2017 synopsis vinithra

Flood management for Chennai: A case of the Adyar River Synopsis

V.Vinithra

Guided by Sandip Patil

Co -guided by Tapan Modi

Masters of Landscape Design Faculty of Architecture

May, 2017

Abstract Water is one valuable natural resource which has been managed and worshipped by our ancestors. Southern part of India, especially Tamilnadu has no perennial rivers flowing across it. So the practice has been to store the rainwater and reduce its runoff to the sea. It is an important resource for peoples’ livelihood and is scarce and should be handled with ultimate caution. Process such as urbanization has disturbed the exiting hydrological cycle of the region. It has drastically increased the volume of runoff and reduced the time of concentration. Water when it precipitates on to the ground has to have a way to drain into the sea and finally evaporate to its original form. This natural way through which the water drains is the course of the river and each river would have a flood plain which is capable of taking the excess water during monsoon. The problem of flooding arises when this flood plain is being encroached upon. Flooding is a natural phenomena and it cannot be stopped. However the effects or impacts on the human settlements should be minimized.

Keywords: Hydrology, Disaster management, Flood Management, Water harvesting, Adyar river, Chennai floods 1. Introduction A major reason for the floods to occur was the disturbance of the natural basins which lead to the absence of way for the excess water to drain into the sea. This made the river take its original course which is now built over and hence lead to flooding of these localities.

Chennai has 3 rivers flowing through it, namely Kosasthalaiyar River, Coovum River and Adyar River.

These are the 5 major watersheds in and around Chennai.

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Source:https://www.ijirset.com/upload/2014/july/43_new.pdf

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Flood Management for Chennai: A case of the Adyar River

The study region for this study would focus on the Adyar River basin.

These are the 4 micro watersheds within the Adyar sub basin and this flood plain is stressed due to unplanned development and encroachment.

2.Aim The thesis aims to analyze the cause of the floods in Chennai in December 2015 through the study of the existing hydrological bodies in the city and to derive flood management strategies for the city, through the case of the Adyar River basin.

3.Objective 

Define a study region, such as a river sub basin and locate the areas along the river which were flooded during the disaster.

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In each of these localities, to locate the recharge zones or the sensitive areas such as marsh lands or open green spaces, which can hold water/ absorb water in emergency situations.

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Analyze the condition of such structures and the carrying capacity of it.

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In case of absence of these zones, devise methods such as rainwater harvesting or allocate spaces for small holding ponds which can reduce the surface run off.

These recharge zones would increase the percolation of the water at these points, thereby increasing the ground water table and to efficiently drain the excess rain water to the sea without flooding the urban localities.

4. Methodology 

The various natural layers of the region would be analysed such as Topography, Land use, Geology, Geomorphology, Soil.

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A watershed map at the city level as well the micro watersheds for the Adyar river basin would be prepared.

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Comparison of the Land use map over the years would be undertaken, to understand the change in Land use and depletion of the water bodies.

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Amount of peak discharge (Surface run off) from each of the sub watersheds within the Adyar River basin and the infiltration rates would be computed.

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These sub watersheds would be ranked according to a priority list (Based on maximum peak discharge)

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Flood Management for Chennai: A case of the Adyar River

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Recharge zones/ sensitive zones within each of these sub watersheds would be identified and appropriate water draining or recharging techniques would be identified for each sub basin.

5.Scope The possibility of identifying recharge zones as a solution to drain the excess water during floods could further be modified depending upon the region and used as a template for managing floods in urban areas 6.Limitations Chennai city has 5 river basins. Adyar River being the main river to carry the flood discharge during monsoon, Adyar river basin is taken as the study area and the solution would be demonstrated.

In the process of the study, if appropriate water recharging techniques are not identifiable in each of the micro watersheds, the thesis would be open to other solutions as well.

