an integral management water strategy.

5.3 METHODOLOGY The work process was based on visits and tours with the different sectors. We conducted interviews, collected cartographic information and made a photographic record of each visit.

Understanding the relation of different actors within the water problem in the territory was an important task. Due to time constraints we worked in more detail with groups among Xochimilco municipality than Tlahuac. We focused on the chinampera zone in San Pedro Tláhuac which gathers touristic and productive activities in “Los Reyes Lake”.

Sectors in the territory 1. Productive Sector. composed of peasant of chinampas and communal lands of Xochimilco and Tláhuac 2. Touristic and entertainment sector. Relates to the touristic activities in the lake, including canoeists guild at different piers in the channels area of the heritage zone. We also included activities related to the rent of the chinampas as football soccer fields or events and socio-cultural celebrations.

3. Irregular settlements. The street of human settlements that adds environmental pressure to the water usually by spilling waste directly into the water.

4. Academic and ONG’s. This sector has a strong presence in the area and focuses on research projects and non-for profit initiatives. The main interest is to register the voices and actual conditions related to water infrastructure and management.

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Once the sectors were defined we pursue with interviews and trials through the different channels of the heritage zone, this work was carried out throughout September and October. With information from this phase we proceed to map the problem considering different sectors perspective.

Figure 5.10 shows where the interviews with farmers and rowers took place. Figure 5.11. shows the different trials we made, piers and several aspects of water infrastructure, as well as the most propense area of flooding in the ejidal zone San Gregorio Atlapulco.

Figure 5.10. Map showing interviews with the producers and touristic guild in the area conforming the two municipalities of Xochimilco and Tlรกhuac.

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Figure 5.11. Map showing field-work trials, main piers, settlements and water infrastructure highlighting main risk flooding area near San Gregorio Atlapulco’s wetlands.

The information collected during the interviews and the attended expert workshops at the Resilience Agency, allowed us to start mapping the actor concerns about the water problem and the activities that they were doing in order to fix the problem.

Problem mapping is a process in which stakeholders collaborate to develop a visual representation of a wicked problem, then identify as many relationships (interdependencies and interconnections) within it as possible. Types of relationships include oppositions and affinities between stakeholders, connections and interdependencies The object of the map is to have a common understanding of multiple actors in the territory and highlight what kind of activities were taking place with each sector. We wanted to validate and then visualize the concept of Xochimilco as an interrelated system from the actors point of view to foster the dialog between sector, especially between the locals and other instances that don’t necessarily belong to the territory but play a fundamental work in the conservation of the area.

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The diagram in Fig. 5.12 is the result of the problem mapping and shows the perceptions as testimonials we collected from the interviews in notes in yellow, in the lower right corner we indicated the initials of the corresponding actors or sector. On the left side of the diagram, we show the stakeholder map and places and issues commonly mentioned by the interviewed. When the border of the note is marked with a dashed line, it means that the interviewed did not paraphrased that statement exactly, but that it was inferred from the interview. Piles of notes indicate that more than one sector has the same perception. A red border frame on a yellow note indicates the opposed point of view or even conflicts between actors. The blue notes refer to actions taken by each sector and follows the same nomenclature procedures.

This diagram acts as an artifact to organize and visibilize the voices, points of view and perceptions of several actors within the sectors that would possibly have not had a chance to interact otherwise. It also shows recurrent perceptions of the problem or actions. It helps to make a hierarchical categorization according to prioritization and also detect small interventions to link with possible solutions.

We grouped these voices in relation to five main topics 1. Political and governance 2. Cultural partner 3. Infrastructure, technological aspects 4. Environmental 5. Economic From each sector or stakeholder, we identify the main problems and actions taken in their daily practices and water management and group them in themes and repetitive practices. The problems and concerns could be linked to general themes that might give some inspiration to solutions or interventions and they also show how the same problem could be seen from different perspectives according to each sector.

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Figure 5.12. Problem mapping showing testimonials from different actors in regard of their relationship with their activities and concern with water.

The themes in general were divided according to the next categories: 1. Political and governance aspects

Legislation and regulations

Governance-Ungovernability

Land tenure

Comprehensive social management

2. Socio-cultural aspects

Life in the Chinampa

Production work -external and internal visions-

Water value perception

Culture and economy

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3. Infrastructure and technology

External political actions

Effects and consequences

Local practices of remediation

Lake cleaning and maintenance

4. Environmental aspects

General causes of degradation

Water biodiversity

Production of chinampa and pollution

5. Economic aspects

Labour and production

Effects on the economy

Socio-political problems

We observed that there are similar perceptions of the problem even if they come from different sectors and we highlighted conflicts or actions between sectors. Those relations are extremely important if we try to develop or implement initiatives in the territory.

DISSEMINATION AND SOCIALIZATION OF THE HYDROLOGICAL MODEL For the dissemination and socialization of solutions we organized a series of focus groups with different sectors of the community the first one took place in november with the production sector of Xochimilco (Fig. 5.13) and touristic sectors of Zacapa-Nativitas and San Gregorio.

The main objectives of the groups was: 1. Validate problems resulting of the interviews and problem map 2. To know their point of view about projects suggested by external sectors in order to achieve agreements and find paths to develop them.

3. Build possible futures based on their own projects and reflect about viable way of amplify efforts.

The general structure of the session consisted in: First part: Introduction to the water problem in the territory Second part: Highlights the main 5 problems

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Water pollution

Water extraction Ungovernability

Biodiversity loss On the third part we introduced three solutions created by other sectors previous validated by the institutional and academic sector, which favours criteria of those projects that attended water quantity, quality and distribution, which attended the main worries of the community, as shown in the problem map 5.12.

Rio Amecameca reconnection

The hydrological model for water control system

Green corridors The session ended with the hierarchization of the proposals according to each group perspective we also encourage them to think of their own solutions and reflect on how it can be connected to others.

Fig. 5.13 the two images above show sessions of focus groups with the productive and touristic sectors.

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RESULTS Traditional tourist sectors The two participant groups of this sector suggested that any proposals should follow an awareness phase, followed by community education and compliance to legislation with a responsable deputy willing to follow up projects. They also made suggestions according to each proposal 1. Rio Amecameca reconnection - This proposal it is not viable, and does not consider the differential sinking of land - First they need to solve the floodgates control levels - Need to review the whole watershed 2. The Hydrological model - It is necessary and urgent but would need a lot of maintenance - Need to study the platform in depth - Necessary because there is just one stream of entry water and three floodgates that control the whole system - The lakes can become closed systems - The transfer in Caltongo is losing a lot of water 3. Green corridors - Need to activate the production areas - There is cattle raising where it shouldn’t be - Need to apply legislation - Need to comply with zonification Proposals of their own 1. Sewage treatment plant in the dam near “16 of September” street.

2. Recovery program of the piers 3. Green corridors along the channels to filer domestic waters 4. Strategies of piers beautification 5. Make infiltration wells 6. Make a carcass (hydraulic mill) and treatment plant in the Archaeological Museum

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Traditional tourist sectors Regarding the production sector arguments to validate the solution proposals we found that there is a strong opportunity for the professionalization and education dimension for this sector, associated to the problem of the chinampa or land abandonment by the new generation of owners and due to the hard labour that the cultivation activity represents.

The lack of knowledge of the dynamic of the water system is associated with activities that cause water pollution and at the same time with the problem of the Chinampa´s abandonment which is also caused by water extraction practices that have been taken place historically over the years. There was a final relation taken into consideration by this group that related the problematic to a political aspect, and works as a vicious circle: the ungovernability of the territory, which started in the administration of Porfirio Díaz government, who nationalized the water resources of the área, disowning authority of the original indigenous people, causing governability conflicts in the zone since then.

These relationships can be expressed in the next diagram (Fig. 5.14):

Fig 5.14. Chain of “Vicious Circle Problems” in the territory expressed by the agricultural sector group.

Another relevant aspect mentioned in the session is the problem of land ownership which has varied according to each government administration, sometimes expropriation and others giving it, land sucesion is resolved by tradition without any document formality sometimes favouring informal invasion and destitution of the land.

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Here are the recommendations suggested by this group.

1. Rio Amecameca reconnection The Rio Amecameca is very polluted already, wouldn’t it be easier to reconnect Rio Magdalena? This project won’t need too much investment in infrastructure because of the natural inclination level.

2. The Hydrological model There has been initiatives in San Gregorio from chinamperos making locks for the water, but it stopped working shortly because the level of the water varies a lot and they need a lot of maintenance.

While the battery of wells from Chalco channel continues to function, it might be impossible to establish the control you suggest.

3. Green corridors We are more worried about the education of the producers and inhabitants in the lake, is they continue to use artificial fertilizers and chemicals and continue to spill the drain into the channels there is no point in doing that.

Proposals of their own 1. This group recommended a Chinampa school as a project of sensibilization and education the community, it could be linked to commercialization circuits.

2 They also recommended strategies to legalize the inheritance of the land in order to recover the chinampa.

3. They also suggested an ecological refuge zone: A “santuario” controlled by natural indicators that recovers prehispanic knowledge of the chinampa.

Materials to continued with the workshops Based on community recommendations we developed divulgation materials for the different proposal of solutions that might be used to continue socialization workshops with the community, we believe that this initiative might bring new knowledge and connections between people with different aspects of their heritage.

The design of the materials were done by students from the bachelor of Design from Metropolitan Autonomous University of Cuajimalpa in Mexico City. It consisted of a 3D topographic cardboard model map of the área and pictograms and signs as reference for the different stakehold-

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ers and activities in the area. An historical timeline showing different interventions from stakeholders in the territory and some brochures explaining the work at the chinampas and the hydrological model.

Final recommendations We conclude with final recommendations to the rest of the team:

• It is fundamental to integrate local actors in the generation of solutions • To socialize the different proposals through a series of didactic and outreach materials • There are also conflicts that fragment cohesion between social groups beyon the local communities, like the academic and government sector.

• It is important to amplify the decision making to achieve consensus.

• There are situations of corruption and clientelism that prevents resources and projects from being used to solve problems.

• Inhabitants are interested and willing to promote initiatives that solve problems in the territory, more and more groups are getting independently organized in order to initiate their own projects outside the institutional channels, which have already ceased to be trusted

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120 A WATERA RESILIENCE PLAN FOR ZONE ZONE OF XOCHIMILCO, TLAHUAC ANDY MILPA ALTA WATER RESILIENCE PLANTHE FORHERITAGE THE HERITAGE OF XOCHIMILCO, TLAHUAC MILPA ALTA

DEVELOPMENT OF A PROJECT PORTFOLIO

IDENTIFIED INSTITUTIONAL STAKEHOLDERS Stakeholder mapping and analysis is a key process for complex projects such as the Water Resilience Plan (WRP) of the Patrimonial Zone (PZ) of Xochimilco, Tlahuac and Milpa Alta (XTMA). Appendix K describes the comprehensive process that we followed in identifying, categorizing, mapping and engaging stakeholder, based on Anderson E., Brown, B. (2013).

The analysis of the stakeholder map can greatly influence the expected result and the success of any important initiative. It can be used during any stage of the project. However, carrying out an analysis of the stakeholder map during the planning stage generally helps to improve the results of the project.

With the information that we currently have, we have selected a list of the stakeholders that we consider to have the greatest interest, mandate and influence in the WRP.

Figure 6.1 Adapted from Ece Utkucan Anderson, M.Sc. and Barrett C. Brown, Ph.D. Integral Stakeholder Mapping.

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SOLICITING PROJECT PROPOSALS In order to arrive at a set of project proposals for the ZP, not only from our own project team, but from a wider group of stakeholders, we initiated a ‘Call for project proposals’ to collect ideas for interventions in the ZP. The call was based on the conclusions of Workshop 1 (See Appendix K) and the Guidelines of ‘Water as Leverage’, which is a program with a similar objective sponsored by the Kingdom of the Netherlands for cities in Southeast Asia.

The Call for proposals was announced by the Resilience Agency of Mexico City and distributed among a wide range of identified stakeholders requesting them to propose intervention ideas to restore the water system of the Heritage Zone in such a way that it can cope with:

• the effects of climate change and seismic risks • the socio-economic challenges that are pressing the water system, • and function as a lever for social, economic and environmental development.

The ideas for intervention could be within the area of the XTMA Heritage Zone or in any other area of Mexico City that has a direct influence on this sub-basin.

The interventions could be of 3 types:

• Infrastructure (blue, green or gray) • Information (e.g., education, indicators) • Institutional (e.g., governance models) The call announced that the selected ideas were going to be studied in more detail by the present project team with experts in water systems, earthquakes, social, environmental and economic cost-benefit evaluation. The most suitable ideas were meant to be included in the Water Resilience Plan of the Resilience Agency of Mexico City.

After collecting up to 20 project ideas, a workshop was organized on November 6, 2018, to present ideas and feedback. .

The Call for proposals included a questionnaire that was based on the guidelines of the Water as Leverage program and modified to the specific needs for the XTMA ZP, with questions such as:

• Description and objectives: What is the central idea and what does it intend to achieve? What are the challenges that this idea is trying to solve?

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• Expected impact: what kind of environmental, social, and economic impacts are expected?

• Site location: Where would the future project be located?

What are the existing conditions and current uses of the site?

• Stakeholders or stakeholders: Who are the key stakeholders in this project and what role does each one play?

• Risk assessment: What are the possible political, social, environmental, legal / regulatory and other barriers to the implementation of the project? What are some strategies to mitigate against them?

• Cost estimation: What is the expected cost of the project and the key elements? What are the potential sources of financing?

• Cost-effectiveness: How does the impact compare (in terms of people, businesses and land area affected by the project) with the investment costs?

• Integration: How does this Conceptual Design fit into national, regional and local strategies and plans?

In addition to the call and questionnaire, a group was formed on LinkedIn with the purpose to stimulate the conversation with the group of experts, around important technical documents for the analysis of the problem and solutions for water in the ZP.

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Figure 6.2

PROJECT SELECTION From the projects we received, the following ones best represent each of the main categories that were defined in Section 6.2.

Water Quantity: Rio Amecameca (Deltares-UAM) Water Quality: Linear wetlands in the border between informal settlements and chinampas (Synergy) Water Distribution: System of locks (Deltares-UAM) Subsidence and Groundwater Recharge: Green corridors (Deltares -UAM) Culture: Regeneration of the Xochimilco Ecological Park (PEX) (Keystone and Taller 13)

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Socio-economic Benefit: All projects.

Figure 6.3

6.3.1 Reconnection of the Rio Amecameca to the ZP 1. General description: The central idea is to reconnect Rio Amecameca (RA) to the ZP in order to increase the volume of water that is available for water management in the ZP. This can be done by allowing (part of) the river water into the ZP in the south east corner of the ZP that now flows along the ZP to the north. Allowing the RA water to flow into the ZP area has to be carried out in combination with additional infrastructure to route the water to the right segments of the ZP. Currently, the water quality of Rio Amecameca is low. This intervention therefore has to be combined with measures to improve the quality of the water that flows into the area.

The objective is to solve the water shortage that is apparent in parts of the ZP. This idea solves the problem of water shortage with which some of the functions in the ZP are currently dealing. Agriculture has shortages of water in some parts of the ZP for irrigation of the chinampas. Navigation is hampered in some parts of the ZP because of low water levels in the channels. Water quality may be negatively influenced by stagnant zones and limited flushing in some parts of the ZP. By providing more water to the area, it becomes possible to serve these functions in the ZP better.

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2. Technical description and Location of project The project location is in the south east corner of the ZP, where the RA arrives at the boundary of the ZP and flows north. Currently, the river is routed north here. The water quality improvement may likely be carried out with localized treatment of RA water, for example, with a combination of decentralized wastewater treatment plants (WWTPs) and purifying wetlands. This could be situated upstream of the inflow point at the SE corner of the ZP.

Figure 6.4 Reconnection of Rio Amecameca

3. Social and environmental impact Environmental: The envisioned impact on the water system is a reduction of water shortage, more controllable water levels, an improvement of water quality and of ecosystems health.

Socio-economic: improved agriculture, navigation, recreation, health.

Wetlands: preservation of biodiversity and emblematic species Agriculture: irrigation and maintenance of chinampa groundwater levels Navigation: maintaining sufficiently high water levels in the channels Water quality: preventing stagnant areas and limited washing

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The (co)benefits may include: • Improved agricultural crop yield • Improved navigation • Increase of recreation • Improved ecosystem health • Improved public health • Reduced pumping costs downstream • Restoration of a river (The new major has declared to restore 8 rivers! ) • Improved public space (examples from Barcelona. Singapore,) Number of persons affected: Surface area: 4. Interested parties: • CORENA • Agricultural communities • Recreational embarkment sites • Conagua • IMTA • AZP 5. Economic Cost • Components to be included in a cost estimation are: • Decentralized WWTP • Purifying wetlands • Building/changing infrastructure to reroute the RA water into the ZP • Additional water infrastructure to route the water to the desired parts of the ZP

Figure 6.5 project cost estimation

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Assumptions: Capex estimation of new WWTP of USD $18.8 Millions per 1 m3/sec based on capex of WWTP Atotonilco, Mexico. https://www.waterworld.com/international/wastewater/article/16200963/mexico-s-new-giant-in-town Capex estimation of purifying wetlands based on experience of Alan Plummer Associates, 345 hectares needed for 1 m3/sec At capex per 1 hectare of 80,000 USD http://tamuk-isee.com/wp-content/uploads/2018/03/Costs-and-Other-Considerations-for-Constructed-Wetlands-Tim-Noack.pdf 6. Risks Possible political barriers may be related to other uses of the RA water. Other parties and/or areas may want to use the water as well.

Increase of flood risk in the area itself The current water flow seems not to be used downstream. It causes flooding and it has to be drained/pumped out of the area to the north.

Conagua/Conseco the Cuenca already has this on the list of program.

They have a plan to deepen the lagoon of Tlahuac/Mizquic and lead the RA to this lagoon.

7. Integration with government plans National: Conagua / Semarnat / Sedatu (They plan to do a lot of work with cities and territorial planning.)

Regional: State of MX / Mexico City itself / SACMEX / CORENA Local: Resilience Strategy / Consejo de Cuenca 6.3.2 Linear wetlands 1. General description Convert and enable the streets bordering the channels at the southern border of the Xochimilco chinampa zone as passive treatment systems for the wastewater of surrounding houses, in order to discharge clean water into the wetland’s canals.

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2. Technical description and Location of project Enable sewage treatment systems of a semi-passive nature under the fixed bed principle (BBR or BED Biofilm reactor) in the streets next to the canals. Prior to fixed beds, enable bio digester batteries to give primary treatment to wastewater. Deliver high quality clean water to the channels (complying with NOM 001 ECOL 96). Modules of 10 houses per limit area are considered, which will generate a flow of 12 m3 / day (1.2 m3 / day / house) and an organic load of 4.8 kg BOD5 / day.

Each module of 10 m3 / day considers anaerobic primary treatment of residual water, intensive mechanical oxygenation, and oxidation field under the fixed bed principle (BBR or Bed Biofilm Reactor).

Figure 6.6 Linear wetlands

Location: southern border of chinampa zone, approximately 12 kilometers.

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Figure 6.7 Location of linear wetlands.

3. Social and environmental impact The inhabitants from irregular settlements adjoining chinampas discharge their black and gray water to the channels contaminating the water used for agriculture. According to figures from the delegate of Xochimilco in 2017, Avelino Méndez, there are 307 irregular settlements, with 18 thousand houses and 140 thousand inhabitants, which represents one third of the total population of Xochimilco. With these figures, it is possible to estimate a total approximate discharge of black and gray water of 250 liters per second.

By solving the main cause of water pollution, this linear wetland project can have multiple positive impacts in the system:

• Regeneration of the chinampa agricultural economy that includes at least 2,000 hectares, 8,000 farmers that own the land, 20,000 jobs and food production sufficient for 5 million people.

• The chinampa technique is an outstanding example of premium quality organic agriculture.

• When the chinampa economy is regenerated it becomes itself a force that contains the growth of irregular urban settlements, thus assuring that this ecological protected area survives.

4. Interested parties:

Chinampa agriculture community.

Informal settlements inhabitants.

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Consumers of organic produce and nature tourism.

Alcaldias de Xochimilco, Tlahuac and Milpa Alta.

Federal government: CONAGUA, SEMARNAT. Mexico City Government, SEMARNAT, CORENA, SEDUVI.

5. Economic Cost Investment estimates: $ 545,000 MXP, comprised of $ 195,000 MXP in civil works and $ 350,000 MXP in equipment.

Figure 6.8 Project cost estimation

Total cost estimation based on Synergy’s unit estimation minus 20% from efficiencies that assume large scale production economies of scale (900 units).

6. Risks

• Social: lack of understanding on the collective benefits of the project, undermining local ownership and operation of the project.

• Political: a lack of trust in the technology that could make authorities opt for a more conventional WWTP. 7. Integration with government plans

• National: Conagua (water quantity and quality) / Semarnat (conservation) / Sedatu (land regularization) • Regional: Mexico City / SACMEX / CORENA • Local: Municipalities (Ayuntamientos) of Xochimilco, Tlahuac and Milpa Alta, AZP

6.3.3 Water control system 1. General description Hydraulic structures to control water flows at the chinampa canals area of Xochimilco to restore hydraulic connectivity and remediate subsidence.

2. Technical description and Location of project

• Hydrodynamics of Channel Systems

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• Hydraulic structures • Segmentation of the chinampa system • 5 locks and 15 dams distributed in the chinampa´s wetlands of Xochimilco.

Figure 6.9 Locks system

3. Social and environmental impact This project aims to control water levels and spills in order to: 1. Regulate the water supply and distribute the water on request to the different regions of the ZP 2. Mitigate floods 3. Improve water quality 4. Support the regeneration of chinampa agriculture.

4. Interested parties:

• • • • • •

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Chinampa agriculture communities Tourism companies (trajineras- canal boats) Conservation groups CORENA Conagua AZP

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5. Economic Cost

Figure 6.10 Cost estimations based on research made by Eugenio Gomez, UAM.

6. Risks Possible social risks related to gaining control of the water distribution decision making.

7. Integration with government plans National: Conagua / Semarnat / Sedatu Regional: State of MX / Mexico City / SACMEX / CORENA Local: Municipalities (cabildos) of Xochimilco, Tlahuac and Milpa Alta, AZP

6.3.4 Green corridors 1. General description This project idea is about the development of green corridors (linear parks) near Volcรกn Teutle-Xochimilco. The central idea is to create and develop green corridors or linear parks from the flanks of the Teutle Volcano down to the Xochimilco parts of the ZP. These green corridors may follow the valleys of the original streams that used to flow from the springs at the foothills of the Teutle Volcano into the Xochimilco chinampa area. The water flowing through these green corridors feed the chinampa area. The former springs that are now dry because of groundwater abstraction can be partially restored in order to feed the corridors. The corridors also function as linear parks that connect the urban public space south of the ZP with the Unesco world heritage zone.

The objective is twofold: (1) to improve the water system of the ZP by restoring part of the inflow from the springs south of the ZP and (2) to improve urban public space south of the ZP by connecting it to attractive linear parks. An improved spatial connection may raise awareness of the urban community to the chinampa area.

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This idea contributes to reducing water shortage and improving low water quality in the ZP as well as to improving public space and connecting the urbanized area with the chinampa area.

2. Technical description and Location of project The project location is south of the Xochimilco chinampa ZP area, at the flanks of the Teutle Volcano. Currently, there is (practically) no runoff from the hillslopes down into the Chinampa area, because the spring water is captured by the pumping stations. To our knowledge, there are currently no creeks, channels or culverts with running spring water feeding into the chinampa area. There are smaller (pockets) parks in the zone between the springs and the ZP, which could perhaps be the start of more linear type parks along former stream beds.

Figure 6.11 Green corridors location

3. Social and environmental impact Environmental: The envisioned impact on the water system of the ZP is a reduction of water shortage and improvement of water quality.

Socio-economic: Envisioned impact on the urban public space south of the ZP is improved attractiveness/liveability and better connection with/awareness of the Xochimilco chinampa ZP area.

Reduction of water shortage and improvement of water quality contribute to improved agriculture, navigation, recreation, health.

The (co)benefits may include:

• Improved urban public space

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• Improved connection with chinampa area • Improved water availability and water quality in the chinampa area 4. Interested parties:

• • • • • •

CORENA Urban communities Sacmex Chinamperos Fundas de la vida AZP

5. Economic Cost Components to be included in a cost estimation are:

• Green corridor space and infrastructure • Building/changing infrastructure to reroute part of the spring water into green corridors

Figure 6.12 Project cost estimation Green Corridors

Assumptions: 12 kilometers of canals (green corridors) at a similar cost of building a street with the following specifications: 15 cm thick hydraulic concrete, reinforced with steel. https://www.gob.mx/cms/uploads/attachment/file/23401/costos_parametricos_pavimentacion.pdf 6. Risks Possible political barriers may be related to the current use of the spring water, as it is the source of drinking water. Other barriers may be the current use of the space that would be needed for the green corridors.

7. Integration with government plans

Local: Resilience Strategy

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6.3.5 Water Culture Centre 1. General description A Centre for Water Culture at the Xochimilco Ecological Park (PEX), with projects such as:

• Tourism: Boat rides with different themes, flowers and landscape, Aztec culture, Day of the Dead, bird watching.

• Agriculture: chinampa technology training and food market.

• Culture: Museum about the importance of water in modern civilization and in the history of Mexico City.

• Technology: demonstration centre for water sustainability technologies.

2. Technical description and Location of project Redesign and rehabilitation of the main building as a museum with environmental education spaces.

Redesign and rehabilitation of the rest of the land as green and blue infrastructure such as natural and artificial wetlands, infiltrating gardens, and food production in chinampas and forests with focus on the ecological regeneration.

The park is located in an area of 277 hectares on the western side of Xochimilco wetlands.

3. Social and environmental impact Create awareness on the strategic importance of the Xochimilco wetland system, in terms of its ecological, historical, economical and water production value:

• The chinampas are one of the most productive and sustainable agricultural systems on the planet.

• Exemplary opportunity for citizen action to regenerate its social, cultural, environmental and economic heritage • Cultural Heritage of Humanity by UNESCO • RAMSAR Site (Wetland Conservation), UNESCO • Agricultural Heritage System, SIPAM, FAO • Areas of Importance for the Conservation of Birds, AICAS, CONABIO • Protected Natural Area, SEMARNAT, SEDEMA • Wetland of 3,500 hectares with very high biodiversity richness

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• Supplier of 35% of the water in Mexico City • Climate stabilizer and key part in flood prevention in the Metropolitan Valley.

• Important traditional tourist destination of Mexico City • Public recreational space of Mexico City • Food production capacity for 5 million inhabitants • Last living museum of the lacustrine culture, origin of Mexico Provide Mexico City residents and visitors spaces for environmental recreation and education on ecological culture particularly on water topics.

Show a success story on urban land regeneration of soils, biodiversity of flora and fauna, as well as water harvesting.

The creation of this Water Culture Centre can empower other water culture parks as a movement of emblematic projects towards water sustainability in Mexico and the world.

Figure 6.13 Xochimilco culture centre topics

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4. Interested parties:

• • • •

Mexico city residents and visitors.

Chinampa agriculture community.

Tourism operators.

Academics and business interested in water technologies.

5. Economic Cost

Figure 6.14 project cost estimation

6. Risks

• Environmental deterioration due to poor management • Proliferation of invasive plants on the lake and wetlands, displacing native plants 7. Integration with government plans Federal and Mexico City’s Secretary of Education, Secretary of Tourism, Secretary of Economy.

6.4 POTENTIAL FINANCING SOURCES AND RESPONSIBILITIES A first estimate of an investment package for the first stage of selected projects has a cost of approximately MXP $1,365 millions.

Figure 6.15 Project Investment summary

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In the vast majority of presentations and interviews, the government of Mexico City and the Federal Government were identified as the natural sources of this financing, either by own resources or by resources managed with international development banks, such as the World Bank, IDB or CAF. The other alternative proposed by the participants in the workshops are donations or investments made by philanthropic institutions funded by private companies. Unfortunately, the scale of the capital available through these channels is insufficient for the magnitude of this project.

In addition to this, our perspective is that with a well-designed plan there is a real possibility of co-financing this intervention with private funds that seek financial returns while regenerating the environment, commonly called Impact Investment .

We believe that in order to receive impact investment for this intervention system, it is necessary to re-interpret part of the water resilience plan as a project that, in addition to bringing broad environmental and social benefits, has a direct financial return for investors. This can be justified from the point of view of increasing the productivity and profitability of chinampa agriculture, the increase of quantity and value of tourism derived from the regeneration of the ecosystem or the increase of real estate value of the surrounding area. Defining the specific mechanisms through which it is possible to capture part of this value must be done in conjunction with the direct beneficiaries of this intervention and the government authorities .

The impact investment sector is usually linked to the services of an impact incubator or accelerator. Although, for legal, cultural and political reasons in Mexico, the regeneration of water systems in itself could not be seen as a private business in the short term, but it can be linked to private businesses of agriculture and tourism through public-private partnerships (PPPs). This model has a high possibility of being profitable and viable, despite the numerous and complicated challenges in relation to the negotiation with multiple actors involved in the ZP and the alignment of their interests. This project is a good candidate to be developed with the advice of international incubators and through them receive co-financing for water infrastructure.

A finance solution at the scale of the XTMA ZP project requires a public-private strategy that includes the whole system, that understands and promotes the public benefits of the strategy and finances them with state resources, and also understands the private benefits of the

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strategy and finances them with private resources. For this reason, a PPP initiative with public and private actors has a better chance of success. As part of this PPP, legal, cultural and logistical obstacles will have to be evaluated and a plan to solve these obstacles and generate synergies among all the actors of this initiative is required.

Financing options Category

Candidates

Terms

Government of Mexico City

SACMEX, SEDEMA

Possible sunk investment. Aligned with the investment priorities of Mexico City government, high social and environmental profitability

Federal government

CONAGUA, SEDATU

Possible sunk investment. Aligned with the investment priorities of the federal government, high social and environmental profitability.

Development Banks

World Bank, GEF, BID, CAF

Rates below market returns , social and environmental profitability.

Long-term repayment scheme.

The following table shows 9 actions of the Government Program of Mayor Claudia Sheinbaum that are directly or indirectly related to the ZP XTMA. Actions include agricultural or tourism promotion and cross-cutting issues such as administrative deregulation or employment promotion.

It can be argued that the Water Resilience Plan for the ZP of XTMA is an ideal case for the government to invest public resources. Public funds seek to leverage capital investment from other financial institutions, such as national or international development banks or private investment funds to develop economic opportunities in the area.

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Government actions Claudia Sheinbaum

Farming

Xochimilco

Water

Allocate 1,000 million pesos per year to the owners of conservation land, under mechanisms that support agricultural activity and its commercialization, that promote traditional technologies in an innovative way and that support corn and milpa (corn, pumpkin, bean, chile), through schemes that protect and restore the conservation land and expand the environmental services it provides.

Much of the ZP XTMA belongs to conservation land; chinampa is ideal for milpa and criollo corn agriculture that generates environmental services such as the incentive to regenerate the wetland system.

Protection of this area as a historical and cultural heritage of humanity.

Protecting XTMA as a world heritage zone begins by restoring its water system.

5 billion pesos more to the budget of the Water System of the City, to repair drainage leaks.

One of the main problems of the water system of XTMA is the contamination due to lack of drainage in informal settlements; wetlands are an excellent way to create an environmentally friendly “drainage� of the XTMA ZP

Duplicate the budget, which is currently 688 million pesos.

The cultural legacy of the XTMA ZP is an extremely rich component of the history of Mexico City and the country, since it is the lake where Tenochtitlan was born, the origin of Mexican culture.

This culture was born being lacustrine, therefore an investment in the recovery of the wetland system of XTMA is an investment in culture.

The goal is to create between 500 thousand and one million new jobs in this administration.

By recovering the XTMA water system, employment in the agricultural and tourism sector of the ZP is directly stimulated.

Strengthen tourism.

By recovering the XTMA water system, employment in the agricultural and tourism sector of the ZP is directly stimulated.

Culture

Jobs

Synergy with the XTMA Water Plan

Tourism

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Government actions Claudia Sheinbaum Microenterprise

Synergy with the XTMA Water Plan

Support to the micro and small businesses with soft loans.

By recovering the water system of XTMA, the creation of micro-enterprises in the agricultural and tourism sector of the ZP is directly stimulated.

Promotion of the social economy.

Investment in water infrastructure and incentives for organic agriculture and nature based tourism can generate thousands of jobs in the ZP, which in turn become protectors of the water system of the ZP.

Economy

6.5 CHAPTER CONCLUSIONS Understanding the Xochimilco water system in relation to water itself, climate change, agriculture, economic growth, urbanization, governance, or ecology, helps to design and implement successful solutions.

Solutions can be implemented by the government and new potential markets, such as organic premium agriculture, nature based tourism or new real estate developments. The investment could partly be financed by private players looking for a market return.

Proactively involving all key stakeholders through innovative collaborative processes such as incentivized competitions for solutions, instead of dictating and controlling top down initiatives, opens a larger field of possibilities for effective disruptive solutions and real change.

The complete investment package required by the project portfolio is substantial in financial terms and also in the complex collaboration with local communities and multiple actors involved. The magnitude and complexity are better managed if projects are implemented in stages, so everybody can learn from iterative implementations.

 

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144 A WATERA RESILIENCE PLAN FOR ZONE ZONE OF XOCHIMILCO, TLAHUAC ANDY MILPA ALTA WATER RESILIENCE PLANTHE FORHERITAGE THE HERITAGE OF XOCHIMILCO, TLAHUAC MILPA ALTA

CONCLUSIONS

A project portfolio for a resilient water system The current project aimed to restore the water system of the Zona Patrimonial of Xochimilco, Tlahuac and Milpa Alta (XTMA ZP) in such a way that it can cope with the effects of climate change and the socio-economic challenges that put pressure on the water system.

Understanding of the XTMA water system in relation to climate change, agriculture, economic growth, urbanization, governance, or ecology, is a prerequisite to design and implement successful solutions. In a resilient water system water is part of the problem but also of the solution.

A geological risk assessment and hydrological modelling study of the Heritage Zone in combination with community engagement and stakeholder consultation were performed to develop a project portfolio to improve and sustain the water system of the XTMA ZP. Geological risk assessment A probabilistic analysis was carried out to estimate the intensities associated to seismic hazard within the XTMA ZP. A catalogue with more than 100 years of historical earthquake data was compiled and about 45 seismic sources were defined along the Mexican territory. Site effects, caused by soft deep strata, may occur in the region, and were therefore included by means of transfer functions, which in general terms, relate the stiffness of the soil (soil dominant period) to the amplification factor. Finally, the influence of subsidence in seismic intensities was studied.

The largest seismic intensities occur in the middle of Tlรกhuac District, just at the Heritage Zone boundary. Besides, it was observed that there is a direct relation between intensity changes and fracturing alignments; especially for the 0.2s spectral ordinate. Seismic intensities range from 0.03 to 1.4g, depending on the spectral ordinate and return period evaluated. These intensities were expected since this zone is characterized by soft strata soil and site effects which considerably amplify the seismic waves.

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We assessed the exposure in Zona Patrimonial (XTMA) for two categories: Hydraulic infrastructure (drinking water wells, water supply network, sewage network, reclaimed water network, water storage tanks, pumping stations, water pollution control treatment plants and drinking water plants) Urban infrastructure (houses, roads, markets, churches, government buildings, healthcare buildings and schools).

However, high seismic intensities do not automatically mean high losses. It is important to include the assets of interest in the analysis and their performance when a seismic event occurs. These results might help to plan where to build and which should be the quality of the structures are planning to place there.

As a primary preventive measure (but not as a final decision), structures placed in high seismic regions (greatest seismic intensities) should have a stricter design and construction, whereas those located in low seismic regions (lowest seismic intensities) the design may be controlled by other aspects (i.e. wind or subsidence). It is therefore necessary to combine the information on the spatial distribution of seismic intensities with the vulnerability functions of objects into integral seismic risk to adequately inform decisions on urban planning, building construction and mitigation measures. In this study, we obtained the seismic risk for the XTMA Zona Patrimonial from hazard and vulnerability models, including site effects and subsidence effects.

We assessed the exposure in the XTMA ZP for two categories: (a) Hydraulic infrastructure (drinking water wells, water supply network, sewage network, reclaimed water network, water storage tanks, pumping stations, water pollution control treatment plants and drinking water plants), and (b) Urban infrastructure (houses, roads, markets, churches, government buildings, healthcare buildings and schools).

Results show that the critical scenario was similar to the 19 September 2017 earthquake, because of its distance to the study area, magnitude and depth. It can be concluded that exposure is affected to a greater extent by this type of earthquakes. However, potential risks from Guerrero and Puebla earthquakes with high magnitudes (from a formal seismic disaggregation) should not be discarded.

From the analysis, it can be concluded that the type of infrastructure which is the most vulnerable, is in our case the sewage pumping stations. These stations presented the highest level of risk because the estimated losses due to the critical scenario was the highest.

Hydraulic and sewage installations should have to contemplate vertical and horizontal displacements at these zones to avoid failures. This could be controlled using flexible connections.

It is recommended to do additional geotechnical and geophysics explorations at abrupt transition zones with the purpose of modeling the depth contact geometry between soft soils and basement. In addition, it is recommended to monitor topographic changes and piezometric

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levels in order to follow-up possible water extraction zones. It is recommended to establish contingency plans based on the estimated loss ranking of hydraulic infrastructure for the critical scenario.

Finally, results obtained in this document correspond to general characterizations of each infrastructure, therefore, it is recommended to carry out a more detailed study that allows to reduce uncertainties in the definition of infrastructure vulnerability of, for example, drinking water system and sewage, like pumping stations, drinking water plants and water pollution control treatment plants.

Regional and local modelling of the water system Regional numerical hydrological models were developed for the Basin of Mexico that defines the boundary conditions for the water system of the Zona Patrimonial. A numerical groundwater model was built based on extensive collection of existing data from various public sources, government agencies and universities. Groundwater is the main source for drinking water supply, sustainable groundwater resources management is, therefore, of paramount importance. The dynamics of the hydrological system simulated with the hydrological Wflow model were used for the top boundary conditions of the model (groundwater recharge). On average, the water balance of the Basin of Mexico is estimated at -138.1 mm/yr. Groundwater abstraction exceeds groundwater recharge with a factor of four, as groundwater recharge is estimated at 43.6 mm/yr, while groundwater abstraction account for -181.7 mm/yr.

No accurate information regarding recordings of water levels has become available during the execution of the present study. The model has therefore not been calibrated and the validation with literature information has been on a visual and qualitative basis. The model should therefore be regarded as a schematic first-iteration representation of the groundwater system of the basin of Mexico that can be updated as soon as more information becomes available. It is known that some information will become available in the near future while there is also information that exists already that can be utilized in the next phase of the project.

Some information on groundwater levels seems to exist. A groundwater monitoring system is operated by CONAGUA that may likely provide extremely valuable data. The level of detail of the information reported on the website is however not enough for the calibration and validation of the current groundwater model. For example, studies conducted by

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Carrera-HernĂĄndez and Gaskin (2007) present records of groundwater levels since 1969 (Figure D.6). During the course of the current project we have not been able to acquire this information.

Information on water levels or discharge recordings is not available in the Zona Patrimonial. The current model was therefore not calibrated or validated against measured data. This demonstrates the need to further develop a sustainable and structural data acquisition and monitoring program.

The models for hydrology, groundwater and hydraulics of the XTMA ZP can be further developed in future projects and programs, in order to make use of the potential capacity these models have to support decision making. A training program should be provided to technical staff of UAM, UNAM and ARCDMX in Mexico City.

Participatory stakeholder analysis The workshops and interviews conducted to promote the understanding of the water socio-ecological system in the heritage zone, resulted in a map showing interrelations and interconnections between stakeholders and visualized the interests and/or conflicts between stakeholders. Involving local communities is essential to develop adequate solutions and create ownership.

Development of a project portfolio A portfolio of project proposals was developed to improve the water challenges of water supply, water quality, water distribution and subsidence, with a positive impact on socio-economic, cultural and governance aspects of the area.

The project selection includes: 1. A reconnection of the Rio Amecameca to the Heritage Zone 2. Construction of linear wetlands in the streets bordering the channels at the southern border of the Xochimilco chinampa zone as passive treatment systems for the wastewater of surrounding houses, in order to discharge clean water into the wetland’s canals.

3. Development of a water control system to control water flows at the chinampa canals of Xochimilco to restore hydraulic connectivity and remediate subsidence 4. Green corridors to improve the water system of the ZP by restoring

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part of the inflow from the springs south of the ZP and to improve urban public space through a connection to attractive linear parks.

An improved spatial connection may raise awareness of the urban community to the chinampa area.

5. Building a Centre for Water Culture at the Xochimilco Ecological Park (PEX) to create awareness on the strategic importance of the Xochimilco wetland system, in terms of its ecological, historical, economical and water production value.

The main characteristics of the identified projects were time scale (short, medium and long term), viability (social, technical, legal and financial), scale (regional, local and site), maturity of the project (high / low), relation to climate change, and intrusiveness (very intrusive / not very intrusive).

Two key areas of impact are the quantity and quality of the water. In terms of water quantity, the most impactful project is Rio Amecameca, which alone could provide three times the amount of water needed in the ZP. Regarding water quality, the three key points of intervention are the illegal discharges of domestic wastewater, the agrochemical contamination of the chinampas and the quality of the treated water that is supplied to the ZP. The most likely project that will have a very significant impact on this indicator is the construction of constructed wetlands, which could mitigate the impact of urban wastewater discharges.

In addition to water supply of acceptable quality, the water must be distributed correctly in the wetlands of the Heritage Zone. One of the project proposals addresses this issue: the water control system. This project is an engineering and infrastructure project that is essential for the rest of the projects to be viable and effective, especially if climate change and subsidence change the landscape and conditions in the future.

The collaborative research and design process made it clear that isolated solutions are unlikely to work, partly because each solution requires the environmental, social and economic benefits generated by the other solutions, and each solution is negatively affected by the problems that are generated in other areas of the system. The solution is therefore to propose and implement a comprehensive set of interventions that will have a meaningful impact because of the synergies between and the collective impact of these individual actions. Due to the intricate web of positive and negative relationships that exist between the

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different projects for the regeneration of the water system of XTMA, we therefore believe that we should propose an integral narrative or system of projects that will collectively lead to meaningful impact.

The design of interventions in the water system should consider the relationship between social, environmental and economic realms. For example, reduction of water contamination by sanitary discharges in informal settlements is much more likely to be solved if, in addition to technological interventions, a social strategy is implemented in these communities that raise awareness of the vulnerability and value of these water bodies. Such a social strategy can be empowered through a mechanism that transfers part of the economic value that is generated by regenerating the water system to those communities, in this way aligning the interests of all stakeholders.

The government of Mexico City and the Federal Government were identified as potential sources to finance these measures, either by own resources or by resources managed with international development banks, such as the World Bank, IDB or CAF. In addition to this, Impact Investment of private funds that seek financial returns while regenerating the environment may represent a co-financing possibility. Public-private partnerships (PPPs) initiatives involving public and private actors (e.g., organic premium agriculture, nature-based tourism, inclusive real estate development) should be considered. Proactively involving key stakeholders through innovative collaborative processes (e.g., incentivized competitions) for solutions, may create a larger field of bottom-up possibilities for effective and disruptive solutions and lasting change. The complete investment package proposed with the project portfolio is substantial in financial terms and complex because of the collaboration with local communities and multiple actors involved.

The magnitude and complexity may be better managed if projects are implemented in iterative stages, which is very well possible.

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158 A WATERA RESILIENCE PLAN FOR ZONE ZONE OF XOCHIMILCO, TLAHUAC ANDY MILPA ALTA WATER RESILIENCE PLANTHE FORHERITAGE THE HERITAGE OF XOCHIMILCO, TLAHUAC MILPA ALTA

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BIOPHYSICAL AREA DESCRIPTION A.1 Basin of Mexico The United States of Mexico (also known as Mexico) is divided into thirteen hydrological-administrative regions [RHA by its acronyms in Spanish] for water management and preservation of water resources. Mexico City and the World Heritage Site of Xochimilco, Tlahuac and Milpa Alta (Zona Patrimonial) belong to the RHA XIII, which is subdivided for management purposes into two sub-regions Valley of Mexico (9,739 km2) and Tula (8,490 km2). The sub-region of the Valley of Mexico (also known as Basin of Mexico) includes seven tributary watersheds (Figure A1): Xochimilco (509 km2), Amecameca-La Compañía (1,166 km2), Texcoco (1,401 km2), Avenidas de Pachuca (2,622 km2), Mexico City (1,818 km2), Cuautitlán (832 km2) and Tochac-Tecomulco (1,312 km2). Moreover, the Basin of Mexico is subdivided into seven aquifers: Cuautitlán-Pachuca, Texcoco, Chalco-Amecameca, Metropolitan Area of Mexico City, Tecomulco, Apan and Soltepec.

The Basin of Mexico is integrated by territorial portions of four Federation entities: Mexico City, State of Mexico, State of Hidalgo and, to a lesser extent, the State of Tlaxcala. It is bounded at the north by the Tula-San Juan and Amajac river basins, tributaries of the Pánuco River basin; at northwest, by Tecolutla River; at south and southeast with the basins of Amacuzac and Atoyac-Zahuapan rivers, tributaries of the Rio Balsas basin; at west, by the Lerma River basin.

Figure A.1. Morphology of the Basin of Mexico.

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A.1.1 Location The Basin of Mexico is located in the center of the country, between 19° 03’ 14’’ and 20° 11’ 25’’ of latitude north, and 98° 11’ 42 ‘’ and 99° 31’ 17 ‘’ of longitude west. It is an endorheic basin of lacustrine character and is sited in the middle of the Transverse Mexican Neovolcanic Axis, approximately 2,400 meters above sea level. It is the uppermost area of the Pánuco River Hydrologic Region, reaching heights above 5,000 meters. It is surrounded by mountains: to the north the Sierra de Pachuca, to the east the Sierra de Calpulalpan and the Sierra Nevada with its volcanoes Iztaccíhuatl and Popocatépetl, to the south by the Sierra del Chichinautzin, while to the west the Sierra de Las Cruces.

A.1.2 Topography Its topography consists of three zones. The lower zone, from the bottom of the basin to the 2,250 meter altitude contour line, with an extension of 1,507 km2.

The hill zone, between 2,250 m and 2,400 m contour lines, with a surface of 2,575 km2. The mountainous zone, between 2,400 m contour line and the Basin’s mountainous summits, with an area of 5,518 km2, and is covered with sacred fir (oyamel), pine and evergreen oak forests, as well as grazing lands and seasonal agriculture, and it is the ecological reservation of Mexico City.

The Basin of Mexico is surrounded by the Trans-Mexican Volcanic Belt. During the Quaternary period, basaltic volcanoes Cerro Gordo, Chimalhuacán, Estrella and Chiconautla appeared. Later on, basaltic lava between the Sierra Nevada and Sierra de Las Cruces ridges formed the Sierra de Chichinautzin ridge and closed basin, up to then draining toward the Balsas River Basin. Later on it was stuffed with pebble stones, gravel, sand, ashes, lake clays and other, and it became water saturated. This stuffing generated an extended plateau with shallow lakes.

These lakes were formed by runoffs from a variety of torrential rivers which for thousands of years carried volcanic ashes, alluvium and other materials, as a result of soil erosion caused by runoffs in their way toward lakes. As they settled, these materials formed the clayish bottom of lakes. The Aztecs gave these lakes the names of Chalco, Xochimilco, Texcoco, Xaltocan and Zumpango. All of them have fresh water, with the exception of Texcoco, which is briny. By the end of the Glacial Age, these lakes, the last vestiges of numerous much larger lakes, formed one only shallow water body.

A.1.3 Climate According to the geographic location and the altitude, the climate of the Basin of Mexico varies from semi-arid and hot, in the northeast, to semi-humid and cold in the mountainous regions of the south of the basin (Inegi, 2010). The average annual temperature is 14 °C and ranges from 12 °C in the mountainous to 18 °C in the northern of the lower zone. The basin is influenced by humid maritime air currents of the Gulf of Mexico, with dry and hot air currents coming from the northwest areas of the Mexican high plateau; The rainfall regime of the Basin of Mexico is derived from the direct influence of hurricanes and cyclones in the Gulf of Mexico and the Pacific Ocean (Secretaría de Medio Ambiente, 2001). The mountain that surround the basin act as condensers, producing orographic rains

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from mid-May to mid-October, with an average annual rainfall of 693 mm and ranging from just under 500 mm in the northern zone to more than 1,000 mm in the Sierra of the Chichinautzi and in the Sierra Nevada. During the rainy season, 80% of the yearly rainfall takes place: about 5,706 millions of cubic meters. The rainy season is not favorable for water utilization; on the contrary, it favors swell runoffs. High intensity, short duration storms are frequent in the region. Sixty percent of yearly rainfall is concentrated in three months.

A.1.4 Main Waterbodies During Aztec times, the basin of Mexico was composed by a lacustrine system of 2,000 km2 formed by five main lakes: Chalco, Xochimilco, Texcoco, Xaltocan and Zumpango. At present, the surface area of lakes had been reduced by urban expansion to about 57 km2. The hydrography consists of 45 rivers, 8 lakes and 3 springs (Figure A1). The melting water of the highest parts of some volcanoes, together with the springs in the high and middle parts of the mountains, form 13 perennial rivers: Magdalena, Santo Desierto-Mixcoac, Tacubaya, Tlalnepantla, Hondo, San Idelfonso, San Pedro, La Colmena, Cuautitlán, Tepotzotlán, Ameca, San Rafael and Texcoco-Aculco; the other 32 rivers are temporary, formed from May to October during the rains (Legorreta, 2009). Among the most important lakes are: Zumpango, Guadalupe, Madín, Chalco and Nabor Carrillo; in the eastern part of the basin there are other lakes such as Tochac, Apán and Tecocomulco. The main springs from which clean water flows all the time, are: Fuentes Brotantes, Santa Fe and Peña Pobre.

A.1.5 Water management CONAGUA is the federal institution in charge of the water management, control, regulation and protection. CONAGUA is organized in two modalities: National Level, and Regional Hydrological - Administrative Level. The later is accomplished through the “Organismos de Cuenca”, i.e., Basin Organizations. The “Organismo de Cuenca Aguas del Valle de México” (OCAVM, Mexico’s Valley Water Basin Organization) is in charge of the development of policies and coordination among the Federal government, State government and Municipal government, as well as the water users, individuals and social organizations, on the RHA XIII. Water management in the State of Mexico is in charge of the State of Mexico Water Commission (CAEM). The CAEM is the institution responsible for planning, maintenance, operation and management of water facilities in the State of Mexico. Hierarchically, the CAEM is above Water Utilities but is regulated by CONAGUA. Additionally, Water Utilities of each municipality are in charge of the provision of services in the metropolitan area. In Mexico City, water supply is undertaken by the Water System of Mexico City (SACMEX), previously known as the Water Commission of the Federal District. It is the institution in charge of the provision of drinking water, drainage, sewerage, wastewater treatment and water reuse services in Mexico City.

A.2 Mexico City Mexico City and its metropolitan area (MCMA) is located within the Basin of Mexico. This area has the highest human concentration of the country (close to 25 million people), and generates about 25.4 percent of the national GDP (Mo-

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rales-Novelo, 2011). Mexico City is the Capital of the Mexico Country, it is also the cultural, economic and industrial center of the country, which houses almost all government offices, national and international business centers, cultural activities, universities and the most important research institutes.

A.2.1 Location Mexico City borders with the State of Mexico and the State of Morelos. It has a territorial extension of 1,485 km2. Mexico City is divided into sixteen boroughs (known as “Alcaldias”): Álvaro Obregón, Azcapotzalco, Benito Juárez, Coyoacán, Cuajimalpa, Cuauhtémoc, Gustavo A. Madero, Iztacalco, Iztapalapa, Magdalena Contreras, Miguel Hidalgo, Milpa Alta, Tláhuac, Tlalpan, Venustiano Carranza and Xochimilco. Mexico City territory -less than 0.1% of national territory- is located in the southeast of the Valley of Mexico. Half of its territorial is urbanized; it has grown mainly towards north, east and west.

The MCMA (conurbation surrounding Mexico City) is constituted by the sixteen “Alcaldias” of Mexico City and 59 municipalities of the neighboring State of Mexico. The MCMA has a territorial extension of 7,854 km2, of which 65% is classified as urban land use (43% of Mexico City and 22% of the State of Mexico) and 35% is used for agriculture, forestry, and conservation (SEMARNAT, 2010).

A.2.2 Climate The climate of the MCMA ranges from humid to semi-arid. Mean annual temperature ranges from 11 - 17°C. During the months of April and May, maximum temperatures may reach 33°C, whereas minimum temperatures reach -1°C in December and January.

Average annual precipitation is estimated at 668 mm in the north of the MCMA and 1306 mm in the south, occurring mainly from June through October.

A.2.3 Population According to the Economic Census of 2015, Mexico City has a population of 8,918,653 inhabitants, of which 52.6% are female. Statistics show that 99.5% of the population lives in urban areas, whose residences are established on the north half part of Mexico City (Figure A2). The southern territorial of Mexico City is a green area and whereby much of the aquifer recharge take place. Iztapalapa (est of city) is the most populated Alcaldia, close by 2 million inhabitants and whereby most difficulties for water distribution is encounter.