7.Case example: Mexico city In order to understand how similar urban flooding scenarios are treated in other contexts, a case of the Mexico City was taken and explored. The history of how the city originated in a closed drainage basin by filling soil on a lake and making it arable land was investigated to understand how the original drainage system was modified. Urbanization of the land lead to flash floods and rising levels of water in the region. In order to rectify the situation, large canals were built, to drain the water out of the city. This lead to the development of artificial drainage system in the city, which when fails, ultimately leads to floods again. The current position of the city stands at the city being flooded after monsoon, as well as parts of the city having water scarcity. In order to solve this case, the entire region was subdivided into various zones based on similar typologies and for each zone, sensitive water management strategies were proposed. A few of those strategies proposed for the city of Mexico in order to mitigate the floods are highlighted below.

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Flood Management for Chennai: A case of the Adyar River

Source: Mexico Water management strategy: De Urbanistan

8. Site introduction It is located in Southern part of India, Chennai, in the Northern part of the state and serves as the capital of Tamilnadu.

Chennai city is surrounded by the districts, Tiruvallur and Kancheepuram.

Initially Chennai comprised of the Chennai Corporation of an area of 174Sq.Km, which in 2011 expanded to include the surrounding areas to a total area of 426Sq.Km.

a) Climate: Chennai is part of the Coromandel Coast with a hot and humid climate.

There is no variation in seasonal temperature with an average temperature of 21 to 38 °C. It exhibits an average annual rainfall of 1300mm. b) Terrain: The Chennai city has a relatively flat terrain with contours ranging from 2m to 10m in Chennai city, with few isolated hillocks in the South West part of the city. c) Hydrology of Chennai Chennai is a city of 22 water courses, which includes 3 major rivers namely Kosasthalaiyar river, Coovum River and Adyar River, 1 canal, namely Buckingham canal, 4 reservoirs, 16minor waterways and a number of lakes. The 3 rivers are non perennial rivers and the canal is a salt water tidal canal built by the British 200 years ago, parallel to the coastline.

Chennai city receives water from the four reservoirs, out of which Chembarambakkam and red hills are the rain fed reservoirs.

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Flood Management for Chennai: A case of the Adyar River

Source: Author

Source: Author

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Flood Management for Chennai: A case of the Adyar River

9.Flood scenario The Man made factors for the floods are due to the filling of low lands and dried water bodies. Other factors include reasons such as reduction in the vents/ bridges, sandbar formation at the mouth of the river, clogging of drains, inadequate capacity of the sewers and encroachments.

One more important factor which can aggravate the flooding scenario depends on the tidal levels, Chennai being a coastal city. But during the floods in 2015, the intense rainfall period and the high tide level does not coincide, due to which the tides did not have a role in the catastrophic situation of the floods.

10. Site analysis a) Documentation of the Adyar River In order to understand the nature of the river which flooded, a photographic documentation was done to understand the transverse nature of the river as it flows from its origin till the end. The river originates from the excess runoffs from agricultural fields and finally gains more quantity after the release of water from the Chembarambakkam reservoir. It is a perennial river which majorly flows during the monsoon and carries sewage in the rest of the seasons. Its nature varies from a pure river which supports varied eco systems in the rural backdrop and changes into a river which is used for draining sewage and that which is ignored as it flows through the urbanized parts of the city. b) Study of holding ponds and its scenario during floods The aim was to study the functions and values associated with holding ponds / detention ponds in relation to its surrounding and impact of urbanization on it and its condition during floods with case examples.

Traditional water systems

Source: Author

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Flood Management for Chennai: A case of the Adyar River

A study of the traditional water systems in the site revealed that the series of water bodies were connected in the past. Each lake has a supply and a surplus channel which ultimately connects to the river. Over the years, due to urbanization, these supply and surplus channels have been broken and the original hydrological system was disturbed. Due to this, when there is heavy rainfall, the regions around the lakes and the rivers overflow and water inundates the surrounding areas.

Urbanization has had a major troll on the water bodies. Most of the water bodies that have been existing in the past have drastically depleted in size and have disappeared too. In order to understand the function of these holding ponds with respect to the context and the condition of it during the floods of 2015, a sample of 5 holding ponds were selected and their scenarios were studied.