The MCMA has an estimated population of 19,239,910 inhabitants of which 51.6% are female.

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Figure A.2. Population distribution in Mexico City. From: Gomez-Reyes et al. (2009).

A.2.4 Water system Potable water supply The Metropolitan Area of Mexico City, with its 18 million population, is supplied with a flowrate of 64.5 m3/s, out of which 47.5 m3/s (74%) come from underground sources, while 17 m3/s (26%) come from surface sources such as the Cutzamala System and some springs. Up to date, the aquifer overexploitation level is about one and a half times its natural recharge.

In order to respond to drinking water requirements of Mexico City inhabitants -8.7 million- a mean flow rate of 35.5 m3/s is supplied. Service through home intakes is available to 98% of the population. The remaining 2% is supplied free of charge by tank cars. Out of the total water supply, 24 m3/s (67%) come from underground sources: 19 m3/s from Basin of Mexico aquifer, and 5 m3/s from Base of Lerma aquifer; while the remainder is obtained from surface sources: 1 m3/s (3%) comes from Magdalena River and springs located at west and south regions of the City, and 10.5 m3/s (30%) come from the Cutzamala System, from where water must be pumped to a height of 1,100 meters and piped through 127 kilometers to reach the city.

The flow obtained from mentioned sources is conducted through 514 kilometers of pipelines of several diameters to 295 storage tanks with a combined capacity of 1,700,000 m3. Water is distributed from these tanks to users through 1400

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km of primary network and 12,000 km of secondary network. Additionally, 196 pumping stations are used to supply water to dwellers of higher zones (Buenrostro-Hernandez, 2006b).

To maintain a suitable quality of supply, 34 water production plants are available, 20 of them built after 1998, with a combined installed capacity of 4.08 m3/s.

Twenty-nine of these plants operate next to wells. There are also 365 automated chlorinating devices, strategically located in the system’s structures (Buenrostro-Hernandez, 2006b).

Water demand in the city keeps growing, and the latest flow contribution was received in January 1995. Besides, the Temascaltepec supply phase, as the last of the Cutzamala System, has been suspended because of some still unresolved difficulties, of a social and political nature in that region. Upon this situation, the Mexico City Government is working on a strategy for the supply of water based on a better utilization of this resource at a user’s level, as well as in infrastructure maintenance to prevent leaks and waste and to recover its efficiency, and the construction of new works to improve collection and distribution of water volumes. The decision is to rationalize handling and use of already available water.

Sewage system Mexico City has a drainage system with a great magnitude and complexity. It is a combination type system collecting residential and industrial waste waters, in addition to the runoffs caused by rains. Its structure comprises 10 240 km of secondary network piping, and 2 087 km of primary networks, 144 km of marginal collectors, 178 pumping stations, open cut channels, piped rivers, storage dams and regulation lagoons, and continues in the general draining system and in the Deep Drainage -currently the backbone of the system- until it discharges at the higher part of the Pánuco River Basin, and later on into the Gulf of Mexico (Buenrostro-Hernandez, 2006a).

Summarizing, the sewerage and drainage system of the Basin of Mexico consists of three strategically located large systems of hydraulic ducts, draining in a noticeably north-south direction (Figure A3). They are the three only outlets to displace the flows of waste and rainfall waters out of the Basin, to protect the City from the risk of floods. These outlets are: Western Interceptor-Emitter (Interceptor-Emisor del Poniente), by the Tajo de Nochistongo; the Great Drainage Channel

by Tequixquiac Tunnels I y II, and the Central Emitter of the Deep Drainage System to the El Salto River (Buenrostro-Hernandez, 2006a).

1. The Western Interceptor - Emitter receives runoffs from the rivers located at the western part of the Basin of Mexico, previously controlled at the Western Dam System (Sistema de Presas del Poniente), this latter being an interconnected system integrated by 36 dams -18 in the State of Mexico and 18 in the Federal District- with a total regulating capacity of 3.1 million cubic meters collecting and regulating runoffs from rivers in the Western Mountainous Country (Serranía del Poniente). The Western Interceptor (Interceptor del Poniente), a 4-meter diameter tunnel with a length of 12.4 km, starts at

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University City (Ciudad Universitaria) and discharges at the Rio Hondo Pumping Station, where it is pumped to discharge into the Hondo River towards the buffer reservoir of El Cristo, where swellings are again regulated. During strong storms it sluices into the Western Emitter, which in its route receives waters from the rivers Tlalnepantla, San Javier, Cuautitlán, Tepotzotlán and other with lower flows, until it reaches the Derivadora Santo Tomás (Santo Tomás Diverter Dam), where swells can be again buffered at the Zumpango Lagoon, or else discharge towards the Tajo de Nochistongo and merge with El Salto River to further discharge waters into Tula River and the Endho Dam, wherefrom water is distributed for irrigation.

2. The Great Drainage Channel starts at Lecumberri, close to downtown, and in its route receives contributions from northern, central and northeastern zones, through the sewer, collector and emitter network, after passing through the City’s pumping stations. At Km 9+450 of Great Channel’s path, the Remedios River merges in, with waters coming from the western part of Mexico City, and the excesses from the Tlalnepantla and San Javier rivers, as well as the municipal waters from Tlalnepantla, Naucalpan, Ecatepec and Netzahualcóyotl. At Km 18+500, the Canal de la Draga also merges in, carrying the flows of La Compañía and Churubusco rivers, through the General Drain of the Valley (Dren General del Valle). The Great Drainage Channel continues its path through the Cuautitlán Valley. In its route, it receives contributions from the municipalities located at its banks; there are also withdrawals of flows utilized in agricultural irrigation in both banks of the Great Channel, mainly at Irrigation District 088, Chiconautla, and in several agricultural development units, altogether occupying about 7 300 hectares. Waters from Great Channel flow through the Tequixquiac Tunnels near Zumpango to merge later on with the Salado River to be distributed to irrigation zones such as Districts 03 (Tula) and 100 (Alfajayucan), in the State of Hidalgo.

3. The Deep Drainage System, currently with 165 km in operation, built to sluice rainfall flows outside the Basin of Mexico, is the third water outlet. In 1975, the first stage was completed, with the construction of the Central Emitter Tunnel (Túnel del Emisor Central), with a diameter of 6.50 meters and a length of 50 km, provided with access shafts (lumbreras). Depths range from 25 to 220 meters. Since then, the length has been increased with the construction, upstream, of the six interceptors that are a part of this System, draining several zones of the City from south to north, with diameters ranging from 3 to 6 meters and depths from 20 to 48 meters. These interceptors discharge at Shaft 0 (Lumbrera 0) of the Emitter, at the border between the Gustavo A. Madero Sector (Delegación), in the Federal District, and the Tlalnepantla Municipality, in the State of Mexico. Through the outlet portal, they pour their flows into the El Salto River, outside the Valley of Mexico, at the high part of the Pánuco River Basin. In 1997, the Deep Drainage had reached a length of 153 km of tunnels in operation. Between 1998 and 2000, this System was expanded by putting in operation an additional length of 12 km. The six interceptors, from west to east, are: The Center West (Centro Poniente) Interceptor, starting at Periférico and Constituyentes, to relieve West

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Interceptor (Interceptor del Poniente). It receives flow from Atzcapotzalco and Benito Juárez collectors, and the Remedios River intake work. Before merging to the Central Emitter, it picks up the discharges from the Tlalnepantla Municipality Interceptor. The Central Interceptor (Interceptor Central), receives flow from downtown collectors, as well as Río de la Piedad, and Center-Center Interceptor collectors, along its path to Shaft 0 (Lumbrera 0) of Central Emitter in Tenayuca, State of Mexico, close to boundary with the Federal District. The East Interceptor starts southeast of the City, and it receives water from collectors in the networks at southern and eastern zones, besides the Churubusco River intakes and the East-East Interceptor. Later on it receives water from the Great Channel Interceptor, built in 1999 and in operation since 2000. The tunnel is at a 20 m depth, with a length of 1 000 m and a 3.10 m diameter. It has a capacity to displace 35 m3/s. This assures the gravity drainage of Centro Histórico (downtown) without the need of pumping equipment, thus eliminating the risk of floods suffered for years in this area of the City. Further on along its path, the East Interceptor captures waters from the Great Channel intake and, before reaching Shaft 0 (Lumbrera 0), it receives waters from the San Javier and Tlalnepantla rivers. EastEast Interceptor (Interceptor Oriente-Oriente) receives water from collectors of the lowest zones in the City and, along its path, those of Churubusco River, on its way to East Interceptor (Interceptor Oriente). These interceptors discharge their flows into the Central Emitter (Emisor Central) starting at Shaft 0 (Lumbrera 0) in Tenayuca, goes across the Tezontlalpan Ridge (Sierra Tezontlalpan) and discharges into El Salto River, in the Municipality of Atotonilco, State of Hidalgo.

Figure A.3 Map showing the sub-catchments and rivers (blue lines) derived with Wflow, and how they link to the urban drainage system (red and orange lines) of Mexico City.

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A.3 Zona Patrimonial: Xochimilco, Milpa Alta and Tláhuac The polygon of “Zona Patrimonial” was originally based on the Historical Monuments Area of Xochimilco, Tláhuac and Milpa Alta, according to the Presidential Decree of December 1986 (Official Gazette of the Federation, 1986). This polygon has an extension of 89 km2. However, recent enquiries carried out by UAM and Xochimilco in 2005, and 2014 shows that the surface area is 69 km2. Thus, clarifications were requested in 2013 by the World Heritage Committee to several States and to the government of Mexico City.

These enquiries resulted in the modification of the polygon, which includes since 2014 three additional fractions. The first one incorporates the Canoeing Track and the chinampera zone of Amalacachico-Toltenco (east); the second one includes the archaeological zone of Cuahilama (south); and, the third one considers the lagoon formed between Tláhuac and the limits with the State of Mexico. In total, the new polygon has a surface area of 75 km2 (González-Pozo, 2016).

A.3.1 Location The polygon of Zona Patrimonial is contained in the territories of the Alcaldias of Xochimilco (49.6% of the polygon is in Xochimilco), Tláhuac (48.8%) and Milpa Alta (1.4%). It borders to the north with the city hall of Tláhuac and in a small part also with the Alcaldia of Iztapalapa and Coyoacán; to the east with the municipalities of Chalco of the State of Mexico; to the west with the mayor’s office of Xochimilco and a small portion with the one of Tlalpan; to the south it borders the Alcaldia of Xochimilco, Tláhuac and a mountainous stretch of the Alcaldia of Milpa Alta (Figure A4).

The Alcaldia of Xochimilco is located in the south-east area of Mexico City and borders with Tlalpan, Coyoacán, Tláhuac and Milpa Alta. It has a territorial extension of 125 km2, approximately 8.4% of the total surface of Mexico City. About 80% of the Xochimilco surface is classified as conservation area (100 km2).

The Alcaldia of Tláhuac extends over a surface of 89.5 km2, accounting 5.8% of the Mexico City territorial extension. The Alcaldia is located in former lakebed of Chalco and Xochimilco. There are some lake areas along with four major canals (Chalco, Guadalupano, Atecuyuac and Amecameca) and wetlands (Cienega de Tlahuac, Bosque de Tlahuac, Los Humedales) under conservation status.

The Alcaldia of Milpa Alta has a territorial extension of 268.6 km2 and it borders with Xochimilco, Tláhuac and Tlalpan, with the state of Morelos and the State of Mexico.

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Figure A.4 Location of the polygon of the ZP with respect to the Alcaldias of Mexico City and the sub-basins of the Basin of Mexico.

A.3.2 Topography The topography of the Zona Patrimonial is mostly flat. The terrain oscillates between elevations of 2,234 to 2,242 meters above sea level, where elevations below 2,238 meters are flooded or at risk of imminent flooding. The southeast part of the polygon of Zona Patrimonial, the terrain rises to 2,260 meters (Figure A5).

Therefore, it is a territory of scarce relief, where small altimetric differences explain the contours of its lake origin, as well as the transformations that continue to occur. This region has an average altitude of 2230 m.

Xochimilco is located on a flat surface of lacustrine origin with slopes that range from 0%–5% and an average altitude of 2,275 m. The southern part of the region is located in a mountainous area formed by the Xochitepec and Cantil mountains, and the Teoca, Zompole and Teuhtli volcanoes. These mountains formed a natural barrier between Xochimilco, Milpa Alta and Tláhuac.

The Alcaldia of Tláhuac is alienated into three zones, 1) flat lakebed, 2) transitional areas and 3) the mountain formed by volcanic sediments. The main elevations of Tecuautzi, Tetecón, the Sierra de Santa Catarina (2,800 m) and Teuhtli Volcano (2,700 m). The Santa Catarina Range serves as an important aquifer recharge area as well as a barrier against urban sprawl.

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The Alcaldias of Milpa Alta has an average altitude of 2413 m. It is part of the Trans-Mexican Volcanic Belt and the Chichinautzin Range. The main elevations of the Alcaldia are Cuautzin (3,510 m), Tulmiac, Ocusacayo (3,220 m), La Comalera (3,230 m), San Bartolo (3,200 m), Tláloc (3,510 m), Chichinautzin (3,470 m), Yecahuazac, Quimixtepec, El Oclayuca (3,140 m), El Pajonal (3,100 m), El Ocotécatl (3,480 m), Acopiaxco (3,320 m), Tetzacoatl (3,310 m), Tehutli (2,800 m) Cilcuayo (3,580 m), Nepanapa (3,460 m), Texalo (3,560 m), Oclayuca (3,390 m), San Miguel (2,988 m).

Figure A.5 Land elevations of the Zona Patrimonial.

A.3.3 Climate The zone has a subhumid temperate climate, with an average annual rainfall of 600-1,100 mm and an average annual temperature of 16 °C, with fluctuations between 8 and 25 °C (Espinosa et al., 2009). Water temperature depends on diurnal night cycles with fluctuations above 15 °C (Zambrano et al., 2009). The hydrological regime depends on seasonal change, with the rainy season occurring from May to October and the dry season from November to April. During the rainy season, the aquatic part of the ecosystem expands and forms temporary wetlands linked to permanent bodies of water (Figure A6).

The climate of the Alcaldia of Xochimilco is humid with precipitation during summer. According to the Köppen’s classification, the climate corresponds to C(W2) (w) b(i’), with average annual precipitation of 620.4 mm concentrated between May and October.

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The temperature ranges between 12°C and 18°C, with little variation of average monthly temperatures. During most of the year the prevailing winds come from the North and Northeast, and from November to February, the wind comes from the Southeast.

For the case of Tláhuac, the predominant climate is semi-humid with an average annual temperature of 15.7 °C. Temperature might range from 8.3°C to 22.8°C. Average annual rainfall of 533.8 mm, and occurs mostly between the months of June to August.

Figure A.6 Water bodies, fluvial network and residual water treated in the polygon of the ZP.

A.3.4 Population The polygon of Zona Patrimonial has a mixture of land uses. The combination of residential areas, agricultural areas, cultural heritage, nature, recreational areas and tourism makes the ZP a unique system (Figure A7). The residential areas are mainly located on the southern border of the ZP and becomes the boundaries of the chinampa areas of Xochimilco and Tlahuac. Although, illegal human settlements are located within the chinampa areas, just passing the residential area.

The Alcaldia of Xochimilco with a population of 415,007 inhabitants is the ninthmost populous alcaldia of Mexico City. According to the Population Survey of 2015 conducted by the National Institute of Statistics and Geography (INEGI), Tláhuac

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has a population of 361,593 inhabitants, whereas Milpa Alta has a population of 137,927 inhabitants.

Figure A.7

Land uses of the Zona Patrimonial.

A.3.5 Water System The aquatic part of the Zona Patrimonial, i.e., the hydraulic system of ZP, corresponds to channels of the chinampas of Xochimilco, Tláhuac and the extinct ones of Mixquic (Figure A8). These channels form a network of 404.14 km (Gónzalez-Pozo, 2016), of which 171.22 km are of main channels (2 to 5 m wide and more than 2.5 to 3.5 m deep) where navigation is possible, especially for the tourist activity developed in the channels of Xochimilco. The rest of the channels are narrow (less than 3 meters wide) and shallow (less than 1 meter deep), known as “apantles” or “acalotes”. Many of the apantles maintain hydraulic communication with the main canals, while others are flooded and sometimes filled to serve as irrigation channels for the chinampas, especially the canal system of San Gregorio and San Luis. The canal system is filled by its south coast with the discharges of the WWTP (wastewater treatment plant) “Cerro de la Estrella” and it is drained by the north coast through the Chalco Canal that runs from east to west. The natural direction of the flow in the channels is, therefore, from south to north, where it is diverted by the tributary channels to the Chalco channel (Figure A8).

The hydraulic connection of the channel system is not free, it has regulations through the gates that are operated by an open and closed mechanism according to the water levels (Figure A9). These hydraulic structures (pillars and locks) have

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been constructed in response to the differential sinking experienced by the ZP, attributed to the overexploitation of the groundwater that underlies the ZP. The hydraulic system of the channels has been segmented into 5 parts: Segment “Laguna del Toro.” It is located in the western part of Xochimilco´s chinampas, next to the southern end of Cuemanco channel. This area is hydraulically connected to the Xochimilco segment, having as control work the Yucatan lock; regulating the flow levels that pour the Laguna del Toro to Xochimilco segment, while allowing navigation between both segments the Laguna del Toro and Xochimilco.

Figure A.8 Channel System of the ZP, indicating the original flow direction.

Segment Xochimilco. Delimited mainly by the channels of Japan and Bordo at north; San Gregorio Atlapulco in the eastern; to west with the channel of Cuemanco and in the south with the channel of Apatlaco that finishes in the Bridge of Urrutia. The main channel network of 100 km in length is navigable in 80.2%, is obstructed in 19.8% and channels blinded with only 0.001%. Therefore, this segment had provided conditions for navigation to tourism and recreation; mainly navigation in canoes (knows as trajineras). This segment drains its flow to the channel of Chalco, through the channel of Japan and its spillway at the confluence of them.

Segment San Gregorio. This segment is comprised between segment of San Luis and Puente de Urrutia (Bridge of Urrutia), approximately has a channel length of

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205.5 km, of which only 20.7% are navigable channels and the rest have condition of obstructed, dry or blinded. The segment is bounded in the north by the Laguna San Gregorio, which has been forming for several decades by flooding chinampas. This segment drains to the channel of Chalco through the San Sebastian channel, and the flow is regulated by the spillway that is located at the confluence with the Chalco canal.

Segment San Luis. This section is next to San Gregorio, formerly it included all the area occupied by the CORENA facilities, which it borders on the north and the east. Its canal network has 43.5 km in length; 29.2% are channels suitable for navigation and the rest are obstructed and mostly blinded. This segment is hydraulically connected to San Gregorio through a spillway located next to the Flower Market of San Luis.

Segment Tlรกhuac. Section located east of the ZP, in line with the Chalco Canal and close to the Chalco-CORENA spillway. The network of canals in Tlรกhuac consists of 9.34 km in length, 82.14% are navigable channels with an apparent health (in terms of the width of their main channels), all flanked by large shafts. Consequently, it has also developed some tourist activity, with only one important pier in the so-called Laguna de los Reyes Aztecas (Gonzรกlez-Pozo, 2016). Tlรกhuac, being one of the highest altitude zones in the ZP, has as its outlet a spillway that is located in the Chalco canal, just in the vicinity of CORENA.

Figure A.9 Hydraulic segments of the channel system of chinampas of the ZP.

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In Xochimilco´s alcaldía, the sink rate is of the order of 15 cm/year (Izazola, 2001).

In general, the lowest areas in the ZP are located around the lagoons (San Gregorio and Lago Tláhuac-Xico), while the highest areas surround the southern perimeter of the ZP. Therefore, the ZP tends to be transformed into an endorheic micro-basin with its drainage towards the San Gregorio Lagoon and towards the Tláhuac-Xico Lake. The evacuation of water from the system through the Chalco Canal requires, therefore, assistance with the pumping system and rectification of its slope. The Institute of Engineering of the UNAM (National Autonomous University of Mexico) together with SACMEX (Water System of Mexico City) recently (2018), have put into operation an Archimedes screw-type pumping system at the confluence of the San Sebastían Canal, to pump water from the lower part of the Chalco Canal. However, the rectification of the Chalco Canal bottom from the pumping point to the west exit is still pending, so that in the rainy season there are floods around this area.

A.3.6 Water Quality In addition to the overexploitation of the aquifer, urban growth, change in land use, and flooding, the ZP is experiencing a great impact on the degradation of water quality. This is very poor and has a high concentration of nutrients, heavy metals, pesticides and microbiological agents such as E. coli and enteric virus (Toranzos et al., 2007). The urbanization process has become a means where the invasion of lands, irregular settlements, clandestine drainages and the removal of vegetal cover proliferate easily. Historically, the urbanization of the area has been a direct cause of the abandonment of agrarian policies, which could otherwise have strengthened chinampa agriculture. The clandestine drainages are an important consequence of the urbanization of the chinampa, especially of the irregular settlements. These discharges of sewage are considered as the main source of contamination of the waters of the channels of the chinampas, especially those associated to the irregular settlements located on the south coast of the ZP, whereby the the circulation transports pollutants to the north coast. This practice not only accelerates the process of eutrophication in the area, but also constitutes a risk to health, given its contribution to a sudden increase in the abundance of bacteria in water bodies (Barrera-Escorcia et al., 2013).

Waste Water Treatment Plants (WWTP) There are seven WWTP within the ZP (Figure A10), operated by SACMEX and, on average, have an operating efficiency of about half of their design capacity (54.2%), i.e., these WWTP can generate approximately twice as much water flow as the current. The Cerro de la Estrella WWTP is the largest, its treatment volume (1,339 L/s) is slightly more than five times the treatment volume of the other six WWTP together. Therefore, the Cerro de la Estrella WWTP provides about 80 percent (81.8%) of the total treated water that is channeled to the ZP (1,324 L / s), both for the filling of the channels of the chinampas and lakes as for the agricultural irrigation of the tables. Only three WWTP (Cerro de la Estrella, San Luis and San Lorenzo) allocate most of their treated water for the filling of the channels of the chinampas and the lakes that are presented in the ZP. The rest of the WWTP (La Lupita, San Nicolás, El Llano and San Andrés Mixquic) reuse their waters for

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agricultural irrigation in the Ejidos of San Juan Ixtayopan and in the Agricultural Fields of the Comalchica Triangle. The seven WWTP process the wastewater with contact filters (activated sludge); those that channel their treated water for the filling of channels and lakes have the greatest scope of treatment (tertiary) (Figueroa-Torres et al., 2017).

The water flow treated by the WWTP, and the corresponding discharge in the channels and lakes of the ZP, is not constant. It fluctuates daily and over the years has also had considerable variations (SACMEX. 2018). The Cerro de la Estrella WWTP had an average flow in the last 27 years (1990 to 2017) of 1,372 L/s, with maximum fluctuations up to 2,000 L/s and minimums of 317 L/s; its annual average of the last 11 years (2006 to 2017) has been of 1,339 L/s. On the other hand, the average flow generated in the last 27 years by the San Luis WWTP has been only 38 L/s, with maximum fluctuations up to 125 L/s during the decade of the 90’s.

Unfortunately, the minimum flows generated by this WWTP are characteristic from 2005 to the present (i.e., last 11 years), and are of an annual average of 20 L/s.

Figure A.10 Location of WWTP and their discharge sites in channels and lakes of the ZP.

The treated water is channeled to the water bodies (channels of the chinampas and lakes) of the ZP, in the manner indicated in Figure A10 (SACMEX. 2014). The channels of the chinampas receive almost three quarters (74.7%) of the total water treated (1,105 L/s) that is sent to the bodies of water. The largest volume of treated water (93.7%) received by the bodies of water comes from the Cerro de la Estrella WWTP. The discharge volumes of the Cerro de la Estrella WWTP in the

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chinampas channels are: 10 L/s at the Fernando Celada pier that feeds the canal system of the Laguna del Toro; 40 L/s at the La Draga site that feeds the Xochimilco channel system on the west side, and 300 L/s on Channel 27 (pier Zacapa) that feeds this system from the south; 80, 5 and 300 L/s feed the San Luis canal system through the Caltongo Canal (Exclusa), Canal Caltongo (Mercado) and Floricultor, respectively. Throughout the distribution network of treated water in the ZP, the Cerro de la Estrella WWTP also feeds the Tláhuac canals, including the Lake of the Azteca Kings, with 30 L/s, aided by the Revolucion rebombeo. Likewise, with the Cuemanco rebombeo, the Canoeing and Rowing Channel of Cuemanco is fed with 50 L/s. This distribution network of Cerro de la Estrella WWTP also has a discharge of 120 L/s that fills the lake area of the Ecological Park of Xochimilco and another of 100 L/s that feeds Lake Tláhuac-Xico. For its part, the San Luis WWTP provides 5 L/s to the San Luis canal system at the Descarga México 70 site, as well as 10 and 5 L/s at the Atenco and La Fábrica sites, respectively, that feed to the San Gregorio Canals system. This last system of channels is also fed through the Moctezuma discharge, with 40 L/s from the San Lorenzo WWTP, which also feeds the Lake of Tláhuac Forest with 10 L/s.

Regarding the quality of the treated water from WWTP that have discharges in the ZP, there is a program of water quality monitoring by SACMEX, which is the entity responsible for the operation of these WWTP. SACMEX has the Central Laboratory of Water Quality Control (LCC) in Xotepingo, Coyoacán, which performs laboratory analytical procedures for the determination of the concentration of water quality parameters and their maximum permissible limits to be met, in accordance to those indicated in the official standards: NOM-001-ECOL-1996; NOM002-ECOL-1996 and NOM-003-ECOL-1997, which define the control of wastewater discharges and the quality of the treated water. The staff of the LCC attends weekly WWTP to collect samples of treated water generated by the plants, except for Cerro de la Estrella and San Lorenzo, which is sampled every two weeks. Once the samples have been collected according to the regulations, they are delivered to the LCC for their physicochemical and bacteriological analysis of water quality. Based on the results of the analyzes, the LCC sends recommendations to the PTAR operators to apply chlorine in the form of hypochlorite, or chlorine gas, as in the Cerro de la Estrella WWTP, to restore the disinfection treatment. It should be noted that several plants have a laboratory, but they are not accredited and lack of personnel and/or chemical reagents necessary to carry out the analyzes.

In general, the treated water produced by the WWTP that channel their discharges to fill the channels and lakes of the ZP, meet the standards established by the SEMARNAT (Secretariat of the Environment and Natural Resources) for such uses.

However, at different times there are fluctuations in the treatment processes that can generate problems in the quality of the treated water. For this reason, to corroborate the quality of the treated water generated by these plants (Cerro de la Estrella, San Lorenzo and San Luis), professors and undergraduate and master students of the Autonomous Metropolitan University at Xochimilco, conducted sampling and analysis of the treated water, both in the WWTP and in the discharge sites in channels and lakes of the ZP (Jiménez-Castillo, 2017). The results show the quality of the treated water produced by these WWTP is practically the same as that obtained in their respective discharge. Therefore, when

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the concentration of some quality parameter of the treated water, generated by the WWTP, exceeds the maximum limit established by the standards, the concentration of this parameter in its discharge also exceeds the maximum limit; as in the case of orthophosphates, their concentrations in the three WWTP examined were detected well above (an order of magnitude) of the maximum limit. On the other hand, the San Lorenzo WWTP observed concentrations of total phosphorus, slightly higher than the maximum limit established by the regulations for the protection of aquatic life and for public use.

Water Discharges without Treatment In general, the treated water produced by the WWTP that channel their discharges to fill the channels and lakes of the ZP, meet the official standards established for such uses. Unfortunately, the inhabitants of the irregular settlements that are located at neighborhoods adjacent to the chinampa zone (Figure A11), have resulted in bad urban practices (v.gr., discharges of black and gray water directly to the channels of the chinampas), contaminating the treated water that fills the hydraulic system of the ZP channels. The causes of the formation of irregular settlements are multiple and have been generated over many years, v.gr., disorderly growth of the CDMX, low capacity of urban planning, permissive attitude of the authorities and society towards the culture of illegality (i.e., lack of enforcement of rules and sanctions), insufficient supply of land for housing that is affordable for lower income sectors, ignorance about the implications of irregularity (SEDESOL, 2010). The main consequence of the proliferation of irregular settlements is the establishment of homes in poverty, with a lack of urban and basic infrastructure, their homes are unhealthy for their inhabitants. In the particular case of the ZP, most of the inhabitants of the irregular settlements have not been accustomed to living between channels, consequently they are shut down to build houses and streets or expand chinampas.

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Figure A.11 Location of irregular human settlements (red dots) in the ZP.

In order to identify the number of untreated wastewater discharges that are discharged to the Xochimilco channels (i.e., black and gray wastewater without treatment), the staff of the Engineering Institute of the National Autonomous University of Mexico (UNAM) navigated most part of the Xochimilco channels, during 2013 and 2014, as part of an integral project for the diagnosis and proposals for channel improvement (Flores-Serrano et al., 2016), sponsored by the Secretariat of Science, Technology and Innovation (SECETI) of the Government of Mexico City. This project generated the register of discharges of black and gray water in the channels of Xochimilco (Iturbe-ArgĂźelles et al., 2015). The expeditions were made by canoe and on foot; the latter was made because there are already many stretches of canal that have been invaded by streets or expansions of chinampa, or are impassable because they are silty or because of the presence of aquatic vegetation. The register covered a total of 38 neighborhoods (including the known as barrios) in an area of 26.5 km2, as well as a total of 247 channels along 116 km. There were 1,374 discharges of raw wastewater (Figure A12); 603 of sewage and 771 of gray water.

The results of the register showed the direct association of the location of the discharges of raw wastewater to the canals, with the location of 917 houses or businesses of irregular human settlements. Untreated wastewater discharges occur along the south side of the Xochimilco, San Gregorio and San Luis chinampa zones, from the same side where the canals are filled with treated wastewater from the Cerro de la Estrella, San Lorenzo and San Luis WWTP (Figure A12). Consequently, poor quality water is conceived from the channels of the chinampas.

In addition, raw wastewater discharges are not confined to the south side of the

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chinampero zone, but move to the north, mixed with the treated wastewater, transported by the flow channels, until it leaves the system through the Chalco channel. Therefore, it is expected that the water circulating through the channels of the chinampas of the ZP is contaminated and that it exceeds the maximum limit established by the norms for the protection of aquatic life and for recreational use.

Figure A.12 Location of raw wastewater discharges (black dots) in the channels of the Xochimilco chinampas of the ZP.

Specifically, the neighborhoods with the largest number of discharges of raw wastewater (i.e., high discharge density) corresponded to Col. Caltongo, Barrio Tlacoapa, Barrio San Lorenzo and Barrio La Asunción. These four neighborhoods as a whole represent 60% of the discharges of sewage without treatment to the Xochimilco channels (Iturbe-Argüelles et al., 2015). The channels associated with these colonies with high density of discharges, also corresponded to channels with the highest number of discharge, v.gr., Canal del Seminario, Canal La Santisima, Canal Telpampa, Canal de Xicanualpan and Canal Caltongo. The main source of the registered downloads was from households, but there were contributions of commercial properties with different business, among which the greenhouses and the sites dedicated to the raising of pigs stand out; discharges into canals are predominantly done through tubes, followed by infiltration (Iturbe-Argüelles et al., 2015).

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Water Quality Conditions The channels of the chinampas are being contaminated by the clandestine discharges of untreated wastewater, a situation that has not only impacted the productive capacity of the chinampas and the touristic value of the region, but also the resilience capacity of the system (Narchi, 2013). In order to describe and analyze the environmental contributions of bioremediation carried out by the chinampa as an agricultural unit, Mendoza-Correa (2018) evaluated the water quality of the Xochimilco channels. To this end, it carried out a bibliographic review of documents about water quality data within channels and apantles of the chinampas (i.e., scientific articles, theses, publications and government projects, as well as associations working on the site). Additionally, it used the CONAGUA databases: “Water quality data 5000 monitoring sites” (CONAGUA, 2017) and “Surface water quality 2012-2015” (CONAGUA, 2015). The information showed here regarding conditions of water quality in the channels of the chinampas represents the work of Mendoza-Correa (2018) in this topic; such information was updated and concluded in our particular way.

All tabular and/or digital information acquired was processed for quality control and thus generate a reliable database that corresponded to measurements made within the channels of the chinampas. Only information from recognized institutions was selected, as well as results previously validated by academic committees, editorial bodies, ad-hoc committees and/or recognized authorities, also it was included official information from government institutions. Check-by-contradiction techniques were applied and verified by possible errors of variation of the sources (checked in each database), numerical by computer (corrected and verified by printing the map). Finally, they were corrected to build the database by records of water quality parameters, integrated with 357 water quality records from 210 sites, taken from 16 different sources.

The database was processed to group records by sectors of chinampas that include canals and bodies of water: Canal de Remo and Canotaje, Ecological Park of Xochimilco (PEX), Laguna del Toro, Xochimilco, San Gregorio, San Luis and Laguna of San Gregorio. In each of the sectors, a weighted average of the values of water quality parameters of Dissolved Oxygen (DO), Turbidity, Conductivity, Nitrates, Ammonium and Phosphates was carried out (Table A1). Once the physicochemical variables were averaged, all of them were used together to generate a composite value, i.e., a water quality index (Table A1), as indicated in Zambrano et al. (2009).

The range of this index allowed to describe the state of the physicochemical conditions of the water quality of the channels in terms of (1) adequate conditions, (2) inappropriate conditions and (3) bad conditions. Likewise, to know the degree and the possible origins of bacteriological contamination in the channels, the index of Toranzos et al. (2007) was applied; where a value greater than 4 is characteristic of human fecal contamination, between 2 and 4 indicates that it is predominantly of human origin, between 0.7 and 2 is predominantly of animal origin and less than 0.7 is indicative of animal waste.

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Table A.1 Water quality in the sectors of chinampas.

From: Mendoza-Correa (2018).

The results show that sites with the best water quality are the Rowing and Canoeing Track, the Xochimilco Ecological Park and the San Gregorio Lagoon (Table A.1). The physical barriers that isolate these bodies of water from the rest of the system have been beneficial for the conservation of water quality, since they only receive water from the discharge of the Cerro de la Estrella WWTP (Figure A10) and are not exposed to the circulation of water in the channels coming from the south coast that cross the points of discharges of residual water without treatment of the irregular human settlements. The areas exposed to raw sewage discharges and circulation in the canals present poor water quality, such as the channels of the Xochimilco, San Gregorio and San Luis chinampas.

Although the channels in general are adequately oxygenated, this is not the case for the rest of the physicochemical parameters of water quality. On average, the concentrations of nutrients registered in the channels and body of water of the ZP, are extremely high in comparison with other bodies of water in the Basin of Mexico, which report for maximum orthophosphate of 4.63 mg/L and minimum of 0.005 mg/L (Figueroa-Torres et al., 2017); the values registered for orthophosphates exceed the maximum limit established by international standards of 1.5 mg/L (RamĂ­rez-Carrillo et al., 2009), the lowest value of phosphorus is higher than any classification value for a hypereutrophic system (Zambrano et al., 2009). The high values of nutrients are due, on the one hand, to the agricultural activities that take place in the area, and on the other hand, to the quality of the treated wastewater coming from the treatment plants that fill the channels and bodies of water of the ZP. Turbidity varies from highly turbid places in the channels and bodies of water with intense navigation activity, such as the Rowing Track and the PEX, to sites of good transparency (Laguna del Toro and San Gregorio, channels of Xochimilco, San Gregorio and San Luis).

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Regarding the bacteriological results of the water samples from the channels of the chinampas, they revealed a critical contamination in 90% of the samples, i.e., practically all the channels of the chinampas of the ZP are contaminated with pathogenic bacteria. Higher bacterial counts than the standards of the World Health Organization and Mexico, v.gr., in the waters where the Cerro de la Estrella WWTP is poured, presence of total and fecal coliform bacteria, with concentrations of 14,000 nmp/100ml, superior to that established by NOM-001-ECOL-1996 of 1,000 and 2,000 nmp/ 00ml (Figueroa-Torres et al., 2017). The high counts of fecal coliforms were attributable mainly to animals, in a second place to human origin.

The fact that human fecal forms can account for 50% of total counts suggests a substantial influence of domestic wastewater on water quality. In particular, the Microcystis aeruginosa species was found in Lake Tlรกhuac Forest, with densities above 100,000 cells/mL (Figueroa-Torres et al., 2017). Their presence and abundance are a potential risk factor for human health and for wildlife, so the body of water is not considered suitable for recreational activities, since the relative probability of acute health effects is high.

B DATA ACQUISITION The construction of hydraulic and hydrological models requires a considerable amount of statistical and geographic information, for example information on land surface elevation, precipitation, temperature, evaporation rate, vegetation cover, runoff coefficients, bathymetry, population density, geology and lithology.

A description of the collected information for the development of the surface and groundwater models is presented in the following sections. All collected data and information is in digital format (shapefiles) and georeferenced to the same coordinate system (longitude, latitude) in the ellipsoidal projection WGS84, for the 14Q zone. The GIS data source of the project acts as an instrument for consulting, storing and managing information.

B.1 Digital Elevation Model The Digital Elevation Model (DEM) for the project area is based on the following topographic maps at scale of 1:50 000: F14D82, F14D81, E14B13, E14B12, E14B11, E14A19, E14B23, E14B22, E14B21, E14A29, E14A28, E14B33, E14B32, E14B31, E14A39, E14A38, E14B42, E14B41, E14A49 and E14A48. The datum uses the geodetic reference system ITRF92 of 1988.0 and ellipsoid GRS80.

B.2 Laser Imaging Detection and Ranging (LIDAR) For the Zona Patrimonial, there is a more detailed DEM available, which was created using LIDAR technology. This exists of a DEM from 2011 (INEGI, 5X5 m resolution), and was obtained from the LIDAR quadrants with keys E14A39F1, E14A39F3, E14A39F4, E14A49C1, E14A49C2, E14B31D3, E14B41A1 and E14B41A2, with datum ITRF92 an UTM projection.

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Additionally, for part of the Zona Patrimonial a LIDAR DEM was created in 2014 by the PAOT (Environmental Procurator’s Office and the Territorial Organization of Mexico City), which covers the Protected Natural Area of Ejidos de Xochimilco and San Gregorio, i.e. the area around the San Gregorio Lagoon. This DEM has a resolution of 0.5x0.5m. See Figure B.1.

Figure B.1 LIDAR DEM for part of the Zona Patrimonial (PAOT, 2014), resolution 0.5x0.5m

B.3 Watershed The delineation of the watershed of the Basin of Mexico and its sub-basins and micro-basins was obtained from the DEM using the techniques of flow direction matrix of runoff, identifying the accumulation of the flow and delineating the partitions of each basin associated with the channels (Band, 1986). It is important to indicate that for the delineation of the watershed, the trace of the micro-watersheds presented by FIRCO (Shared Risk Trust) was also used as a reference.

Likewise, to draw the watershed of the flat areas of the valleys, where the algorithm of demarcation of the watershed is not adequate, the drainage network of Mexico City was used as a guide for the manual digitization of the watershed. In Figure B.2 the watershed, sub-basins and micro-basins are shown.

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Figure B.2 Map indicating the watershed of the Basin of Mexico, its sub-basins and micro-basins, and the area of the Zona Patrimonial

B.4 Drainage network The information of the primary and secondary networks of the combined drainage of the 16 Alcaldias of Mexico City, is provided by the Water System of the City of Mexico (SACMEX) in digital form at a scale of 1:10,000, with an update from 2001 in AutoCAD format. The information was compiled from previous studies carried out by the UAM for the SEDUVI (Secretariat of Urban Development and Housing) and processed to transform into the corresponding GIS format.

B.5 Water treatment network The information of the primary and secondary treated wastewater networks of the 16 city Alcaldias of Mexico City is located in the SACMEX in digital form at a scale of 1: 10,000, with an update from 2001 in AutoCAD format. The information was compiled from previous studies carried out by the UAM for the SEDUVI and processed to transform into the corresponding GIS format.

B.6 Meteorological data A dataset of 30 years of rainfall, temperature and average evapotranspiration data was used, based on 136 weather stations operated by the National Meteorological Service (SMN) in the Basin of Mexico. The information was extracted from the climatological database of the 1987-2016 period, prepared by the SMN. With this information average annual rainfall, rainfall intensity, temperature and average evaporation were calculated. See Appendix C.4 for an elaboration of how the rainfall data was applied in the surface water modelling.

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Recently also records from 10 automatic weather stations within the Basin of Mexico have become available, providing climatological information every 10 minutes from 2017 up until today.

B.7 Hydrography The rivers and surface water bodies in the Basin of Mexico were obtained from the INEGI Hydrographic Network at scale 1:50,000, edition 2.0, in geographic coordinates with datum ITRF92 epoch 1988.0. The information is contained in the folders RH26Dn, RH26Do, RH26Dp, RH26Dt and RH26Du. B.8 Chinampa channel network The system of georeferenced channels of the chinampas of the ZP was derived from information obtained from digitized cartography and orthophotos (1985, 1994, 2002 and 2014) provided by the Alcaldia of Xochimilco and other sources.

These orthophotos were digitized and geo-referenced and a mosaic was created (i.e., aerial photogrammetric surveys). All this information was verified in the field with more than 150 visits in order to clarify several aspects of the aerial photographs.

Additionally, it was used the bathymetric survey of the ZP channels provided by the Secretariat of Science, Technology and Innovation, and carried out by The Institute of Engineering of the UNAM in 2016. Such survey provided an actualized channel shapes and bathymetric data.

B.9 Bathymetry of channel network In 2016, the Secretariat of Science, Technology and Innovation (SECETI) conducted a bathymetric survey of the Zona Patrimonial using sonar scan. In this survey, bathymetric cross-sections were obtained from point cloud depth information.

We recently acquired this data. A great deal of data processing was required to debug the bathymetric curves, especially in narrow and close channels where interpolating algorithms usually overpass channel boundaries (i.e., chinampas).

This bathymetric survey covered the channels from the network of Laguna del Toro, Xochimilco, San Gregorio and San Luis. An additional survey was carried out to get depth information of the channels in the Tlahuac area. Channel cross sections were derived at locations where bathymetric curves are well defined and the depth points across the section were selected at interception with the bathymetric curves to avoid further interpolation (Figure B.3).

The bathymetric survey of Tlahuac channels was performed with GPS map sounders. In this case, depth data were sampled only at channel cross sections of the Tlahuac network (Figure B.3). Most of the Tlahuac channels are dry and it was only possible to measure their depth where it was deeper than 50 cm. During the bathymetric survey clandestine discharge of sewage into the canals was observed.

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Figure B.3 Location of cross sections with depth measurement of the ZP channels

B.10 Water quality of channel network Several data sources were consulted in order to assess the water quality of the channel network, as stated in Mendoza-Correa (2018), which includes a bibliographic review of documents about water quality data within channels and apantles of the chinampas (i.e., scientific articles, theses, publications and government projects, as well as associations working on the site). Also two CONAGUA databases related to water quality were consulted: “Water quality data 5000 monitoring sites” (CONAGUA, 2017) and “Surface water quality 2012-2015” (CONAGUA, 2015). All tabular and/or digital information acquired was processed for quality control in order to generate a reliable database that corresponds to measurements made within the channels of the chinampas. Only information from recognized institutions was considered, as well as results previously validated by academic committees, editorial bodies, ad-hoc committees and/or recognized authorities.

Conditions of the water quality in the channels of the chinampas were verified through physical-chemical and biological analyses of water samples from the channels, obtained during measurement campaigns. For this purpose, professors and undergraduate and master students of the Autonomous Metropolitan University at Xochimilco conducted sampling and analysis together with CORENA and with the Civil Associations, through the work plan entitled “Monitoring and evaluation of water quality and aquatic fauna in the Protected Natural Area Ejidos de Xochimilco and San Gregorio”. For this study, integrated samples of water were collected on September 24, 25 and 27, 2018, in eight sampling stations of the Protected Natural Area Ejidos de Xochimilco and San Gregorio (Figure B.4).

The physical parameters pH, temperature, dissolved oxygen, salinity, total dissolved solids and conductivity were measured in situ with a portable YSI ® model 585 multiparameter. In the case of nutrients (nitrites, nitrates, ammonium,

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phosphates) samples of 500 mL were collected in glass or polyethylene bottles and were analyzed in the UAM-Xochimilco laboratory using a HANNAŽ spectrophotometer model HI83200, following the manufacturer’s instructions for each method. Determination of total and fecal coliforms followed the methodology established by NMX-AA-42-1987, whereby the most probable number (mpn) of total coliforms, fecal coliforms (thermotolerant) and presumptive Escherichia coli are determined. With the help of an Olympus CH30 optical microscope, the composition, distribution and abundance of the species were evaluated, with emphasis on toxic or noxious species. The taxonomic identification of the species according to different morphological characteristics was done through the use of keys and taxonomic descriptions of specialized literature (Bourrelly, 1968 and 1970, Figueroa and Moreno 2003, Figueroa et al., 2008, and Figueroa et al., 2015).

Figure B.4 Location of sampling sites for the water quality study in the channels of the Zona Patrimonial

B.11 Runoff coefficient Runoff coefficients Ce represent the proportion of surface water in the basin as a percentage of rainfall; they are grouped in ranges: [0% to 5%], [5% to 10%], [10% to 20%], [20% to 30%] and [30% to 100%]. Values of the spatial distribution of runoff coefficients over the Basin of Mexico were obtained from the vector data set of the hydrological charts of superficial waters 1:250000, available in the INEGEI.

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B.12 Hydrometry Information on hydrometry is available in the digital hydrometric database “BANDAS” (National Surface Water Bank). The database is developed by CONAGUA through the Mexican Institute of Water and Technology (IMTA). It contains daily average flows and water levels for varying periods ranging from 1921 to 2006, for rivers and water bodies of the Basin of Mexico. The stations for which discharge and water level data was received are shown in Figure B.5.

Figure B.5 Map showing the measurement stations with data (blue triangles), indicating station names and rivers (Source: Deltares, 2017)

B.13 Aquifers Information on the administrative limits of the seven aquifers within the Basin of Mexico are defined in the Official Gazette of the Federation of the Ministry of the Interior. These aquifers are: Soltepec (2902), State of Tlaxcala; Cuautitlán-Pachuca (1508), State of Mexico; Texcoco (1507), State of Mexico; Chalco-Amecameca (1506), State of Mexico; Ápan (1320), State of Hidalgo; Tecocomulco (1319), State of Hidalgo and Metropolitan Area of the City of Mexico (0901), Mexico City.

B.14 Geology The geological information of the distribution of the lithological units and structural elements of the Basin of Mexico is available from the SMN. The cartography has a scale of 1:250,000. Additionally, it was possible to track 13 stratigraphic

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cross sections within the basin (from A to L), especially around the vicinity of the ZP (Figure B.6). Most cross sections were obtained from published literature, but others directly from lithology profiles available at well perforations. > Check: is this in line with the piece written by Tommer?

Figure B.6 Location of stratigraphic cross sections B.15 Groundwater abstractions Water Rights Public Registry or REPDA (known in Spanish as Registro Publico de Derechos del Agua) contains water concession permits issued upon request by CONAGUA. The REPDA records compile the location, ID, annual volume of extraction, hydrological region, basin, sub-basin, state, municipality and water use of these water abstractions. Water uses can be classified into 12 categories: urban, agricultural, agro-industrial, domestic, aquaculture, services, industrial, livestock, electric power generation, trade, a combination, or other uses. According to the latest census, conducted by CONAGUA, the Basin of Mexico contains 2,278 registered wells.

Furthermore, groundwater extractions per hydrological region per year were obtained from the ‘Cubo Portátil de Información de los Usos de Agua’ of CONAGUA. The database provides total annual water extraction per municipality (m3/yr) for 2005 to 2008.

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C HYDROLOGICAL MODEL FOR THE BASIN OF MEXICO A hydrological model was constructed to estimate total surface runoff for the Basin of Mexico and for the Zona Patrimonial. Required data for the development of the model is rainfall and potential evapotranspiration data, as well as information on land use and soil characteristics, and if available the hydrological drainage network and catchment delineation.

C.1 Hydrological concept: Wflow The open-source distributed hydrological modelling framework Wflow, developed by Deltares, was used to simulate rainfall-runoff processes at the catchment scale. This distributed model can be constructed with available global datasets or with local information. A full description can be found at http://wflow.readthedocs.org.

Wflow solves the governing equations for the surface and subsurface flow routines in time for each grid cell, providing continuous simulated values for the hydrological state variables (runoff volumes, saturation etc.) for each grid cell.

Channel flow is simulated using the kinematic wave model. Wflow contains a library of several different hydrological process conceptualizations, that all assume different ways to describe the relationships between rainfall, storage compartments in the soil and open water, and runoff. For the Mexico Basin the SBM model concept was applied. This concept explicitly accounts for differences in open water, saturated water and soils, all producing different slower and faster runoff components. It has a physics based accounting of groundwater infiltration and uses subgrid variability in terrain to estimate runoff, using a time-variable saturation fraction for each grid cell. The SBM model concept includes only the first aquifer (phreatic layer). See Figure C.1.

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The wflow-sbm model includes the following processes: • Snow routine • Interception routine (based on a modified version of the Gash model) • Soil routine (based on topog_sbm, Vertessy 1999) • Kinematic wave routing to simulate transport and attenuation of water through the channel network

Figure C.1 Schematization of the wflow-sbm model concept

Snow Based on temperature, precipitation falls either as rain or as snow, or a mix of both. The rates of snowmelt and refreezing are calculated based on pre-defined temperature thresholds.

Interception The interception routine is based on the analytical approach by Gash (1979) and uses canopy characteristics to determine interception evaporation. The inter-annual variability of vegetation (storage capacity of the canopy) is taken into account with monthly distributed value of the Leaf Area Index (LAI) derived from

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MODIS imagery. Part of the hourly rainfall that enters the model evaporates when it is intercepted by the canopy.

Soil The soil is considered as a single reservoir with a certain depth, divided into a saturated store and an unsaturated store. Water infiltrates in the soil according to a fraction of paved and unpaved area. Water that cannot infiltrate is added to the kinematic wave reservoir for routing. Soil evaporation, transpiration through vegetation, and open water evaporation occur from the soil. Transfer from the unsaturated store to the saturated store is controlled by the saturated hydraulic conductivity at water level depth and the ratio between the unsaturated store and the saturation deficit. Vegetation may extract water from the saturated store which results in a flux from the saturated store to the unsaturated store. Recharge is defined as the net flow of water from the unsaturated to the saturated store (transfer – capillary flux – saturated-store evapotranspiration). Lateral flow from the saturated store from one grid cell to another is based on the difference of water level of both pixels. When the water level of the saturated store is at the surface, water may exfiltrate and will be added to the kinematic wave reservoir.

Kinematic wave The resulting water in the kinematic wave reservoir is routed downstream in the rivers using the kinematic wave equation, based on the simplified momentum equation combined with the continuity equation. For this purpose, a Strahler order is determined for every grid cell. For river cells, the river width is estimated following the approach described by Finnegan et al. (2005).

The data necessary for the hydrological modelling can be subdivided into two types of data: ‘static’ data and ‘dynamic’ data. Static data are used to define the structure of the hydrological model. Static input data comprise the following data sources: • digital elevation model • land cover characteristics & parameters • soil characteristics & parameters • river network (derived from the DEM if not available) • sub-catchments (derived from DEM + river network if not available) The model is forced with meteorological data for each model time step (‘dynamic input’): • Rainfall • Potential evapotranspiration These model inputs are described in more detail in the following sections.

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C.2 Model structure The regional Wflow-SBM model for the Mexico Basin was set up in geographic coordinate system WGS1984 with a spatial resolution of 0.001ยบ, which corresponds to a pixel size of approximately 100 x 100 m, and hydrological simulations were carried out with a daily model time step.

The Digital Elevation Model (DEM) used for the hydrological model is the local DEM as described in Section B.1. The DEM was resampled from the original resolution of ca. 15 x 15 m to the model resolution of ca. 100 x 100 m.

The river shapefile obtained from INEGI was burned into the DEM, before the local drainage direction network (LDD) was derived. The LDD gives a flow direction for each cell (1-9), indicating the topographic flow direction of each cell, based on the topography. By burning the river network into the DEM, the drainage direction was forced to be towards the rivers and streams, ensuring a drainage pattern to be in agreement with the river network layer, also after resampling of the DEM to a larger cell size (100 m instead of the original 15 m).

Based on the derived local drainage network, the sub-catchments for the Valley of Mexico were created. Figure C.2 shows the sub-catchments for the whole Valley of Mexico; in total 120 sub-catchments were identified. The creation of the sub-catchments was done by determining the locations where the natural drainage system of rivers and streams on the hills changes into the artificial urban drainage system of the urban zone of the Valley of Mexico. Wflow is a hydrological model in which water is routed downstream based on the kinematic wave; it is therefore not suitable to use for flow calculations in an artificially controlled drainage system with channels, bridges, control structures, etc. The urban drainage system can be modelled with a hydrodynamic model, such as SOBEK, as was done for the Zona Patrimonial (see Chapter 4). This method results in the sub-catchment map as shown in Figure C.2, where each sub-catchment comprises the area upstream of a point where its main stream changes from a more or less natural stream into an artificial channel. These sub-catchment outflow locations are approximations and have been determined by examining the data and discussing results with experts from UAM.

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Figure C.2 Map showing the sub-catchments and rivers (blue lines) derived with Wflow, and how they link to the urban drainage system (red and orange lines) of Mexico City

C.3 Soil, land cover and model parameters Model parameters in Wflow can be spatially distributed, or linked to land-cover, soil or sub-catchment characteristics. Local data on soil parameters has not been available, and therefore parameter values were based on a global soil parameter dataset developed by Dai et al. (2013). These global parameter maps have varying spatial resolutions. Global soil thickness is available at ca. 1 x 1 km, other soil parameters are available at ca. 5 x 5 km, and other parameters are linked to land cover and thus depend on the resolution of the land cover map used. For this project a simplified land cover classification map was used derived from INEGI data, in which the original 23 classes were converted to 9 more general land cover classes (see Figure C.3). For examples of maps of the spatially distributed soil parameters, see Figure C.4 (soil thickness) and Figure C.5 (saturated hydraulic conductivity).

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Figure C.3 Land cover map for the Mexico City Basin as used in the hydrological modelling, simplified to 9 classes from the original 23 classes by INEGI

Figure C.4 Soil thickness in mm as used for the hydrological modelling, original spatial resolution 1 x 1 km (source: Dai et al., 2013)

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Figure C.5 Saturated hydraulic conductivity in mm/day, as used in the hydrological model, original spatial resolution 5 x 5 km (Source: Dai et al., 2013)

The mentioned global parameter set (Dai et al. 2013) represents a best estimate at the global level; however, the quality and applicability for local models are not guaranteed. Some adjustments have been made in an effort to best resemble the local conditions. Discharge data to calibrate the model was very sparsely available (see Section 4.2.1), and an actual calibration of the hydrological model based on these data was not possible. With the available measurements some basic model performance checks were done. As a result from these actions some parameters were slightly adjusted with respect to the initial global parameter set

by Dai et al. (2013): • The M parameter was decreased, so that the hydraulic conductivity decreases more steeply with soil depth (see Figure C.6); this causes a more rapid decline of the baseflow recession during periods without rainfall; • The infiltration capacity of the soil was increased to decrease the infiltration excess overland flow; • Rooting depth was adjusted according to regional land cover classes based on the mentioned land cover map by INEGI (Figure C.3), in order to better resemble the regional vegetation evaporation pattern; • Crop factors to convert reference potential evapotranspiration to land-cover-specific evapotranspiration were assessed per land cover class and adjusted in agreement with local land cover types.

All final model parameters used in the hydrological model are described in Table C.1. These parameters represent the best estimate with the data available, but have a high uncertainty, due to a lack of local parameter information and a lack of model validation data.

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Figure C.6 Graph showing how in the Wflow model the M parameter governs the decrease of the saturated hydraulic conductivity (K) with soil depth

Table C.1 Model parameters related to soil, used in the regional Wflow model

Parameter

Description

Values

Soil parameters SoilThickness

Maximum depth of the soil [mm]

Based on Dai et al. 2013 Distributed; 1460-2880 mm

SoilMinThickness

Minimum depth of the soil [mm]

Based on Dai et al. 2013 Distributed; 980-1920 mm

InfiltCapSoil

Soil infiltration capacity [mm/d]

Based on Dai et al. 2013 Distributed; 15-95 mm/d

InfiltCapPath

Infiltration capacity of the compacted areas [mm/d]

Based on Dai et al. 2013 Uniform; 10 mm/d

CapScale

Scaling factor for capillary rise calculations [mm/d]

Uniform; 100 mm/d

KsatVer

Saturated hydraulic conductivity [mm/d]

Based on Dai et al. 2013 Distributed; 100-750 mm/d

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197

Parameter

Description

Values

M

Soil parameter determining the decrease of saturated conductivity with depth [-]

Based on Dai et al. 2013 Distributed; 130-250

rootdistpar

Parameter determining how roots are linked to water table [-]

Uniform; 500

thetaR

Residual water content of the soil [mm/mm]

Distributed; 0.02-0.09

thetaS

Porosity of the soil [mm/ mm]

Based on Dai et al. 2013 Distributed; 0.35-0.55

RootingDepth

Rooting depth of the vegetation [mm]

Distributed; 10-2800 mm

WaterFrac

Fraction of open water in non-river cells [-]

Distributed; 0-1

PathFrac

Fraction of compacted area per grid cell [-]

Distributed; 0-0.8

LC to SpecificLeafStorage

Specific leaf storage [mm]

Distributed; 0-0.13

LC to BranchTrunkStorage

Branch trunk storage [mm]

Distributed; 0-0.5

LC to ExtinctionCoefficient

Coefficient to calculate the canopy gap fraction based on LAI [-]

et_reftopot

Crop factor converting reference ET to specific potential ET [-]

Canopy parameters

198

Distributed; 0.6-0.8

Distributed; 0.75-0.88

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Parameter

Description

Values

Surface water parameters

N

Roughness coefficient: Manning’s N (non-river cells)

Uniform; 0.1

N_River Roughness

coefficient: Manning’s N (river cells)

Uniform; 0.045

C.4 Rainfall As one of two dynamic model inputs, rainfall is used to force the model. The decision to run the hydrological model on a daily time step is partly based on the temporal resolution of the available rainfall data. The sub-catchments surrounding Mexico City are mostly relatively small (most are smaller than 200 km2, and also many are smaller than 150 km2), which means that the response time of the catchments could be less than a day. Best model results would thus be achieved by running the model on a sub-daily time step (e.g. hourly), but sufficiently long rainfall measurements are only available on a daily time step for the 136 meteorological stations operated by SMN, hence the hydrological model was run on a daily time step. The rainfall dataset covers the years from 1979 to 2016.

For the selection of meteorological stations two situations were considered. First, only stations with continuous measurements during the period of analysis were selected. Second, to reduce uncertainty in the assessment of the meteorological variables, the use of stations with gaps smaller than five years of measurements were also included.

Validation of the rainfall measurements was performed as follows. Unreliable measurements in the time series were removed by simple statistical analysis. Two or more consecutive stations with similar characteristics (e.g. elevation, latitude and longitude) were compared. By doing this, extreme events were corroborated.

Additionally, total precipitation per year was calculated, and this was compared to statistical reports of the stations published by CONAGUA. In Figure C.7 a validation example is given for station Españita – in this case, an outlier was removed, that was too high when considering the rainfall pattern of station Españita itself, and also too high when considering the surrounding stations for that date/ period.

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199

Figure C.7 Validation example showing original (blue, underlying) and corrected (red, on top) rainfall time series for station Españita. In this example, an outlier in May 1979 was removed.

For use as input into the hydrological model, the rainfall recordings were interpolated to maps, using inverse distance weighted interpolation. An example of a rainfall map is shown in Figure C.8.

C.5 Potential evapotranspiration For potential evapotranspiration, a dataset was used which has been developed in the EU project EartH2Observe (www.earth2observe.eu), based on the WFDEI dataset. Penman-Monteith reference potential evapotranspiration with a daily time step was used. The dataset was downscaled from the original spatial resolution of 0.25° (ca. 27 km) to 0.083° (ca. 10 km) and was then resampled from 0.083° to the hydrological model resolution (100 m).

An example of a potential evapotranspiration map is shown in Figure C.8.

Figure C.8 Example of a daily rainfall map from interpolated station measurements (left) and a potential evaporation map derived from EartH2Observe (right)

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C.6 Model validation data For 15 gauging stations in the Mexico City Basin, measured discharge time series were available for this project, giving mean daily discharge in m3/s (as indicated on the map in Figure B.5). The available discharge measurements have been used for a broad performance check of the hydrological model. However, the quality of these discharge measurements is uncertain. The stations that are clearly under strong influence of regulating structures have not been used for this model check (these are the stations Desfogue Presa Guadalupe, Huehuetoca, Presa Iturbide). For the remaining station measurements a comparison is made with the modelled discharges at those locations. However, the usefulness of some of the remaining stations remains uncertain due to causes like data gaps, uncertain locations, possibly more regulation, apparent data interpolations like in El Salitre, etc. In El Salitre (see Figure C.9, Figure C.10, Figure C.11) the rainfall peak of the near-by upstream rainfall stations in mid-August 1988 is not visible in the discharge measurements; instead the measurements seem to have been interpolated over that period. This is a relatively short period and serves as an example to show that the discharge measurements might not be reliable everywhere.

Figure C.9 Location of discharge station El Salitre and its two closest rainfall stations, San Luis Ayucan and El Salitre rainfall station.

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201

Figure C.10 Measured and simulated discharge time series for El Salitre gauging station (top) and measured rainfall time series for its two closest rainfall stations, San Luis Ayucan and El Salitre (bottom)

Figure C.11 Discharge measurements for El Salitre around August 1988, close-up

Another example of data quality issues can be seen for station Molino Blanco.

Some stations do show patterns that indicate some form of regulation, even though they are not clearly located downstream of a regulating structure. See for example station Molino Blanco in Figure C.12. Where the simulated time series

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for this location (red line) shows a natural pattern of baseflow recession, the measurements (blue line) show a pattern with sudden increases and decreases in discharge, which could indicate some form of regulation.

Figure C.12 Measured (blue) and simulated (red) time series for gauging station Molino Blanco.

Another example can be seen for multiple stations. There is a very large rainfall peak at 3-4 March 1988, which is not seen in any of the discharge measurements, but is seen in many rainfall measurements. This could mean two things. The catchments are small and therefore have a very short response time, and the discharge peak had already passed before the next gauge reading (assuming that the reading happens more or less once per day). The other possibility is that the discharge measurements are not correct, for example, there have been no readings during this heavy rainfall event. The latter is likely, since there is no signal at all in the discharge measurements, if there was a reading there should at least be a little signal.

It is also important to note that there are no discharge measurements available in the catchments upstream of the Zona Patrimonial, which is the area of primary interest of the project. The measurements of the other stations in the Mexico City Basin have been used for model validation, assuming that there is some continuity/similarity over the basin, but for this project it would have been better to have measurements upstream of the Zona Patrimonial area itself.

 

APPENDICES

203

D GROUNDWATER MODEL FOR THE BASIN OF MEXICO D.1 Hydrogeological conceptual model Geology The geological formation of the Basin of Mexico can be summarized in four main events: (1) deposits of Cretaceous limestones; (2] deposition of volcanic and alluvial sediments; (3] deposits of lava flows and pyroclastics during the Tertiary age; and (4] development of the lake and closure of the basin (González-Morán, Rodríguez, & Cortes, 1999). The watershed can be divided into three major hydrologic zones: (1) the lacustrine valley floor; (2) the transition zone and (3) the hillslope area. According to Gonzalez-Moran et al. (1999), the lacustrine valley floor has elevations ranging from 2,230 to 2,250 m a.m.s.l. This zone has a surface of 1,431 km2 and represents about 23% of the basin floor. The transition zone is an area with high permeability located between the lake and the hillslope region.

Moreover, the hillslope region corresponds to the primary recharge area.

The basin can be divided into four hydrogeological units: the upper aquitard, upper aquifer or granular aquifer, lower aquitard or volcanic aquifer, and the lower aquifer. The upper aquitard has an average thickness of 60 m, it is composed of lacustrine sediments (clays) with good porosity, and high storage capacity but with low permeability; and volcanic rock, sandy-clay deposits at a depth of 10 m. The aquitard is divided by a thin layer of sand (3m) with high permeability at 33 m.

The upper aquitard or granular aquifer (main unit of exploitation) has an average thickness of 600 m. However, it can reach up to 1,000 m in some areas. It is formed by alluvial deposits, basaltic rocks, and volcanic clays with small layers of sand, alluvial and pyroclastic deposits from the Quaternary. The aquifer is phreatic in the mountain ranges and alluvial plains, whereas it is semi-confined in the central region of the basin. In the towns of Zumpango, Chalco and Texcoco the aquifer is considered as confined.

The volcanic aquifer has an average thickness of 1,500 m. The aquifer is formed by pyroclastic materials, sedimentary clasts, volcanic clays fractured volcanic rocks at a depth of 3,000 m from the Pliocene; andesitic volcanic rocks from the plio-quaternary; basaltic and andesitic rocks of the Upper Pliocene; stratified series, volcanic rock, lacustrine deposits of the lower Pliocene; volcanic rocks from the Tertiary; lacustrine sediments from the Upper Tertiary; acidic igneous rock from the Miocene; intermediate igneous rock of the Oligocene; and conglomerates from the Eocene.

The last unit (lower aquifer) is constituted by volcanic rocks from the mid-Tertiary to upper Tertiary; carbonate rocks from the Cretaceous. It has an average thickness of 500 m.

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Hydrogeological schematization The estimation of the thickness of the hydrogeological units that constitute the hydrogeological model was based on the studies conducted by Herrera et al.

(1989). This study describes the geological formations of the south region of the Basin of Mexico and presents a detailed overview of the hydrogeological units.

Figure D.1 presents an example of the data reported by Herrera et al. (1989), showing the depth of the granular aquifer.

Figure D.1 Depth of the granular aquifer in m. The granular aquifer is considered as the main layer for groundwater extraction (Source: Herrera et al. 1989).

Information on geology and hydrogeology is very scarce in the northern region of the basin, including areas of the state of Tlaxcala, Hidalgo and parts of the State of Mexico. Information recovered from CONAGUA (2002a, 2002b, 2002c, 2002d, 2002e, 2013, 2015), INEGI (2015a, 2015b), Herrera et al. (1989) and, Mosser and Molina (1993) were used as database for the hydrogeological model.

Hydrogeological properties per unit, such as vertical resistance (VCW), horizontal permeability (KHV), vertical permeability (KVV) and specific storage coefficients (SSC), were based on the study by conducted by Herrera et al., (1989) and CONAGUA. For example, Figure D.2 presents the specific storage for the first aquitard underlying Mexico City. This information was digitized and included in the groundwater model.

APPENDICES

205

Figure D.2 Specific storage of the upper aquitard (10-2m-1) retrieved from the report by Herrera et al. 1989)

Table D.1 summarises the hydraulic properties reported by Herrera et al. (1989) for the different hydrological units. No information is available on the lower aquifer.

The same values were therefore used for the volcanic aquifer. This assumption needs to be corroborated and validated during the next phases of the project.

Table D.1 Hydraulic properties of the hydrogeological units of the Basin of Mexico (Source: Herrera et al., 1989)

Vertical (m/s)

Unit

Horizontal Permeability (m/s)

Upper aquitard

5-20 x10-9

Granular aquifer

1.0x10-5 – 15x10-5

1x10-5

Volcanic aquifer

1x10-7

3x10-6

---

---

Lower aquifer

Storage coefficient (/m) 5.73x10-3

---

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Figure D.3 shows a map of the horizontal permeability of the granular aquifer used for the groundwater model. Similar maps were created for the vertical and horizontal permeabilities of all four layers. For the storage coefficient, a constant value for the entire basin was used in each layer.

Figure D.3 Horizontal permeability (m3/d) for the granular aquifer.

Borehole cross-sections An overview of the available boreholes is given in Figure D.4. For some cross-sections the major units have been mapped, see Figures D.5 – D.10. The following layers are indicated: bottom of the upper aquitard (dashed green line), bottom of the upper aquifer (dashed orange line), bottom of the more permeable part of the second aquitard (blue dashed line). Model layers are indicated by solid lines: topography (black), bottom upper aquitard (green), bottom upper aquifer (red), bottom lower aquitard (bright green) and bottom lower aquifer (orange). A legend for the units of the boreholes is presented in Figure D.11.

APPENDICES

207

Figure D.4 Overview map of available boreholes and names of the cross-sections

Figure D.5 Cross-section AA, see Figure D.4 for position and text and Figure D. 11 for legend

Figure D.6 Cross-section DD, see Figure D.4 for position and text and Figure D. 11 for legend

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Figure D.7 Cross-section GG, see Figure D.4 for position and text and Figure D. 11 for legend

Figure D.8 Cross-section JJ, see Figure D.4 for position and text and Figure D. 11 for legend

Figure D.9 Cross-section KK, see Figure D.4 for position and text and Figure D. 11 for legend

APPENDICES

209

Figure D.10 Cross-section MM, see Figure D.4 for position and text and Figure D. 11 for legend

Figure D.11 Legend for units in boreholes of Figure D.5 – D.10

D.2 Groundwater abstractions Groundwater wells and abstraction rates were derived from the publicly available database of the Registro Público de Derechos de Agua (REPDA) (refer to section B15). For the Basin of Mexico, REPDA records report 2,872 wells, of which 620 wells have an industrial use, 1,095 an agricultural use and 1,156 wells a domestic/public use (Figure D.4). The vast majority of the wells provide information regarding

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their location (x,y,z), ID, water use, annual volume of extraction (millions of m3), diameter, required extraction rate (l/s) and maximum allowed extraction rate (l/s). In the case of a well for domestic purposes, additional information is reported concerning the institution responsible of the management (e.g. SACMEX) and the system or batteries of wells (e.g. PAI, Norte, Centro). Wells located in the Apan aquifer and Cuautitlan-Pachuca aquifer provide less information concerning physical characteristics of the wells.

Water extraction rates for industrial purposes range from 18 to 8880 m3/d, agricultural extraction rates range from 9.6 to 8400 m3/d, whereas domestic extractions vary from 1 to 13,824 m3/d.

According to the REPDA database, the wells have an average screen depth of 217 m. In total 55 wells do not report the screen-depth. For these wells, a screen depth of 200 m was assumed.

Figure D.4 Locations of abstraction wells from the REPDA database that were used in the groundwater model. Water wells outside Mexico City are mainly used for agricultural purposes and within the city mainly for domestic and public purposes.

A number of wells have incomplete data regarding abstraction rates or annual volume of extraction. In total 793 wells do not report this information; 3 of these wells have an industrial use, 63 have an agricultural use, and 735 wells have a domestic use. A number of 176 wells with a domestic use only report annual volumes of extraction. Of these wells, 39 are located in Hidalgo, 92 in the State of Mexico, 39 in Tlaxcala and 6 in Mexico City. The annual volume of extraction of these wells was used to estimate the abstraction rates in m3 per day. The calculated value was then compared with the average rates of extraction reported in the REPDA records. This comparison was done depending on the location of the well e.g. State and municipality.

APPENDICES

211

After calculating a potential abstraction rate for the wells with domestic use, in total 91 wells have a calculated abstraction rate of less than 1,000 m3 per day. The remaining 85 wells have a calculated value between 1,000 m3 and 10,000 m3 per day and are located mostly outside Mexico City. According to the REPDA records, the maximum water extraction rate is around 1,000 m3 per day. Since there is not enough information to validate our calculations and taking into account that the area of attention is Mexico City and its Zona Patrimonial, the remaining 85 wells were not included in the simulations. Hence, the groundwater model only includes a total of 2,170 wells (Figure D.4).

Validation There are two other sources of information from which annual abstraction rates for the Basin of Mexico can be derived. CONAGUA reported annual records of abstraction rates per municipality in the Cubo Portátil de Información de los Usos de Agua from 2005 to 2009. UAM developed a database of annual abstraction rates for wells with domestic or public purposes. This database covers 18 years (19902008) and divides water extraction depending on the institution in charge of the wells or operational system (e.g. PAI system). These two sources were compared with the annual abstraction rates as calculated from the daily abstraction rates given by the REPDA records.

The comparison between the CONAGUA and UAM records on the one hand and the REPDA records, on the other hand, showed that the calculated extraction rates from the REPDA records exceed the reported volumes from CONAGUA and UAM if it is assumed that water is abstracted 365 days per year. We, therefore, assumed that shortages and disruptions in water provision are frequent and that many wells are not operated during the weekends. A good comparison between the records was found for a number of 260 operational days.

Figure D.5 shows the evolution of water abstractions during the period 1979 – 2014. For comparison, also calculated groundwater recharge and evapotranspiration are shown. On average, groundwater abstraction in the Basin of Mexico for the period 1979-2014 is estimated at 1,455 106 m3/yr (46 m3/s). During this period, average groundwater abstraction is around twice as high as groundwater recharge.

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Figure D.5 Time series of annual groundwater abstraction (wells) in the Basin of Mexico (1979-2014). For comparison, also groundwater recharge (RCH) and evapotranspiration (EVT) are given. All values in m3/yr.

D.5 Model limitations No accurate information regarding recordings of water levels has become available during the execution of the present study. The model has therefore not been calibrated and the validation with literature information has been on a visual and qualitative basis. The model should therefore be regarded as a schematic first-iteration representation of the groundwater system of the basin of Mexico that can be updated as soon as more information becomes available. It is known that some information will become available in the near future while there is also information that exists already that can be utilized in the next phase of the project.

Some information on groundwater levels seems to exist. A groundwater monitoring system is operated by CONAGUA that may likely provide extremely valuable data. The level of detail of the information reported on the website is however not enough for the calibration and validation of the current groundwater model.

For example, studies conducted by Carrera-Hernรกndez and Gaskin (2007) present records of groundwater levels since 1969 (Figure D.6). During the course of the current project we have not been able to acquire this information.

APPENDICES

213

Figure D.6 Time series of groundwater levels of 40 wells located in the Basin of Mexico, for different locations: a) Pachuca, b) Apรกn, c) ร rea Metropolitana de la CDMX, d) Texcoco, e) Sur de CDMX, f) Tlรกhuac, g) Chalco y h) Ecatepec (Source: Carrera-Hernรกndez y Gaskin, 2007).

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E HYDRAULIC MODEL FOR THE ZONA PATRIMONIAL E.1 Model setup The features of the hydraulic model for the Zona Patrimonial (ZP) were modelled with the network schematisation as presented in Figure E.1 with various network objects as discussed in the next sections. The hydraulic model contains the following main features: • Main channel network • Channel profiles (cross sections) • Upstream discharge boundaries • Downstream water level boundary • Lateral flows in the region The inflows into the hydraulic model are: • Inflows from the sub-catchments of the surrounding hills (upstream flow boundaries) • Inflows from the wastewater treatment plant (outlet points of the Cerro de la Estrella WWTP) (lateral flows) • Direct runoff from rainfall within the Zona Patrimonial itself (lateral flows) The outflows of the model are: • Outflow at the Canal Nacional No hydraulic structures such as weirs and sluices were included.

Figure E.1 Detailed hydraulic model schematisation for the Zona Patrimonial in SOBEK, showing the main channel network (dark blue lines), lateral flow nodes (yellow) and boundary nodes (pink)

APPENDICES

215

Network: channels and cross sections For the 1D network schematisation of the Zona Patrimonial, the main channels of the area were incorporated, see Figure E.1. For these channels a uniform cross-section was assumed; see Figure E.2. The assumed dimensions of the channels are a width of 12 m, a bed level width of 9 m and a depth of 3 m. For a more accurate modelling of the area, it is advised to update this with measured cross sections.

Figure E.2 For the detailed hydraulic model a uniform cross-section was defined

The bed levels of the channels were derived from the LIDAR digital elevation model that is available for the Zona Patrimonial, with a 5 m horizontal resolution. This DEM was manually adjusted in some places to smooth out high outliers (these high-elevation outliers might have been caused by trees or buildings that were not filtered out during LIDAR DEM processing).

Boundary conditions In the model the following types of boundary conditions were specified: • Upstream flow boundaries • Downstream water level boundary • A number of ‘dead channel ends’ with zero-flow boundaries The upstream flow boundaries are the inflows from the surrounding sub-catchments. These were calculated with the regional hydrological Wflow model (see Section 4.2) and are a function of time (daily average in m3/s per day). In Figure E.3 the sub-catchments are shown that drain into the Zona Patrimonial. The orange sub-catchment (Río Amecameca) is currently not draining into the Zona

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Patrimonial, but the geography of the catchment does make it a possibility for the RĂ­o Amecameca to discharge into the ZP. The orange line connects it to the green ZP network.

Figure E.3 The sub-catchments currently or potentially draining into the Zona Patrimonial

Figure E.4 The sub-catchments currently or potentially draining into the Zona Patrimonial – zoomed in

The downstream boundary of the model is a water level boundary placed on the Canal Nacional, somewhat downstream of the output location of the model, see Figure E.5. This water level boundary is placed far enough downstream so that there can be gravitational outflow from the Zona Patrimonial.

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217

Figure E.5 Hydraulic ZP model schematisation with the output location of the model results on Canal Nacional (red dot) and the downstream water level boundary further downstream (pink square, top left)

Lateral inflows Two forms of lateral inflows are schematised: the inflows from the wastewater treatment plant, and the runoff from rainfall that is generated within the Zona Patrimonial itself.

The inflows from the Cerro de la Estrella wastewater treatment plant were assumed to be constant inflows, as given in Table E.1. The locations of these outlets are shown in Figure E.6.

Table E.1 Outlet locations of the Cerro de la Estrella wastewater treatment plant, given in l/s.

ID

Name

1

Canal 27 (Embarcaderos Zacapa)

300

2

Los Galeana

77

3

Fernando Celada

10

4

La Draga

40

218

Discharge (l/s)

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ID

Name

Discharge (l/s)

5

Parque Ecologico

120

6

Canal Caltongo

77

7

Canal Caltongo (Mercado)

5

8

Mexico 70

5

9

Floricultor

300

10

Canal Caltongo (exclusa)

80

11

Other -

77

12

Other --

77

13

Other ---

77 Total:

1245

Figure E.6 Map showing the Zona Patrimonial with the main channel network (green lines), intermittent streams (blue dotted lines) and the outflow points of the Cerro de la Estrella wastewater treatment plant (purple dots).

APPENDICES

219

The rainfall runoff that is generated within the ZP itself was calculated with Wflow model. See Figure E.7. For all the areas in between the main channels, the runoff was calculated and then schematised as lateral inflows into the hydraulic model (time-dependent, average m3/s per day).

Figure E.7 Schematisation of lateral inflows within the ZP itself (channels in green) representing the rainfall runoff.

E.2 Model limitations Navegables and cauces The model describes the main channel network of the ZP. At this stage, there was no need to include the detailed network of numerous smaller channels (as shown in Figure E.8 and Figure E.9).

Figure E.8 Figure showing the detailed system of navegables and cauces in the ZP

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Figure E.9 Figure showing the San Gregorio system of navegables and cauces in the ZP

Bathymetry Information on bathymetry and dimensions of the canal profiles has become available very recently (see Appendix B.9), but is at this stage not yet included in the model. A logical next step in follow-up projects is to replace the current schematic cross-sections with this new information.

Model calibration Information on water levels or discharge recordings is not available in the Zona Patrimonial. The current model was therefore not calibrated or validated against measured data.

F PROBABILISTIC SEISMIC HAZARD ASSESSMENT F.1 Historical Earthquake Catalog The historical earthquake catalogue for this study used data from different international sources such as the v5.0 of the ISC-GEM global instrumental catalogue (Storchak et al., 2013) and the USGS-NEIC. For validation and verification purposes of particular events, other data sources (e.g., Engdahl and VillaseĂąor, 2002) were used. The working catalogue only accounts for instrumental seismicity and covers the 1902-2017 period, having a minimum threshold magnitude equal to MW4.0.

To comply with the requirements of a Poissonian framework, a de-clustering process, following a magnitude and time dependent procedure as the proposed

by Gardner and Knopoff (1974) was employed to include in the final version only those events that can be classified as mainshocks. A homogenization of the magnitudes to moment magnitude (MW), using the global relationships proposed by Scordilis (2006) for events with original magnitudes reported in Ms and/or mb was also performed.

APPENDICES

221

Finally, a completeness verification process was performed for different magnitudes (4.0, 4.5, 5.5, 6.5, 7.5 and 8.0), following the procedure proposed by Tinti and Mulargia (1985). For this working catalog, the following time periods were obtained: • 1909-2017 for Mw≥7.5 • 1934-2017 for Mw≥6.5 • 1973-2017 for Mw≥4.0 Figure F.1 shows the different periods of completeness for the working catalogue of this study.

Figure F.2 shows the epicentral locations of the earthquakes included in the working catalogue classified by magnitude ranges whereas Figure F.3 shows their depth distribution.

For the estimation of the seismicity parameters that account for the future earthquake occurrence at each one of the seismic sources, only those earthquakes from 1902 onwards were considered. This is justified by the lack of completeness for earthquakes before that date, even if it is known that instrumental records exist in the region from approximately 1870. Nevertheless, it is worth noting that the historic earthquake information for the domain under study has been reviewed and used as an indicator for the assignment of the Mmax values to all the modelled seismic sources.

Figure F.1. Periods of completeness for the working catalogue

F.2 Characterization of the Seismic Sources A combination of different approaches was used to assign each of the events included in the working catalogue into the seismic sources. The first step was to plot the epicenters shown in Figure F.2 and Figure F.3 to identify the areas where seismicity is concentrated, followed by the review of previous seismo-tectonic zonations performed in the region under study (Pérez-Rocha and Ordaz, 2008; Alvarado et al., 2017; Salgado-Gálvez et al., 2018).

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All earthquakes in the working catalogue were classified first into two main categories: crustal and subduction. The first category includes the intraplate and shallow seismicity whereas the second one includes interface, intraslab and outer-rise events. Also, in this case, the geometry and characteristics of the subduction process was inferred, when possible, from the Slab dataset (Hayes et al., 2012) which contains a set of contour levels that describe it in detail (depth, strike values, among others).

The classification of the seismic activity was performed using the following criterion: • All events with epicenters located along the subduction trench within a 50 km buffer are considered as interface earthquakes.

• All events outside the 50 km buffer are classified as: - Intraslab earthquakes if the hypocenter of the event is deeper than 50 km - Crustal earthquakes if the hypocenter of the event is equal to or shallower than 50 km.

In addition, all the events located up to 100 km to the West of the subduction trench were classified as outer-rise and were assigned to two dedicated seismic sources in the model that account for the particularities of these types of events.

Regarding the crustal sources, the sub-regions we adopted in this model are based on the geometrical proposals of two previously available studies: the Mexican zonation developed by Pérez-Rocha and Ordaz, recently reviewed and adjusted by Salgado-Gálvez et al., 2018 and the one developed by Benito et al., (2012) under the framework of the RESIS II project, recently updated and adjusted

by Alvarado et al., (2017).

As mentioned before, the interface and intraslab sources were defined using as input data the depth contour levels published by Hayes et al., (2012). These depth contour levels were used to generate several cross sections, as shown in Figure F.4, complemented and validated against the focal mechanisms of the events obtained from the GCMT catalogue (Ekström et al., 2012), also shown in Figure F.4.

Figure F.5 shows the geometries of the seismic sources included in R-CRISIS for performing the PSHA.

APPENDICES

223

Figure F.2. Magnitude of the events included in the working catalogue

(a) (b)

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(c) (d)

(e) Figure F.3. Depth of the events included in the working catalogue: a) 0-25 km, b) 25-50 km, c) 50-100 km, d) 100-200 km, e) >200 km

Figure F.4. Cross sections of the subduction zone in Mexico

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225

Figure F.5. Geometries of the seismic sources considered in the model

Once all the geometric characteristics of the seismic sources were defined, every single event included in the working catalogue was associated to a unique seismic source using a 3D spatial matching process. As per the seismo-tectonic setting of the area under study, there was no need to consider any background source since, in the earthquake assignation process, all events could be assigned to a unique source without any problem using the previously explained criterion.

F.3 Seismicity Models In this study, the seismicity of all sources is assumed to follow a Poissonian process, which briefly assumes independency in time and space between events (reason why a de-clustering process of the historical catalogue was needed when developing the working catalogue). Two Poissonian seismicity models are commonly used in the development of a PSHA:

Modified Gutenberg-Richter (Cornell and VanMarcke, 1969) and, Characteristic earthquake model (Youngs and Coppersmith, 1985)

More details about both models are provided next.

Modified Gutenberg-Richter (G-R) In the modified G-R seismicity model, the future earthquake occurrences are defined by the following relationship:

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where λ0 corresponds the exceedance rate of the minimum threshold magnitude, M0; β is a parameter equivalent to the “b-value” for the source and MU is the maximum magnitude for the source. λ0 and β (a and b) parameters were calculated using a maximum likelihood approach like the one described by McGuire (2004). Since the completeness window varies as a function of M0, those complete sub-catalogues were later combined using the Ordaz & Giraldo (2018) methodology to obtain the final values used in the model.

MU (or Mmax) was assigned using different criteria, that range from the review of the historical magnitudes in the working catalogue assigned to each source, to statistical analyses (e.g., Kijko and Singh, 2011), to previous studies (Pérez-Rocha and Ordaz, 2008), and to the use of the Wells and Coppersmith (1994) relationships between magnitude and rupture lengths. In some cases, expert criteria were also employed.

Characteristic earthquake model There may be cases, mainly related to the interface and intraslab sources, where the modified G-R seismicity model underestimates the occurrences of large magnitude events (Singh et al., 1983). On those cases it is recommended that the modified G-R model is combined with the characteristic earthquake (CE) model, as proposed by Youngs and Coppersmith (1985). Such combination uses the modified G-R model for a magnitude range between M0 and MUGR and the CE model from that value onwards. The CE model uses the following magnitude recurrence relationship:

Here Φ[·] is the standard normal cumulative function, M0 and MU are the minimum and maximum characteristic magnitudes respectively, and EM and s are parameters defining the distribution of M. EM can be interpreted as the expected value of the characteristic earthquake and s is its standard deviation. λ0CH is the exceedance rate of magnitude M0CH. F.4 Ground Motion Prediction Equations One of the components of any PSHA that is affected by a large uncertainty is the prediction of the ground motion that future events may generate. Also because of this, GMPEs are known to be critical components of any PSHA even if there are different approaches for their development.

In areas of moderate and high seismicity, given the availability of strong motion recordings there is the possibility to derive local models via empirical approaches. When available, these local models should at least be considered within the set of base GMPEs used in the population of logic-trees or in the creation of composite (or hybrid) GMPEs.

APPENDICES

227

The selection of GMPEs for this study followed the recommendations by Cotton et al., (2006), incorporating the most recent versions of each candidate GMPEs. Also, to explicitly account for the above-mentioned uncertainties, it was decided to combine, for each tectonic environment, a set of GMPEs using a weighted average approach. For interface and intraslab environments, two local GMPEs have also been considered as explained with more detail next.

In this study, the seismic hazard results are obtained at rock level and for that reason, in the definition of the parameters that account for the soil type in each of the used GMPEs, rock condition was used. Subsequently, hazard results are obtained at soil level by means of the site effects auxiliary files, however, this topic will be discussed further. Besides this, no truncation to any σ value is used on any GMPE during the performance of the PSHA. Interface sources For the interface sources, these two different GMPEs were used: • Arroyo, García, Ordaz, Mora, & Singh (2010) • Reyes (1999) Intraslab sources For the intraslab sources, a GMPE developed by Dr. Singh and Dr. Ordaz, as an internal research, was used.

Intraplate sources For the intraplate (crustal) sources, GMPE developed by Abrahamson & Silva (1997) was used.

As an example, Figure F.6 shows the distance attenuation of all GMPEs (PGA) for different magnitudes, additionally, Figure F.7 shows the median pseudo-acceleration response spectra (rock) for RRUP=15km and different magnitudes.

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Figure F.6. Distance attenuation of GMPEs selected in this study for different magnitudes

Figure F.7. Median pseudo acceleration response spectra on rock for RRUP=15km and different magnitude events

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229

F.5 PSHA Methodology This section provides the information about the computational tool used for the PSHA together with the methodological approach that was followed to obtain the results.

Computation tool The PSHA was performed using the computer program R-CRISIS V19.0 (Ordaz et al.

2019) which implements all the required geometrical and seismicity models for this study. R-CRISIS is a tool with more than 30 years of continuous developments and improvements and is well-known tool at global level (see Figure F.8).

Methodological framework When a probabilistic approach is used to estimate seismic hazard at a site of interest, results are usually expressed in terms of intensity exceedance rates from where also, exceedance probabilities for any intensity value during an observation timeframe can be derived (e.g., 10% in 50 years).

Figure F.8. Main screen of computational tool used for the PSHA of this study

Once the seismicity and attenuation patterns of all seismic sources is known, seismic hazard can be calculated considering the sum of the effects of the totality of them and the distance between each seismogenetic source and the site of interest. Seismic hazard, expressed in terms of the intensity exceedance rate, ν(a), can be calculated as follows (Ordaz, 2000):

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where the sum covers all seismic sources N, and Pr(A>a|M, Ri) is the probability that the intensity exceeds certain value given the magnitude M and the distance between the source and the site of interest Ri of the event. li(M) functions are the activity rates of the seismic sources. The integral is performed from the threshold magnitude M0, to the maximum magnitude Mu of each source, which indicates that for each one, the contribution of all magnitudes is accounted for.

The previous equation would be exact if the seismic sources were points, but in reality those are treated as volumes and because of that, epicenters cannot occur at the center of the sources but, with equal spatial probability within any point inside its corresponding volume. This is considered in the PSHA model by subdividing each seismic source into triangles and then assuming that at each gravity center, seismicity is concentrated. The subdivision is performed recursively until reaching a small enough triangle size to guarantee the precision in the integration of expression (F.3).

Since it is assumed that once magnitude and distance are known the intensity follows a lognormal distribution, the probability Pr(A>a|M, Ri) can be calculated with the following expression (Ordaz, 2000):

where F(¡) is the normal standard distribution, MED(A|M, Ri) is the median of the intensity, given by the associated GMPE for known magnitude and distance, and ĎƒLna accounts for the standard deviation of the natural logarithm of the intensity. Full details of the methodologies implemented in R-CRISIS for performing a PSHA can be found in Ordaz and Salgado-GĂĄlvez (2018).

The return period of any intensity corresponds to the inverse value of the exceedance rate therefore, it can be calculated as:

APPENDICES

231

F.6 Modification of the Seismic Hazard Caused by Site Effects Generally, when a PSHA is carried out, the calculated intensities are determined for “rock” or “firm site” without the influence of site effects. The reason is that most of the attenuation models are developed for rock and not for soil, hence, the dynamic response of deeper strata and the topographic irregularities are not being taken into account, which modify the frequency content and the amplitude of the seismic waves.

A solution to consider the influence of soil particularities is by means of response spectra ratios (RSR), from which it is possible to know the local amplification that will directly modify the intensities assessed at rock. These RSR are able to consider the non-linear effect in the soil through different maximum acceleration values of the ground.

The RSR are interpreted as follows: during the PSHA, the median intensity is calculated for a structural period , located in site , caused by an event with magnitude , at a distance away. Generally, is calculated by attenuation models elaborated to predict the ground motion at “rock”. To include the site effects, the median intensity , will be the product of and the amplification factors del RSR, which depend of the site location , the structural period and maximum acceleration of the ground (factor to consider the non-linearity of the soil) (Ordaz M. et al., 2008).

IS (S,T,M,R) = I(S,T,M,R)×A(S,T,I0 ) (F.6)

Figure F.9. Procedure to obtain intensities including site effects

The site effects are included to the seismic hazard assessment by two files: the first one (with extension *.grd) contains the soil dominant periods in seconds, the other one (with extension *.ft) contains the response spectra ratios (RSR) associated to each point in the period grid (Huerta et al., 2011). An example of these files is presented in Figure F.10.

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Figure F.10. Graphical example of response spectra ratios, RSR, (right) and soil dominant periods (left) for Mexico City

F.7 Seismic Hazard Maps for Rock Tr=43 years

Figure F.11. Peak ground acceleration (PGA) on rock and Tr=43 years

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233

Figure F.12. Acceleration for T=0.2s on rock and Tr=43 years

F.7.2 Tr=250 years

Figure F.13. Peak ground acceleration (PGA) on rock and Tr=250 years

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Figure F.14. Acceleration for T=0.2s on rock and Tr=250 years

F.7.3 Tr=475 years

Figure F.15. Peak ground acceleration (PGA) on rock and Tr=475 years

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235

Figure F.16. Acceleration for T=0.2s on rock and Tr=475 years

F.7.4 Tr=2475 years

Figure F.17. Peak ground acceleration (PGA) on rock and Tr=475 years

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Figure F.18. Acceleration for T=0.2s on rock and Tr=2475 years

F.8 Seismic Hazard Maps for Soil F.8.1 Tr=43 years

Figure F.19. Peak ground acceleration (PGA) on soil and Tr=43 years

APPENDICES

237

Figure F.20. Acceleration for T=0.2s on soil and Tr=43 years

F.8.2 Tr=250 years

Figure F.21. Peak ground acceleration (PGA) on soil and Tr=250 years

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Figure F.22. Acceleration for T=0.2s on soil and Tr=250 years

F.8.3 Tr=475 years

Figure F.23. Peak ground acceleration (PGA) on soil and Tr=475 years

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239

Figure F.24. Acceleration for T=0.2s on soil and Tr=475 years

F.8.4 Tr=2475 years

Figure F.25. Peak ground acceleration (PGA) on soil and Tr=2475 years

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Figure F.26. Acceleration for T=0.2s on soil and Tr=2475 years

F.9 Seismic Hazard Results Influenced by Subsidence Problem F.9.1 Subsidence for 2020 Tr=43 years

Figure F.27. Peak ground acceleration (PGA) on soil, Tr=43 years and subsidence for 2020

241

Figure F.28. Acceleration for T=0.2s on soil, Tr=43 years and subsidence for 2020

F.9.1.2 Tr=250 years

Figure F.29. Peak ground acceleration (PGA) on soil, Tr=250 years and subsidence for 2020

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Figure F.30. Acceleration for T=0.2s on soil, Tr=250 years and subsidence for 2020

F.9.1.3 Tr=475 years

Figure F.31. Peak ground acceleration (PGA) on soil, Tr=475 years and subsidence for 2020

243

Figure F.32. Acceleration for T=0.2s on soil, Tr=475 years and subsidence for 2020

F.9.1.4 Tr=2475 years

Figure F.33. Peak ground acceleration (PGA) on soil, Tr=2475 years and subsidence for 2020

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Figure F.34. Acceleration for T=0.2s on soil, Tr=2475 years and subsidence for 2020

F.9.2 Subsidence for 2030 Tr=43 years

Figure F.35. Peak ground acceleration (PGA) on soil, Tr=43 years and subsidence for 2030

245

Figure F.36. Acceleration for T=0.2s on soil, Tr=43 years and subsidence for 2030

F.9.2.2 Tr=250 years

Figure F.37. Peak ground acceleration (PGA) on soil, Tr=250 years and subsidence for 2030

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Figure F.38. Acceleration for T=0.2s on soil, Tr=250 years and subsidence for 2030

F.9.2.3 Tr=475 years Figure F.39. Peak ground acceleration (PGA) on soil, Tr=475 years and subsidence for 2030

Figure F.40. Acceleration for T=0.2s on soil, Tr=475 years and subsidence for 2030

247

F.9.2.4 Tr=2475 years

Figure F.41. Peak ground acceleration (PGA) on soil, Tr=2475 years and subsidence for 2030

Figure F.42. Acceleration for T=0.2s on soil, Tr=2475 years and subsidence for 2030

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F.9.3 Subsidence for 2040 Tr=43 years

Figure F.43. Peak ground acceleration (PGA) on soil, Tr=43 years and subsidence for 2040

Figure F.44. Acceleration for T=0.2s on soil, Tr=43 years and subsidence for 2040

249

F.9.3.2 Tr=250 years Figure F.45. Peak ground acceleration (PGA) on soil, Tr=250 years and subsidence for 2040

Figure F.46. Acceleration for T=0.2s on soil, Tr=250 years and subsidence for 2040

F.9.3.3 Tr=475 years Figure F.47. Peak ground acceleration (PGA) on soil, Tr=475 years and subsidence for 2040

Figure F.48. Acceleration for T=0.2s on soil, Tr=475 years and subsidence for 2040

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F.9.3.4 Tr=2475 years Figure F.49. Peak ground acceleration (PGA) on soil, Tr=2475 years and subsidence for 2040 Figure F.50. Acceleration for T=0.2s on soil, Tr=2475 years and subsidence for 2040

F.9.4 Subsidence for 2050 F.9.4.1 Tr=43 years

Figure F.51. Peak ground acceleration (PGA) on soil, Tr=43 years and subsidence for 2050 Figure F.52. Acceleration for T=0.2s on soil, Tr=43 years and subsidence for 2050

F.9.4.2 Tr=250 years Figure F.53. Peak ground acceleration (PGA) on soil, Tr=250 years and subsidence for 2050 Figure F.54. Acceleration for T=0.2s on soil, Tr=250 years and subsidence for 2050

F.9.4.3 Tr=475 years Figure F.55. Peak ground acceleration (PGA) on soil, Tr=475 years and subsidence for 2050 Figure F.56. Acceleration for T=0.2s on soil, Tr=475 years and subsidence for 2050

F.9.4.4 Tr=2475 years Figure F.57. Peak ground acceleration (PGA) on soil, Tr=2475 years and subsidence for 2050 Figure F.58. Acceleration for T=0.2s on soil, Tr=2475 years and subsidence for 2050

251

F.9.5 Subsidence for 2070 F.9.5.1 Tr=43 years

Figure F.59. Peak ground acceleration (PGA) on soil, Tr=43 years and subsidence for 2070 Figure F.60. Acceleration for T=0.2s on soil, Tr=43 years and subsidence for 2070

F.9.5.2 Tr=250 years Figure F.61. Peak ground acceleration (PGA) on soil, Tr=250 years and subsidence for 2070 Figure F.62. Acceleration for T=0.2s on soil, Tr=250 years and subsidence for 2070

F.9.5.3 Tr=475 years Figure F.63. Peak ground acceleration (PGA) on soil, Tr=475 years and subsidence for 2070 Figure F.64. Acceleration for T=0.2s on soil, Tr=475 years and subsidence for 2070

F.9.5.4 Tr=2475 years Figure F.65. Peak ground acceleration (PGA) on soil, Tr=2475 years and subsidence for 2070 Figure F.66. Acceleration for T=0.2s on soil, Tr=2475 years and subsidence for 2070

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G BUILDING OF VULNERABILITY FUNCTIONS G.1 Methodology used Two methodologies were used, the first one consist in the conversion of fragility curves into vulnerability curves, the second one, is a simplified calculation of loss due to the estimation of parameters that describe the structural behavior. There are specific cases in which a detailed analysis was performed; this is described in the development of the corresponding function.

G.1.1 Fragility to vulnerability Fragility curves (Shinozuka et al, 2010) represents the probability of a structure to reach or exceed a performance level with an associated damage level. This state of damage is generally defined as a subjective manner (Figure G.1).

One assumption is the N states of damage (EDi‌N), the fragility functions estimates the probability to reach or exceed that damage state given a value of seismic demand. The fragility curve function is as follows:

Where, EDi is the damage state i; s is the seismic demand and qi is a parameter vector of the fragility function fi.

Figure G.1. Fragility curves associated to a parameter of seismic performance

In the literature, is common to find fragility curves associated to different structural systems. However, fragility curves, given its condition to be associated to a qualitative state of damage, does not allow the calculation of an average damage state that allows subsequent operations. (Ordaz et al, 2000).

Due to this, it is necessary to transform this qualitative damage states into quantitative damage states using the existing relation between visible physical damage with its associated repair cost and the calculation of the probability of damage matrices.

253

Unlike fragility curves, where is expressed the probability to reach or exceed a damage state, the damage matrices represents the probability to be exactly in a state of damage. This probability is calculated as follows:

Once the probability that represent a specific damage state is obtained, it is necessary to associate a repair cost value for each damage state. This process can be carried out through a detailed study of the cost of implementing certain repair actions to each state of damage considered in the fragility curves. The change from qualitative to quantitative approach allows to calculate an average repair cost through lineal properties of the value of mathematical expectations as follows:

Where E(β | s) is the expected value of the loss given an intensity; Li is the loss state or the repair value normalized to the total cost of the structure and Pr(Li | s) is the probability to present that loss state given an intensity.

On the other hand, dispersion, represented by standard deviation, is calculated as:

G.1.2 Simplified estimation of loss state associated to a performance parameter The second methodology used is based in the estimation of a loss value associated to a performance parameter through the expression by Ordaz et al (2000):

Where E (b | gi IM) is associated loss given a performance parameter and a measure of intensity; gi is the performance parameter; g50 is the value associated performance parameter related to the 50% of loss and r is a setting parameter of the function at the beginning and end of the damage. The standard deviation is calculated as follows:

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Where

Vmax, Do and r are parameters that depends on the type of structure; Vmax is the maximum variance, Do is the damage level where maximum variance is presented and r is equal to three (Ordaz et al, 2000).

Once the two simplified methodologies are defined, a summary of the estimation of the vulnerability functions associated to each type of structure for two main hazards, earthquake and subsidence, are presented.

G.2 Vulnerability Functions for Drinking Water Network G.2.1 Seismic vulnerability curves for drinking water network Vulnerability curves for previously mentioned assets are estimated through fragility functions from ATC-13 (ATC, 1985) and ATC-25 (ATA, 1991) (from Figure G.2 to Figure G.6), which are adequate to Mexico.

Figure G.2. Fragility curves associated to canals

Figure G.3. Fragility curves associated to electrical equipment

255

Figure G.4. Fragility curves associated to mechanical equipment

Figure G.5. Fragility curves associated to underground pipelines

Figure G.6. Fragility curves associated to medium and low height masonry structures

ATC-13 (ATC, 1985) uses Modified Mercalli scale (MMI) from VI to XII to represents seismic intensity. The damage states are classified in 6 levels: slight, minor, moderate, extensive, major and complete. The following table shows the definition for these damage states.

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Table G.1. Definition of damage states (ATC-1985)

DAMAGE STATE

DEFINITION

Slight

Slight damage that does not needs any repair

Minor

Damage of any general component, does not require repair

Moderate

Damage of one or several components, require repair

Extensive

Extensive damage that requires further repair

Major

Major damage that could result in demolition or repair of the installation

Complete

Total destruction in most of the installation

The following tables represent the damage matrices corresponding to each of the components mentioned above.

Table G.2. Damage matrices for Canals (ATC-1985)

DAMAGE STATE

DAMAGE FACTOR

CDF (%)

INTERVAL

MMI

VI

VII

VIII

IX

X

XI

XII

0

34.9

21.7

5.4

1.4

0.2

0.4

-

23.2

-

None

0

Slight

0-1

0.5

47.3

42.4

31.1

17.3

4.6

Minor

1 -10

5

17.8

35.6 62.0

75.4

56.2 58.3

3.3

Moderate

10- 30

20

-

0.3

1.5

5.9

36.9

17.8

37.8

Extensive

30-60

45

-

-

-

-

2.1

0.3

52.4

Major

60-100

80

-

-

-

-

-

-

6.5

Complete

100

100

-

-

-

-

-

-

-

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Table G.3. Damage matrices for electrical equipment (ATC-1985)

DAMAGE STATE

DAMAGE FACTOR

CDF (%)

INTERVAL

MMI

VI

VII

VIII

IX

X

XI

XII

0

0.5

0.2

-

-

-

-

-

1.8

-

-

-

-

16.9

0.5

-

-

9.3

1.3

None

0

Slight

0-1

0.5

25.4

10.3

Minor

1 -10

5

74.1

86.8 64.3

Moderate

10- 30

20

-

2.7

33.5

71.5

44.9

Extensive

30-60

45

-

-

0.4

11.6

54.1

86.3 50.5

Major

60-100

80

-

-

-

-

0.5

4.4

48.2

Complete

100

100

-

-

-

-

-

-

-

Table G.4. Damage matrices for mechanical equipment (ATC-1985)

DAMAGE STATE

DAMAGE FACTOR

CDF (%)

INTERVAL

MMI

VI

VII

VIII

IX

X

XI

XII

0

8.0

-

-

-

-

-

-

None

0

Slight

0-1

0.5

79.1

8.8

0.8

-

-

-

-

Minor

1 -10

5

12.9

91.2

87.9

36.0

7.6

1.1

-

Moderate

10- 30

20

-

-

11.3

63.3

73.6

41.5

10.5

Extensive

30-60

45

-

-

-

0.7

18.8

55.6

67.2

Major

60-100

80

-

-

-

-

-

1.8

22.3

Complete

100

100

-

-

-

-

-

-

-

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Table G.5. Damage matrices for underground pipelines (ATC-1985)

DAMAGE STATE

DAMAGE FACTOR

CDF (%)

INTERVAL

None

0

Slight

MMI

VI

VII

VIII

IX

X

XI

XII

99.8 20.9

8.7

-

-

-

0

100

0-1

0.5

-

0.2

54.1

34.2

1.3

89.5

-

Minor

1 -10

5

-

-

17.2

36.1

7.9

0.5

-

Moderate

10- 30

20

-

-

7.8

21.9

Extensive

30-60

45

-

-

-

-

1.1

29.6 56.4

Major

60-100

80

-

-

-

-

0.2

3.3

37.9

Complete

100

100

-

-

-

-

-

0.1

1.2

89.5 66.5

4.5

Table G.6. Damage matrices for medium and low height masonry structures (ATC-1985)

DAMAGE STATE

DAMAGE FACTOR

CDF (%)

INTERVAL

MMI

VI

VII

VIII

IX

X

XI

XII

0

1.2

-

-

-

-

-

-

3.1

0.3

-

-

-

-

1.0

-

-

12.2

2.8

48.6 71.6

46.3

16.2

50.9

-

-

None

0

Slight

0-1

0.5

47.0

Minor

1 -10

5

51.8

Moderate

10- 30

20

-

0.3

Extensive

30-60

45

-

Major

60-100

80

Complete

100

100

96.6 57.2

16.2

42.2

75.6 49.9

-

0.3

8.2

-

-

-

-

0.5

-

-

-

-

-

259

As is observed in tables above, the intensity scales are presented in Modified Mercalli (MMI) scale, however, actually, seismic hazard is represented by peak ground acceleration, acceleration or spectral displacement. The measure of intensity to represent vulnerability in this type of structures is peak ground acceleration (PGA), so to transform Mercalli scale, presented in damage matrices (previously shown in tables), the relation between MMII and PGA is presented in the following table: Table G.7. PGA values associated to MMI scale (BIS, 1999)

MMI

VI

VII

VIII

IX

X

XI

PGA (g) 0.12

0.12

0.21

0.36

0.53

0.71

0.86

XII 1.15

With this information, fragility curves methodology is used and the vulnerability curves associated with each of the mentioned assets are obtained.

Figure G.7. Seismic vulnerability curve associated to canals

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Figure G.8. Seismic vulnerability curve associated to underground pipelines

For the case of pumping stations, it is considered that these structures consist in electrical and mechanical equipment as pumps, control panels and other devices using for the operation and medium and low height masonry buildings. These assets are included directly in one only function that represents the whole station, with the following participation factors:

Pumping stations 30% Buildings 20% Electrical equipment 50% Mechanical equipment

Figure G.9. Seismic vulnerability function associated to pumping station

G.2.2 Subsidence vulnerability curves for drinking water network The calculation of vulnerability functions for subsidence associated to canals, was carried out using studies presented by Giardina (2015), in which the presented damages curves are associated to subsidence in masonry structures. These curves were calculated under the study of several parameters such as wall configuration, modulus of elasticity, among others.

To obtain damage curves associated to concrete lining canals, an extrapolation was performed between the modulus of elasticity in the curves proposed by Giardina (2015) to the values of characteristic modulus of elasticity for concrete.

The calculation of vulnerability curve was made for 12 meters canal sections. The next figure shows this vulnerability curve.

Figure G.10. Subsidence vulnerability curve for sections canals

261

Vulnerability curve for underground pipelines was estimated under the hypothesis that connections between pipes are the most susceptible from subsidence, causing rupture in pipes and leaks.

Figure G.11. Diagram of decoupling connections of pipes due to subsidence

Differential subsidence () necessary to decoupling a pipeline is directly related with the diameter of the pipe (D), to length of pipe (L) and the deformation capacity of the connection (x). This subsidence is calculated as: Δ=(L . x)/D

(G.8)

Once diameter of the pipe is defined, the damage in this type of structures starts when the connection reaches the deformation limit. Generally, drinking water pipes have connections with low ductility, for this reason it is considered a maximum ductility of two.

Once this parameters are defined (Sundberg, 2017), the vulnerability function by subsidence is calculated with the expression proposed by Ordaz et al (2010). The next figure shows the resulting vulnerability curve.

Figure G.12. Subsidence vulnerability curve associated to underground pipelines

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In the case of pumping plants, they consider the same elements as the case of seismic hazard. It is necessary to mention that, given the configuration (Figure G.13) of the electrical and mechanical equipment, it is very difficult to turn over by differential subsidence, reason why the study is focused in the connections that this equipment have with others.

Figure G.13. Pumping plants common equipment

Considering the above, vulnerability curve was calculated for connection between equipment and this was calculated in a similar manner than the calculation of the vulnerability curves for underground pipe connections. The following figure shows the vulnerability curve for electrical and mechanical equipment due to subsidence, represented in terms of repair cost respect total cost of the equipment.

Figure G.14. Subsidence vulnerability curve for electrical and mechanical equipment

For the case of the estimation of the vulnerability curves for settlement, associated to medium or low height masonry structures, the studies by Giardina et al (2015) were used.

The authors determinate that subsidence caused cracks in the superior part of walls exposed to shear stress, which were increasing in function of the level of

263

subsidence. They establish damage functions for masonry structures under different configurations based on a deflection ratio, which is calculated as follows: Δ=Δ/L (G.9) Where D is the deflection ratio, L is the length of the supported base, and D is the subsidence.

These authors obtain different damage curves given an opening percentage (windows and doors) in walls of the structure. For this study, a medium level opening percentage was used (Figure G.15).

Figure G.15. Wall with a medium level opening percentage

Giardina et al (2015) damage curve is presented in the following figure:

Figure G.16. Damage curve by differential subsidence associated to masonry structures with medium opening percentage

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The damage levels presented in the previous curve are described in the table below.

Table G.8. Damage classification for masonry buildings based on crack width

DAMAGE LEVEL

DAMAGE CLASSIFICATION

CRACK WIDTH, D (MM)

1

Negligible

0.1

2

Slight

0.1 < d < 1.0

3

Minor

1.0 < d < 5.0

4

Moderate

5.0 < d < 15.0

5

Severe

15.0 < d < 25

6

Extensive

> 25

Based on the damage classification of the table above, the corresponding vulnerability curve was calculated (Figure G.17).

Figure G.17. Subsidence vulnerability curves for medium or low height masonry structures

With the individual functions obtained, the corresponding vulnerability curve for pumping plants was calculated.

265

Figure G.18. Subsidence vulnerability curves for pumping stations

G.3 Vulnerability Functions for Wells Wells are buried structures which are used to extract water of subsoil. These wells are generally constructed by excavation with shovels or using backhoes and then to be lined with masonry. Wells as they are undergrounded structures, its performance is linked to soil performance that surround them. Wells can suffer failures by compression and by shear related to soil deformation, such as soil failures.

G.3.1 Seismic vulnerability curves for wells Seismic vulnerability curves are calculated through the analysis of fragility curves and damage matrices associated to underground structures. This fragility curves are taken from ATC-13 (ATC, 1985) and ATC-25 (ATC-1991) reports. Damage matrices presented in this reports are developed for United States, however, they are suitable to represent this kind of structures in Mexico. The Table G.9 shows value of the damage probability matrix associated to this type of structure.

Table G.9. Damage probability matrix associated to structures that crosses alluvial strata (ATC-1985)

DAMAGE STATE

DAMAGE FACTOR

CDF (%)

INTERVAL

MMI

VI

95.9 91.6

VIII

IX

X

XI

XII

8.3

0.4

-

-

-

0.2

-

None

0

Slight

0-1

0.5

4.1

8.4

62.7

26.0

3.8

Minor

1 -10

5

-

-

29.0

73.6

85.5 43.6

Moderate

10- 30

20

-

-

-

-

266

0

VII

10.7

11.8

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DAMAGE STATE

DAMAGE FACTOR

CDF (%)

INTERVAL

MMI

VI

VII

VIII

IX

X

XI

XII

Extensive

30-60

45

-

-

-

-

-

1.2

27.8

Major

60-100

80

-

-

-

-

-

-

0.6

Complete

100

100

-

-

-

-

-

-

-

The following figure shows the fragility curves for wells and underground tanks.

Figure G.19. Fragility curves for wells and underground tanks

As shown before, it is possible to calculate vulnerability curves through fragility curves. The following figure shows the vulnerability curve associated to wells.

Figure G.20. Seismic vulnerability curves for wells, showing expected value of loss (continuous line) and standard deviation (discontinuous line)

267

G.3.2 Subsidence vulnerability curves for wells Given the construction type of the analyzed wells, the damage caused by differential subsidence will be reflected in the containing walls of the well. This containing walls are masonry made. The following figure shows the way it fails.

In this same figure, it is observed how the sinking in one side of the well causes stress and shear stresses.

Figure G.21. Sinking in wells

Using Giardina et al (2015) studies, it was possible to calculate the vulnerability curve associated to subsidence for this type of structures. In the case of exposed assets, the damage curve for masonry walls without gaps was used. The following figure present the vulnerability curve.

Figure G.22. Subsidence vulnerability curve for wells, showing expected value of loss (continuous line) and standard deviation (discontinuous line)

G.4 Vulnerability Curves for Tanks Tanks serve for regulation and storage of transported water, these can be undergrounded, superficial or elevated. Surface or underground tanks are constructed

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with reinforced concrete and have regular geometry. Its foundation are made of sand or concrete bases that supports them. Elevated tanks consists in steel tanks which are supported by columns with the same material.

The damages in underground tanks include damage in walls and roof support system. Surface tanks directly supported on ground also have many kinds of damage. In the case of steel tanks, the damage can be presented in the union between the base and walls, wall bulging, rupture of the rigidly connected pipes, implosion due to sudden loss of content, differential subsidence, failure in the anchoring system, roof system failure and total collapse. For concrete tanks, the most common failures are the roof system failure, cracking and sliding in construction joints. Elevated tanks generally fails because of the incorrect support system, if the damage condition exceeds a specific flexion condition or damage in connections, this generally results in a total damage.

Next, the parameters used in calculating the vulnerability functions corresponding to the mentioned assets.

G.4.1 Seismic vulnerability curve for tanks Seismic vulnerability functions for tanks are calculated with the study of fragility curves. Such curves were taken from ATC-13 (ATC, 1985) and ATC-25 (ATC, 1991).

The next table shows the damage matrix associated to surface and underground tanks.

Table G.10. Damage matrix associated to surface tanks

DAMAGE STATE

DAMAGE FACTOR

CDF (%)

INTERVAL

None

0

Slight

MMI

VI

VII

VIII

99.8 20.9

IX

X

XI

XII

8.7

-

-

-

0

100

0-1

0.5

-

0.2

54.1

34.2

1.3

-

-

Minor

1 -10

5

-

-

17.2

36.1

7.9

0.5

-

Moderate

10- 30

20

-

-

7.8

21.9

Extensive

30-60

45

-

-

-

-

1.1

29.6 56.4

Major

60-100

80

-

-

-

-

0.2

3.3

37.9

Complete

100

100

-

-

-

-

-

0.1

1.2

89.5 66.5

4.5

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Table G.11. Damage matrix associated to underground tanks

DAMAGE STATE

DAMAGE FACTOR

CDF (%)

INTERVAL

MMI

VI

VII

VIII

IX

X

XI

XII

0

93.6

92.7

2.8

-

-

-

-

6.4

7.3

80.8

-

-

-

-

-

-

14.4

98.0

87.9

4.5

-

None

0

Slight

0-1

0.5

Minor

1 -10

5

Moderate

10- 30

20

-

-

2.0

2.0

12.1

90.2

65.7

Extensive

30-60

45

-

-

-

-

-

5.3

34.0

Major

60-100

80

-

-

-

-

-

-

0.3

Complete

100

-

-

-

-

-

-

-

100

To transform the values from MMI to maximum ground acceleration the values of Table G.7 were taken.

The following figures shows the fragility curves for surface and underground tanks.

Figure G.23. Fragility curves for surface tanks

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Figure G.24. Fragility curves for underground tanks

The following figures shows the vulnerability curves for this type of structures.

Figure G.26. Seismic vulnerability curves for undergrounded tanks For the calculation of vulnerability function associated to elevated tanks, the study of non-linear behavior of elevated steel tanks was used. For this, Ntibaziyaremye (2016) studied the seismic behavior of elevated tanks obtaining both, the curve and capacity spectrum of a typical structure, which are similar to those built in Mexico (Figure G.27).

271

Figure G.27. Elevated tank (Ntibaziyaremye, 2016)

The height of the tank is 20 m with a fundamental period of vibration of 0.78s with water, the width of the tank is 7 m. The estimated capacity spectra refers to a fluent spectral displacement of the tower of 4.4 cm, while the last displacement is 5.5 cm. In order to adapt these values of the structures with the ones constructed in Mexico, it was decide to use a maximum ductility of two, considering the P-D effects which this type of structures are subjected once they are deformed. The resulting capacity spectrum for these type of structures constructed in Mexico is shown in the next figure.

Figure G.28. Capacity spectrum for elevated steel tanks

With the capacity spectrum it was possible to calculate maximum distortion using the formula proposed by Miranda (1997, 2001), to then, calculate vulnerability curve with the expression proposed by Ordaz (2000). The following figure shows the resultant vulnerability curve.

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Figure G.29. Seismic vulnerability curve for elevated steel tanks

G.4.2 Subsidence vulnerability curve for tanks In the case of damage estimation due to subsidence in underground and surface tanks, same concepts defined by Giardina et al (2015) in calculation of subsidence in masonry structures was used.

Giardina et al (2015), presents several damage curves due to subsidence for structures with different modulus of elasticity. Through a lineal regression of different value for each modulus of elasticity, it was established a relation that allows extrapolating the results to structures of different material, so the values of differential subsidence were established. The following figure shows the vulnerability curve reinforced concrete storage and regulation tanks.

Figure G.30. Subsidence vulnerability curve for underground and surface tanks

In the case of elevated tanks, there is very little or no information about the structural behavior of this type of infrastructure due to differential subsidence, which is why it was defined as follows.

273

The model for calculation of the differential subsidence vulnerability curve used is defined using as a base the model used for the calculation of seismic vulnerability curve for this structure (Figure G.31).

Figure G.31. Model analyzed (left) and deformed (right) of the elevated tank analyzed

Figure G.31 shows in a schematically form how sinking in one of the supports generates stresses in the opposite point the support and in all the bars that support the tower.

For damage analysis, it is considered as a structural element the main column opposite to sinking. With this, the increase of the present moment in the support opposite to subsidence in function of the differential subsidence was analyzed. Accepting that the whole structure works together and the effort caused throughout it are distributed, the tower is modeled as a cantilever (Figure G.32).

Figure G.32. Idealization of a strenuous column

The resulting moment of displacement in the superior part of the structure is given by:

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Where Di is displacement in the highest part of the tower due to subsidence, M is the assumed moment as a strenuous column with a cantilever behavior, E is the modulus of elasticity, I is the moment of inertia of the section and L is the length of the column.

Analyzing the geometry of the tower, it was determined that the relation between the displacement in the highest point of the tower and the one presented in the support that sinks is approximately 1/3. This relation is valid only for this type of tanks.

The subsidence that causes Dy creep, and the Du last state were determined with the following expressions:

Where My, is the moment of creep and Mz is the plastic moment, calculated as:

Where S is the elastic section modulus and Z is the plastic section modulus of the column and fy is the steel yield strength.

Once defined the last and differential creep subsidence, loss expected value and standard deviation were calculated by the formulation proposed by Ordaz et al (2000).

Figure G.33. Subsidence vulnerability curve for elevated tanks

275

H EXPOSURE SUMMARY This section describes information given to ERN by the client and shows statistics of the final database considered in the current risk study, that comprises structural characteristics and geography location of records.

H.1 Information given to ERN Information of water and urban infrastructure were facilitated by Deltares and The Universidad Autónoma Metropolitana (UAM) to ERN. Information and files given to ERN are described next and was divided into two groups: hydraulic infrastructure concerning wells, drainage, tanks, distribution pipe networks and similar items; and Urban Infrastructure concerning houses, churches, markets, roads, healthcare buildings, schools and public transport stations.

H.1.1 Hydraulic infrastructure Information given by Deltares describes drinking water wells, water storage tanks and sewage network of Mexico City, this information is listed next: Geospatial files for drinking water wells: • Agricultural_Wells.shp • Domestic_Wells.shp • Industrial_Wells.shp Geospatial files for sewage network of Mexico City at The Heritage Zone: • Drainage_CDMX.shp • Deep_Drainage_Exits.shp • Hydraulic_Infraestructure_CDMX_System.shp Geospatial files for water storage tanks: • Regulation_tanks.shp • Water_tanks_Xochimilco.shp Geospatial files for water treatment plants: • Water_treatment_plantst.shp Information given by UAM is listed next.

Geospatial files for water supply network, drinking water wells and drinking water treatment plants: • MILPA-ALTA200P.dwg • TLAHUAC200P.dwg • XOCHI200P.dwg Geospatial information for sewage network: • MILPA-ALTA200D.dwg • TLAHUAC200D.dwg • XOCHI200D.dwg

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Geospatial information for reclaimed water network and water pollution control treatment plants: • Red_Primaria_AguaTratada.shp • PTAR_S.shp H.1.2 Urban Infrastructure Information about markets, medical centers, hospitals, state government offices, churches, subway stations, schools, squares and cemeteries: • 09sip.shp H.2 Complementary Information Finally, complementary information is listed next which was consulted in order to obtain a more complete database.

H.2.1 Hydraulic Infrastructure • Drinking Water Plants: Sistema Nacional de Información del Agua, CONAGUA. http://sina.conagua.gob.mx/sina/tema.php?tema=plantasPotabilizadoras • Sewage network information: Inventario Nacional de Obras de Protección contra Inundaciones. https://www.gob.mx/cms/uploads/attachment/ file/105634/18_AGUAS_DEL_VALLE_DE_M_XICO.pdf H.2.2 Urban Infrastructure • Houses: Vectorial de localidades amanzanadas y números exteriores, urbanas. Cierre de población del Censo de Población y Vivienda 2010. INEGI. • Schools: Directorio Estadístico Nacional de Unidades Económicas (DENUE) marzo 2018.

• Roads: Marco Geoestadístico Nacional, febrero 2018. http://www.beta. inegi.org.mx/app/biblioteca/ficha.html?upc=889463526636

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H.3 Exposure per infrastructure type

Figure H.1. Water storage tanks, drinking water plants, drinking water wells, pumping stations and water supply network distribution at The Heritage Zone

Figure H.2. Reclaimed water network, sewage network, sewage pumping stations and water pollution control treatment plants at The Heritage Zone

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Figure H.3. Urban Infrastructure distribution at The Heritage Zone

Figure H.4. Urban infrastructure distribution at The Heritage Zone

279

Figure H.5. School distribution at The Heritage Zone

Figure H.6. Road distribution at The Heritage Zone

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Figure H.7. House distribution at The Heritage Zone

I ANNUAL AVERAGE LOSS MAPS

Figure I.1. Seismic annual average loss distribution for sewage network

281

Figure I.3. Seismic annual average loss distribution for drinking water plants and water storage tanks

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Figure I.4. Seismic annual average loss distribution for water supply network and pumping stations

Figure I.5. Seismic annual average loss distribution for hydraulic infrastructure

283

Figure I.6. Seismic annual average loss distribution for houses

Figure I.7. Seismic annual average loss distribution for urban infrastructure

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Figure I.8. Seismic annual average loss distribution for healthcare buildings

Figure I.9. Seismic annual average loss distribution for schools

285

Figure I.10. Seismic annual average loss distribution for churches and markets

Figure I.11. Seismic annual average loss distribution for roads

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J RISK ANALYSIS FOR SEPTEMBER 19TH EVENTS AND CRITICAL SCENARIO J.1 September 19, 1985 earthquake The overall expected seismic loss of September 19, 1985 earthquake as calculated was 0.01% of the total exposure for the 2020 subsidence scenario. The expected loss of this scenario was 0.05% and 0.01% to hydraulic and urban infrastructure respectively.

Expected seismic loss of September 19, 1985 earthquake to hydraulic infrastructure for the 2020 subsidence scenario is detailed in Table J.1. Sewage Pumping Stations stand out with the highest expected loss with 0.51%. Additionally, expected loss distribution per hydraulic infrastructure type is presented in Figure J.1.

Table J.1. Expected loss of September 19, 1985 earthquake to hydraulic infrastructure

INFRASTRUCTURE NAME

EXPECTED LOSS - 2020

Sewage Pumping Stations

0.51%

Pumping Stations

0.28%

Reclaimed Water Network

0.05%

Sewage Network

0.05%

Water Supply Network

0.05%

Water Pollution Control Treatment Plants

0.00%

Drinking Water Plants

0.00%

Drinking Water Wells

0.00%

Water Storage Tanks

0.00%

287

Figure J.1. Expected loss of September 19, 1985 earthquake to hydraulic infrastructure

Expected seismic loss of September 19, 1985 earthquake to hydraulic infrastructure for the 2020 subsidence scenario is detailed in Table J.2. Markest stand out with the highest expected loss with 0.50%. Additionally, expected loss distribution is presented in Figure J.2.

Subsidence scenarios at 2030, 2040, 2050 and 2070 were omitted due to the small or null variations observed.

Risk maps with expected loss distribution due to September 19, 1985 earthquake are presented from Figure J.3 to Figure J.13.

Table J.2. Expected loss of September 19, 1985 earthquake to urban infrastructure

INFRASTRUCTURE NAME

EXPECTED LOSS - 2020

Markets

0.50%

Healthcare Buildings

0.13%

Public Statin Transport

0.08%

Sport Centers

0.04%

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INFRASTRUCTURE NAME

EXPECTED LOSS - 2020

Roads

0.03%

Houses

0.01%

Goverment Buildings

0.01%

Schools

0.00%

Churches

0.00%

Town Squares

0.00%

Cemeteries

0.00%

Figure J.2. Expected loss of September 19, 1985 earthquake to urban infrastructure

289

Figure J.3 .1985 Earthquake expected loss distribution for drinking water wells

Figure J.4. 1985 Earthquake expected loss distribution for drinking water plants and water storage tanks

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Figure J.5. 1985 Earthquake expected loss distribution for drinking water wells

Figure J.6. 1985 Earthquake expected loss distribution for hydraulic infrastructure

291

Figure J.7. 1985 Earthquake expected loss distribution for sewage network

Figure J.8. 1985 Earthquake expected loss distribution for urban infrastructure

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Figure J.9. 1985 Earthquake expected loss distribution for healthcare buildings

Figure J.10. 1985 Earthquake expected loss distribution for schools

293

Figure J.11. 1985 Earthquake expected loss distribution for churches and markets

Figure J.12. 1985 Earthquake expected loss distribution for roads

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Figure J.13. 1985 Earthquake expected loss distribution for houses

J.2 September 19, 2017 earthquake The overall expected seismic loss of September 19, 2017 earthquake as calculated was 0.42% of the total exposure for the 2020 subsidence scenario. The expected loss of this scenario was 0.48% and 0.41% to hydraulic and urban infrastructure respectively.

Expected seismic loss of September 19, 2017 earthquake to hydraulic infrastructure for the 2020 subsidence scenario is detailed in Table J.3. Sewage Pumping Stations stand out with the highest expected loss with 2.95%. Additionally, expected loss distribution per hydraulic infrastructure type is presented in Figure J.14.

Table J.3. Expected loss of September 19, 2017 earthquake to hydraulic infrastructure

INFRASTRUCTURE NAME

EXPECTED LOSS - 2020

Sewage Pumping Stations

2.95%

Pumping Stations

1.43%

295

INFRASTRUCTURE NAME

EXPECTED LOSS - 2020

Reclaimed Water Network

0.60%

Water Pollution Control Treatment Plants

0.50%

Sewage Network

0.49%

Water Supply Network

0.46%

Drinking Water Plants

0.35%

Drinking Water Wells

0.24%

Water Storage Tanks

0.11%

Expected seismic loss of September 19, 2017 earthquake to urban infrastructure for the 2020 subsidence scenario is detailed in Table J.4. Markets stand out with the highest expected loss with 6.08%. Additionally, expected loss distribution per urban infrastructure type is presented in Figure J.15.

Subsidence scenarios at 2030, 2040, 2050 and 2070 were omitted due to the small or null variations observed.

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Risk maps with expected loss distribution due to September 19, 2017 earthquake are presented from Figure J.16 to Figure J.27.

Table J.4. Expected loss of September 19, 2017 earthquake to urban infrastructure

INFRASTRUCTURE NAME

EXPECTED LOSS - 2020

Markets

6.08%

Healthcare Buildings

3.23%

Public Transport Station

1.22%

Roads

0.73%

Sport Centers

0.66%

Government Buildings

0.46%

Houses

0.36%

Schools

0.05%

Churches

0.05%

Town Squares

0.00%

Cemeteries

0.00%

297

Figure J.15. Expected loss of September 19, 2017 earthquake to urban infrastructure

Figure J.16. 2017 Earthquake expected loss distribution for sewage network

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Figure J.17. 2017 Earthquake expected loss distribution for drinking water wells

Figure J.18. 2017 Earthquake expected loss distribution for drinking water plants and water storage tanks

299

Figure J.19. 2017 Earthquake expected loss distribution for water supply network and pumping stations

Figure J.20. 2017 Earthquake expected loss distribution for hydraulic infrastructure

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Figure J.21. 2017 Earthquake expected loss distribution for urban infrastructure

Figure J.22. 2017 Earthquake expected loss distribution for healthcare buildings

301

Figure J.23. 2017 Earthquake expected loss distribution for schools

Figure J.24. 2017 Earthquake expected loss distribution for markets

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Figure J.25. 2017 Earthquake expected loss distribution for churches

Figure J.26. 2017 Earthquake expected loss distribution for roads

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Figure J.27. 2017 Earthquake expected loss distribution for houses

J.3 Critical Scenario The scenario with highest expected loss is considered as critical in this study. The overall expected seismic loss of critical scenario as calculated was 24.16% of the total exposure for the 2020 subsidence scenario. Expected loss of critical scenario to hydraulic and urban infrastructure was 17.17% and 25.01% respectively.

The top five scenarios with highest expected seismic loss for the 2020 subsidence scenario to hydraulic and urban infrastructure are listed in Table J.5. Expected loss distribution for critical scenario per hydraulic infrastructure type is detailed in Table J.6 and Figure J.28. Sewage Pumping Stations stand out with the highest expected loss with 30.18%.

Table J.5. Top five scenarios with highest expected seismic loss to hydraulic and urban infrastructure

Scenario

Hydraulic Urban Frequency Infrastructure Infrastructure Expected Loss Expected Loss

Prof. int. centro nueva_ SF56_M=7.66

3.00E-04

17.17%

25.01%

Prof. Interm Oeste nueva_SF11_M=7.57

2.57E-04

16.04%

22.79%

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Scenario

Hydraulic Urban Frequency Infrastructure Infrastructure Expected Loss Expected Loss

Prof. Interm Oeste nueva_SF12_M=7.57

2.57E-04

10.57%

13.71%

Eje volcรกnico_SF37_ M=7.02

1.98E-05

10.41%

20.30%

Prof. int. centro nueva_SF55_M=7.66

3.00E-04

9.84%

13.10%

Table J.6. Critical scenario expected loss to hydraulic infrastructure

INFRASTRUCTURE NAME

EXPECTED LOSS - 2020

Sewage Pumping Stations

30.18%

Reclaimed Water Network

20.01%

Water Pollution Control Treatment Plants

18.53%

Sewage Network

17.52%

Pumping Stations

16.96%

Water Supply Network

16.67%

Drinking Water Plants

16.21%

Drinking Water Wells

8.22%

Water Storage Tanks

7.27%

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Figure J.28. Critical scenario expected loss to hydraulic infrastructure

Expected loss distribution for critical scenario per urban infrastructure type is detailed in Table J.7 and Figure J.29. Markets stand out with the highest expected loss with 64.25%.

Table J.7. Critical scenario expected loss to hydraulic infrastructure

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INFRASTRUCTURE NAME

EXPECTED LOSS - 2020

Markets

64.25%

Healthcare Buildings

35.76%

Government Buildings

28.45%

Public Transport Station

27.68%

Roads

17.66%

Schools

6.00%

Churches

5.48%

Town Squares

0.00%

Cemeteries

0.00%

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Figure J.29. Critical scenario expected loss to hydraulic infrastructure

Risk maps with loss distribution due to critical scenario are presented from Figure J.30 to Figure J.40.

Subsidence scenarios at 2030, 2040, 2050 and 2070 were omitted due to the small or null variations observed.

Figure J.30. Critical scenario expected loss distribution for drinking water wells

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Figure J.31. Critical scenario expected loss distribution for drinking water plants and water storage tanks

Figure J.32. Critical scenario expected loss distribution for water supply network and pumping stations

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Figure J.33. Critical scenario expected loss distribution for hydraulic infrastructure

Figure J.34. Critical scenario expected loss distribution for sewage network

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Figure J.35. Critical scenario expected loss distribution for houses

Figure J.36. Critical scenario expected loss distribution for urban infrastructure

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Figure J.37. Critical scenario expected loss distribution for healthcare buildings

Figure J.38. Critical scenario expected loss distribution for schools

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Figure J.39. Critical scenario expected loss distribution for churches and markets

Figure J.40. Critical scenario expected loss distribution for roads

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K THE PROCESS OF PORTFOLIO DEVELOPMENT K.1 Systemic analysis Map 1: Original system Originally water from the southern wetlands came from two main sources: springs and rivers. The springs were the product of water infiltration in the recharge areas of the mountains that surround the Valley of Mexico. The rivers that fed the wetland area came from the south-west mountains and the Sierra Nevada.

Map K.1 ORIGINAL SYSTEM

Map 2: Current system The original water system was affected by human activity since the Colony in several ways. On one hand, the use of south springs by means of wells was increased until the flow of these towards the wetlands was eliminated. On the other hand, in more recent times, to prevent flooding in the Valley of Mexico, the flow of rainwater has been diverted from the south and west ravines by sending it to the State of Hidalgo.

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With this, the wetlands of Xochimilco, Tlรกhuac and Milpa Alta remained practically without water income except for the rain that falls on site. To compensate for this lack of water, treated water is supplied from the Cerro de la Estrella treatment plant, which has a relatively high salt content and whose quality is at least questionable.

Currently, almost all of the water that would otherwise go to the southern wetlands is diverted for urban use, and almost the only source of water to the wetlands is a fraction of this same water, treated after its urban use.

The current system does not provide the amount neither the quality of water necessary to keep the ecosystem healthy and the agriculture that depends on that ecosystem.

Map K.2 CURRENT SYSTEM

Map 3: Complete current system At present, the inputs of water to the Valley of Mexico system are the rainwater that is captured and stored in the forests of the south, west and Sierra Nevada; the pluvial water that infiltrates the aquifer in the recharge areas of forests and stony areas south of the Valley; the rivers that enter the Valley of Mexico mainly by the west ravines and the south, but also to a lesser extent by the east towards

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Texcoco; and the aqueduct of the Cutzamala system that brings water at a very high cost in energy and environmental and social impact.

Most of the water that arrives through the ravines and rivers is extracted from the Valley by pumping it to Hidalgo to avoid floods, and the water deficit generated by this extraction is only partially compensated by what is introduced by the Cutzamala. The net effect is a deficit of water that is the cause of differential land subsidence.

By diverting the water from the rivers and by extracting all the water from the springs, the water from these two sources no longer reaches the wetlands of the south. This deficit has to be compensated for by feeding the treated water to the wetlands.

With this, currently the main flow of water in the Valley of Mexico is artificial, and consists largely of the import of water from Cutzamala and the extraction of water by the two emitters of pluvial and residual drainage to Hidalgo. The current system makes a “short circuit” and removes much of the water before it can be stored (in forests, ravines and aquifers) or used, and eliminates all natural flow to the wetlands, compensating them insufficiently with the artificial flow of treated water.

Map K.3: COMPLETE CURRENT SYSTEM

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Map 4: Systemic map The complete systemic map shows mainly two major groups of factors relevant to the Water Plan: the systemic description of the current water problem, and its relation to the decision-making process for the creation of infrastructure projects and interventions in the water system.

In this map you can see the variety of possible intervention points with infrastructure projects, the multiple relationships between these points and projects, and the complexity of the current environment of the project generation process.

Map K.4: SYSTEMIC MAP

Map 5: Infrastructure Projects In addition to the existing infrastructure projects, there are several possible intervention points for new infrastructure projects that seek to contribute to solving the water problems of the Xochimilco, Tlรกhuac and Milpa Alta wetlands. These possible intervention points can be grouped into areas of rain catchment, water storage, use and treatment.

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The main capture areas are the mountains and forests around the Valley of Mexico, mainly in the south-west of the Valley, where there are more forests, less population and greater rainfall. In these areas there are currently several problems that make the collection of water ineffective. On one hand, these areas are being populated, especially in the form of informal settlements between the forests and the City. The result of this is that the water that is captured clean in the forests is contaminated by irregular discharges of wastewater. In addition, due to lack of water storage capacity and infrastructure, in the rainy season the main problem for the City is to get rid of surplus water to avoid flooding. Thus, most of this water, which contains rainwater mixed with sewage, is captured by the drainage and sent to Hidalgo.

The storage points include the forests themselves, the ravines, regulatory vessels and the aquifer. Especially to the south and west of the Valley of Mexico, forests have been reduced and their soils eroded. This has reduced their ability to store water, especially during storms when forests could serve as buffers for the flow of water that causes flooding. The ravines, especially to the west of the Valley, are deforested, contaminated, and in some cases invaded by slums. With this, their capacity as infiltration zones and especially as possible regulatory vessels through infrastructure, is untapped. The infiltration areas are mostly affected and threatened by deforestation and urbanization, so their capacity to recharge the aquifer is greatly reduced. This causes that the main water storage in the Valley of Mexico is underutilized and at risk of becoming too salty and therefore unusable.

Regarding the use of water, there is a lot of water that enters the Valley and is suitable for irrigation, and is not used in part due to lack of infrastructure to separate it from sewage and to store it, and partly because before it can be used, it is sent to Hidalgo down the drain.

From the water that is used in the Valley of Mexico, a part is treated and reused, including its use in the wetlands of the Heritage Zone, but most of it is taken from the Valley to Hidalgo. Being that the Valley of Mexico was all wetland area, it is in its vocation and capacity the natural treatment of water by means of natural and constructed wetlands.

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Map K.5: INFRASTRUCTURE PROJECTS

Map 6: Infrastructure projects generation process In this map it can be seen the complication, duplicity and lack of clarity in the current environment of the generation process of infrastructure projects.

They highlight the low importance and influence of the social sector in decision-making and the redundancy in governance as factors that exacerbate the need for a clear process for project generation, from its proposal to its creation and even to its operation.

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Map K.6: INFRASTRUCTURE PROJECTS GENERATION PROCESS

Map 7: Improved decision-making process on infrastructure projects and interventions in the water system This improved process is one of the results of the workshop carried out with the participation of experts in water and the Heritage Zone. It proposes to be based on real information, validated and organized in a collection of relevant information; feed this information to the relevant actors in the process; create a Master Plan that aligns the interests of the relevant actors; make decisions under a model of synergy and not zero-sum; and socialize the process to turn the proposals into real projects.

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Map K.7: IMPROVED DECISION-MAKING PROCESS ON INFRASTRUCTURE PROJECTS AND INTERVENTIONS IN THE WATER SYSTEM

MAP K.8: ORIGINAL SYSTEM

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Map 9: Key intervention points to influence the decision-making process on water infrastructure projects In the workshop it was proposed that the main points of intervention are: 1. Collection of Information, since this is what feeds all decision-makers 2. Government plans, since currently the government is the one that generates the most relevant plans, and generates a large number of plans on infrastructure mainly without much input from those outside the government 3. Bridge actors: people who can serve as liaison or influencers with the government 4. Alignment of interests: propose and seek the alignment of interests among all actors 5. Instruments that facilitate the socialization of the proposal and approval of the projects, in order to increase the level of social participation, approval and appropriation of the projects by the citizens. â&#x20AC;&#x192; Map K.9: Key intervention points to influence the decision-making process on water infrastructure projects

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K.2

Mapping institutional stakeholders

Stakeholders mapping and analysis is key process for complex projects such as the Water Resilience Plan (WRP) of the Patrimonial Zone (PZ) of Xochimilco, Tlahuac and Milpa Alta (XTMA). This section is using an adaptation of Ece Utkucan Anderson, M.Sc. and Barrett C. Brown, Ph.D. of the method used by the Sustainable Management Development Program of the United States Center for Disease Control and Prevention. The theoretical descriptions of this method of analysis cited in this document are a synthesis of Utkucan and Brown’s study.

The analysis of the stakeholder map can greatly influence the expected result and the success of any important initiative. It can be used during any stage of the project. However, carrying out an analysis of the stakeholder map during the planning stage generally helps to improve the results of the project.

Many projects receive the promise of stakeholder participation, but getting their support in reality is not always easy. Careful and thorough planning is essential to identify the right stakeholders and to ensure that stakeholders participate appropriately and effectively.

K.2.1 Integral stakeholder analysis We followed the following steps: • Step 1- Identification of all potential actors • Step 2- Categorization of actors • Step 3- Integral mapping of actors • Step 4- Determination of recruitment strategy Step 1- Identification of all potential actors Stakeholders or stakeholders are individuals, groups or institutions that may be affected by a proposed project (in a negative or positive way), or those that may affect the outcome of the project.

This is an analysis of institutional actors that may affect or be affected by the Water Resilience Plan (WRP). One of the fundamental objectives of this plan is to offer a portfolio of viable, bankable and socialized projects that contribute to restoring the water system of this Heritage Zone in a way that can cope with: 1. the effects of climate change and seismic risks 2. the socio-economic challenges that are pressing the water system, 3. and function as a lever for social, economic and environmental development.

The intervention ideas may be within the XTMA Heritage Zone or in any other area of Mexico City that has a direct influence on this sub-basin and may be of 3 types: 1. Infrastructure (blue, green or gray) 2. Information (eg, education, indicators, certifications) 3. Institutional (eg Structures, technologies and governance processes)

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Sectors that may affect or be affected by the WRP: 1. Mexican, federal, state and municipal government. In the context of the WRP ZP XTMA its function is to design, approve and execute water regeneration programs.

2. Public or private financiers, their function is to analyze the Cost-Benefit profitability of the projects and provide the necessary capital for its implementation.

3. International cooperation organizations contribute their experience in similar projects to optimize the impact of the projects.

4. Academia and consultants, are experts in social, environmental, economic and political issues related to water projects.

5. Local population are the direct beneficiaries of improvements in the water system of ZP XTMA. 6. Civil Society Organizations, groups with philanthropic interests organized to improve the water quality of the XTMA system.

7. Companies, organizations of the private industry that offer market solutions to the water problems of the XMTA system 8. Nature, these actors make up the XTMA ecosystem, such as water, trees, terrestrial or aquatic animals. In theory, they are represented by other governmental or civil society organizations, but we believe it is important to name them directly.

This is the first general list of interested parties:

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Step 2- Stakeholder Categorization The second step includes providing greater clarity by categorizing stakeholders as Beneficiaries, Supporters, Opponents, Resource Providers, Vulnerable Groups or Indifferent Groups: • Beneficiaries: Institutional actors that will benefit directly from the Water Resilience Plan for XTMA. • Supporters: Parties that may not directly benefit from the WRP, but support it by being aligned with their vision.

• Opponents: Parties that oppose the WRP due to real or perceived negative impacts.

• Resource providers: people, groups, organizations that have resources that they are willing to share for the WRP. • Vulnerable groups: Parties that could be adversely affected by the WRP, and that have no direct power.

• Indifferent groups: the parties that are under the direct or indirect influence of the PRH but do not have preference over the outcome of the project.

Step 3 - Integral mapping of stakeholders Integral matrix of stakeholder profiles: The Integral matrix of stakeholder profiles includes the following information: • Motivation to be in the project: How will the project benefit them?

• Expectations and perceived objectives in relation to the project: Do the goals and expectations of stakeholders support or are in conflict with the objectives of the project?

• Possible negative impact on the project: Are there interests of the interested parties that conflict with the objectives of the project?

• Projected use of the Project or Project Results: How will the actor directly benefit from the project and how will this affect the motivation of the stakeholders?

Matrix of Power / Influence and Interest: This matrix maps stakeholders according to the level of impact they could have on the project when considering the power and influence they have compared to their level of interest in the project through the use of a grid.

This matrix is especially useful for creating a well-informed participation strategy for stakeholders. By placing interested parties in the MPII, it is possible to determine what level of participation is best for each one.

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When filling out this matrix, the following questions can be considered: • Intent to participate in accordance with the Project Design: Does the interested actor want to participate or simply need to be informed? How much does the actor need to participate to make the project a success?

• Level of influence on the project for decision making: What is the power and status of the stakeholders in relation to the project? Does the actor have informal influence or personal connections that will affect the project? What power do the actors have over the implementation of the project or over other actors?

• Level of importance for the success of the project: What resources could the actor contribute to the project?

List of actors classified by: 1. Estimated level of power or influence 2. Estimated level of interest

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With the information that we currently have, we have selected a list of the stakeholders that we consider to have the greatest interest and power in the WRP.

Adapted from Ece Utkucan Anderson, M.Sc. and Barrett C. Brown, Ph.D. Integral Stakeholder Mapping.

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Step 4- Determination of the engagement strategy The implementation of an engagement strategy is based on the individual profiles of each stakeholder and is related to the level of interest and influence of each participant. In the method described by Anderson and Brown a â&#x20AC;&#x153;ladder of participationâ&#x20AC;? is proposed, which is a list of options, ordered from lowest to highest level of involvement and responsibility, from the exchange of information (simplest level) to active and active / inclusion-empowerment (more complex level): Participation ladder - Self-empowerment / active inclusion (ownership of the initiative) - Interactive collaboration - Functional participation: Formation of work groups - Participation through material incentives.

- Participation by Consultation - Exchange of information Integral stakeholder profile matrix (top 10)

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The analysis of WRP actors allows us to recognize the importance of each and define their best form of participation. For the case of the WRP it is a continuous process, since new actors will be added and the roles of the current stakeholders throughout the project cycle can change. This mainly responds to the fact that it is a complex project with multiple angles; that the WRP is being defined during a change of municipal, state and federal government, both executive and legislative; and that, in the process of receiving and feedback of ideas, the proponents will enrich the list of actors from their own experience.

The adoption of a comprehensive approach in the analysis of actors allows us a deeper understanding of the system of actors, and promotes a healthier and more effective participation throughout the project.

K.3 Participatory workshops K.3.1 Workshop 1 Workshop 1 was held on September 24, 2018 in the morning at the offices of the Agencia de Resiliencia de la Ciudad de Mexico (ARCDMX) with the following participants:

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The conversation involved all the participants and was generated around the following questions:

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â&#x20AC;˘ What is the relevant geographical area to do projects that solve the water issue of Xochimilco and why? â&#x20AC;˘ What results should the projects pursue and what are their metrics, to determine their viability? â&#x20AC;˘ Who are the decision makers and what process should be developed with them for the solutions to be carried out?

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Conclusions map workshop 1

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K.3.2 Workshop 2 The objective of workshop 2 is the presentation and discussion of the projects received. Nine projects were presented and the dynamics of dialogue focused on sharing the perspectives on which are the most important projects for the Water Plan of the ZP, and which are the most important improvements for these projects.

List of projects presented Proyecto

Ponente

Documento

1. Reforestation of forests, water recharge

Agua Capital

NA

2. Cienega Grande Wetland

Synergy, Agua y Energía

NA

3. Linear wetlands in streets of adjacent settlements

Synergy, Agua y Energía

NA

4. Environmental regeneration of the Tarango micro-basin

Ectágono

PDF y ppt

5. Revaluation intervention

ProNatura

PDF

6. Transition edge

Moisés Vargas

PDF y ppt

7. Reconnection of the Amecameca River with the ZP

Deltares

PDF y ppt

8. Automatic control system for water management (locks, level meters)

UAM

ppt

9. Development of green corridors linear parks, Teutle-Xoch Volcano

Deltares

PDF y ppt

10. Water Lighthouse

Fundación Faena

PDF

11. Sustainable Tepepan

Uexotl A.C.

PDF

12. Agricultural Chultón

Fundación Faena

PDF

13. Xochimilco Ecological Park

Taller 13

PDF

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K.3.3

Workshop conclusions on proposed projects

• The main characteristics of the project identified were time scale (short, medium and long term), viability (social, technical, legal and financial), scale (regional, local and site), maturity of the project (high / low), relation to climate change, and intrusiveness (very intrusive / not very intrusive).

• The main areas of impact identified were water amount, water quality, water distribution and governance.

• It is of key importance that the projects can be built and start operating in the next 6 years. Otherwise, it is possible that the next government in Mexico City will not commit enough to execute and support the projects if they are not visible within their 6-year mandate.

• The feasibility of some projects was questioned. Some of the workshop participants believe that Cinema Grande could be a complex project due to the variety of pollutants found in urban wastewater, while other participants argue that there are a multitude of successful cases of wetlands that are used for treatment of urban waters around the world. It was considered that the Tarango had a low viability due to the possible difficulty to solve the problem of transferring water physically or “on paper” from Tarango to Xochimilco.

• The discussion on the regional, local or site scale of the projects allowed the participants to understand that the proposed projects should form a balanced portfolio in different scales.

• The projects presented vary widely in their maturity or degree of development of their design. It was found that some projects were incipient designs or, rather, seemed a proposal of ideas to develop, but definitely not proposals mature enough to be close to executable.

• The projects were classifiable by their degree of intrusion, which affects the physical complexity and the difficulty in obtaining public approval for the construction. The projects considered intrusive were green corridors and street wetlands, which would require construction in dense urban areas. The projects that are considered to have a low degree of intrusion are the control system, Cienega Grande and Amecameca River, although the latter could involve construction works in a large number of municipalities in different jurisdictions.

• While costs are a key area of impact, there was not a full discussion on this issue, beyond mentioning that the cost impact of the proposed projects should be evaluated in terms of cost-benefit analysis.

• The two key areas of impact are the quantity and quality of the water.

In terms of water quantity, the most important project is Amecameca, which alone could provide 3 times the amount of water needed to solve the problem. The next one order of magnitude is Cienega Grande, which could also provide enough water to solve the problem.

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• Regarding water quality, the three key points of intervention are the illegal discharges of domestic wastewater, the agrochemical contamination of the chinampas and the quality of the treated water that is supplied to Xochimilco. The most likely project that will have a very significant impact on this indicator is street wetlands, which, despite its complexity, could solve the problem of urban wastewater discharges.

• Having a lot of good water is not enough. The water must be distributed correctly in the wetlands of the Heritage Zone. Only one of the projects addresses this issue: the control system. This project is an engineering and infrastructure project that is essential for the rest of the projects to be viable and effective, especially if climate change and subsidence change the landscape and conditions in the future.

•• Although it is a key area of impact, none of the projects addressed governance directly. It is essential that the project portfolio be enriched with at least one important project that addresses the issue of how to make decisions in complex challenges from the Heritage Zone.

The water system of ZP XTMA has a complex interaction with the immediate surroundings of the ZP and the surroundings of the Valley of Mexico. This water system affects and is affected by other systems such as the following: • Informal urban drainage, particularly that related to areas with informal settlements. These human settlements are informal urbanization, and because they do not have an authorized urban plan, do not have permits for the installation of drinking water infrastructure or drainage, so the cost in welfare for this population is high. At the same time the cost for

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the water system, particularly the canals of the Chinampa zone, is also very high. These informal settlements or slums directly discharge the black water into the channels, contaminating their water with coliforms.

Due to the natural flow of the system, they tend to disperse in all the channels. This has a perverse effect on the perceived value of the production of vegetables of the chinampas, because when exposed to the possibility of contamination of bacteria such as e.colli, the market overreacts negatively to such risk and generates a cascading effect that closes the doors to the most of the vegetables produced in the chinampas to markets of high price or high added value. Reversing this wasted opportunity is one of the key pieces of the regeneration of the chinampas zone, which we will explore later. â&#x20AC;˘ Tourism is another industry that is negatively affected by the destruction and contamination of the water system of the ZP. Pollution, water scarcity and differential subsidence are factors that make the visit to the channels less attractive for tourism, in addition to the fact that the chinampas are abandoned due to their low economic profitability, which makes the tourist visit less attractive. This is a clear example of negative feedback loops, a factor such as pollution and scarcity of water generates negative effects on agriculture and tourism, which in turn reinforce the cycle of water pollution because very few people appreciate its value in the current system and so on. â&#x20AC;˘ Water extraction for urban consumption. Before deciding to use the Xochimilco springs as a source of drinking water for Mexico City, Xochimilco had an abundant supply of clean water, which made this place a natural beauty site for tourism. With the growth of the population in the Valley of Mexico and the poor policies for infrastructure and water consumption, the government felt forced to use the clean water of Xochimilco to meet the needs of the city and return treated water to the channels as a measure to prevent drying. Today the quantity of drinking water for Mexico City is again insufficient for the same reasons as before, bad infrastructure and consumption policies, but the Xochimilco springs in particular, are exploited to their maximum capacity under the current model and cannot be a source of more water for the City or for themselves. â&#x20AC;˘ Wetlands. The lacustrine-agricultural system of XTMA is a human built wetland with a high agricultural productivity and capacity to clean water by passive biological methods at an almost zero operating cost. Also, the Valley of Mexico has water from rainfall that far exceeds the consumption needs of the metropolis settled in this valley. Wetlands constructed for the purpose of remediation of rainwater and human sanitary discharges are a viable alternative to convert this abundance of rainwater into a highly valuable asset, instead of being treated as dirty wastewater that, in addition to not being able to be used for human or productive purposes, pumping it out of the city has a high cost of operation. As a design principle, the wetlands built within ZP are a good alternative for systemic intervention because they simultaneously serve key dimensions of water

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ecology such as quantity, quality and distribution; of environmental issues as habitat for endemic and migratory species; of social dimensions, such as public parks; and as a productive input for the agricultural, tourism and real estate industry. â&#x20AC;˘ The collaborative research and design process made it clear that in the face of a complex problem such as water plan of XTMA, isolated solutions are unlikely to work, partly because each solution, inherently complex in itself, requires the environmental, social and economic benefits generated by the other solutions, and each solution is negatively affected by the problems that are generated in other areas of the system.

This dilemma is due to the existence of conflicting conditions in many of the functional relationships between projects. A case where this conflict is evident is in the project to bring water from the Amecameca River that would be cleaned by a large constructed wetland before being incorporated into the Xochimilco channel system. However, one of the main sources of water pollution in Xochimilco is the approximately 1,400 sanitary discharges from informal settlements in the southern periphery of the canal zone. A project to incorporate clean water to Xochimilco like Amecameca has a great positive impact, but this will be hindered if the pollution generated by the discharges of informal settlements is not resolved in parallel.

Due to the intricate web of positive and negative relationships that exist between the different projects for the regeneration of the water system of XTMA, we consider that instead of proposing a list of projects vulnerable to the existence or not of other projects, it is better idea propose an integral narrative or system of projects that together solve the whole problem from the beginning.

Each project has a true but partial perspective of the integral solution that the ZP needs. In this sense, a project that can function as a catalyst and integrator of the other essential projects would be the key project with which the regeneration of the ZP can begin.

The relationship between social, environmental and economic systems is inevitable. The design of interventions in the water system has to consider or even take advantage of these three systems. The case of water contamination by sanitary discharges to the Amecameca River or to informal settlements is much more likely to be solved satisfactorily if, in addition to the technological intervention on the water system, a social strategy is implemented with these communities that encourage them to take care of these water bodies.

This social strategy can be empowered through a mechanism that transfers part of the economic value that is generated by the regeneration of the water system of the ZP to the stakeholders that currently destroy value in the zone, thus aligning the interests of all the stakeholders towards a regeneration that is good for everyone.

The categories to propose projects were originally three: Quantity, quality, and distribution. Projects that increase the amount of water in the system, projects that improve the water quality of the system, and projects that make a more

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functional water distribution within the ZP XTMA system. These categories are clear and indispensable; however, they do not cover all the subtle aspects of the human dimension, which clearly is the main factor that positively or negatively affects this water system. After several conversations in the workshops and in the interviews, we decided to add the categories of water culture and water governance. In this context we define culture, as “the way we are used to doing things”, our perceptions and ways of valuing water or not, and the positive or negative effects of a healthy or polluted water system.

Many of the actions that have been taken on the water system of XTMA have to do fundamentally with the way in which we value or not the water itself and its system of relations, such as tourism, migratory birds or chinampa agriculture.

Closely linked to the issue of culture is governance, which we define here as “the structures and processes with which we make and execute collective decisions “, or simply “the way in which the groups decide and execute these decisions”. Traditionally, government, whether municipal, state or federal, is the most common form of governance, but it does not mean that it is the only nor necessarily the best for certain situations. The format of land ownership in the case of chinampas, or the dynamics of informal settlements are two clear examples of governance that function in a gray area that is not formally regulated by conventional government procedures. It is important to recognize that there are these parallel forms of governance in order to better understand the problem and thus be able to design more efficient and more effective interventions.

For the integral narrative or project system, we list below the projects that we consider to be most aligned with the regeneration needs of the water system of the ZP and that have characteristics that complement the other projects and vice versa. The criteria we have used to build this system are the following: • Redundancy: The system has at least 2 projects that perform similar functions and that have different possibilities of failing at the same time; or the project itself has a distributed design that makes it less vulnerable to failures of the entire project.

• Low Seismic vulnerability: the projects are evaluated by risk experts and seismic threats ERN, to determine the changes required to reduce their exposure to risk.

• Mitigation of or adaptation to climate change / drought / flood.

• They are complementary to each other, by themselves they are relatively vulnerable to problems of the XTMA that the project does not solve directly.

• They are designed to start giving results in the short term, that is, during the administration of the new Government of Mexico City.

• They have a high, real and directly measurable impact on the water system of the ZP. The call for project ideas was made in such a way that all kinds of projects that had some short or medium-term impact, and in any area, that had an effect on the XTMA water sub-system could be proposed. This in order to maximize the type of projects. On the other hand, for the first stage of implementation of a system of projects we are giving preference to those that have high impact and begin in the short term.

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342 A WATERA RESILIENCE PLAN FOR THE ZONE ZONE OF XOCHIMILCO, TLAHUAC ANDY MILPA WATER RESILIENCE PLAN FORHERITAGE THE HERITAGE OF XOCHIMILCO, TLAHUAC MILPA ALTA