The case examples clearly demonstrated the importance of such holding ponds which acts as a breather for the city and the ignorance of such vital structures lead to the flooding of the surrounding localities.

c) Study of natural resources in the study region

Hydrology being a major component for the study, the site was delineated based on the river and its river basin. The entire catchment for the Adyar River was narrowed down as the site for this study. The downstream of the river is the city of Chennai, where the river meets the sea and an estuarine landscape is formed. While Chennai is heavily urbanized, the upstream of the region is still not developed fully and comprises of agricultural fields and has potential to develop in a sensitive manner.

The Sub basin of the river has an area of 720Sq.Km and the Sub basin comprises of 7 micro watersheds within it and has been demarcated in the map.

The various natural layers such as elevation, slope, land use, soils, geology, geomorphology, hydrology, ground water prospects and hydrogeology were studied.

Detailed water analysis based on reports by the CGWB such as depth of water pre and post monsoon, status of ground water were analysed.

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Flood Management for Chennai: A case of the Adyar River

Source: Author

The flood situation was analyzed in depth using flood modelling softwares, from which the soil and water entities were analyzed and the level to which each of the areas got inundated with water were inferred.

Various sections were taken along the river and the terrain of the river was studied and the most critical sections of the river were identified. Finally the natural layers such as Landuse, soil, geology, lineaments and geomorphology layers were overlapped and the ground water potential map was prepared.

11. Management strategies After a complete analysis of the existing situation in the site, it clearly reveals that flood is a natural phenomena and cannot be avoided. Instead of avoiding it, ways of finding means to tackle with the extra water should be analysed. During periods of intense rainfall, with more areas being urbanized, the water does not have time to percolate into the soil, but rather flows at a faster rate and reaches the outlet. The quantity of this surface run off is also huge, which floods the site.

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Flood Management for Chennai: A case of the Adyar River

Hence the key to this issue would be: 1) Reducing the quantity of surface run off 2) Increasing the time taken for the water to reach the outlet of the respective basin, so that there is more percolation and the ground water gets recharged in the process.

This being the basic outline for the proposal, the flood index (Peak discharge/ Area) for each of the sub basins was calculated and the sub basins were accordingly prioritized according to this value. This helps in understanding the sub basin which requires the most immediate and weighted proposal.

The 4 ideas listed above were further elaborated in terms of the various ways of achieving each of those and the most appropriate way was analysed based on the conditions posed by the sub basin. Various water harvesting techniques and structures such as retention ponds, recharge shafts, bio swales, cascading ponds, Field ponds were explored as part of the proposal and based on the need of the sub basin, was proposed.

For example, the most prioritized basin in the region is that of S5. It majorly comprised of agricultural fields and is yet to fall under the hammer of urbanization. The study of the natural layers in this basin revealed that the ground water table is full up to its potential and further recharge to the immediate aquifer would be impossible. Hence in such a situation, the water should be recharged to the unconfined aquifer present below the immediate aquifer layer and hence a water harvesting structure which goes deep into the

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Flood Management for Chennai: A case of the Adyar River

soils such as a recharge shaft could be proposed. In parts which are incapable of recharging to the ground, storing and retaining water in the surface could be an option.

These agricultural fields could also transform and hold the excess water post monsoon.

Each of the field could have such field ponds at the lowest point of the grid. Post monsoon, there could be a way to drain the excess water at a constant pace out of the farms, or could be stored and used for irrigating the fields in the drought period.

Similar such solutions were explored as part of the process for the other sub basins based on the initial analysis and study of the natural layers.

12. Conclusion The world without humans, without the touch of humans is that in the most natural form. Nature has a cyclic process and it’s a vicious cycle. Also the earth that we currently see now, is something which has been formed through various process over so many years. Hence any modification that we make to the earth or its natural resources should be done in a very cautious manner. Any modification that we make for our comfort should be done considering the natural factors and should have minimal effect on it. Water is one such resource which is scarce and should be handled with ultimate caution. Process such as urbanization has disturbed the exiting hydrological cycle of the region. Water when it precipitates on to the ground has to have a way to drain into the sea and finally evaporate to its original form.

The ideal way to deal with this scenario of flooding is to make the land to act as a sponge and blur the boundaries between the two interfaces of land and water rather than treating them as separate entities. The problem was addressed through the ideology of Delaying the amount of runoff, retaining the water as surface water bodies, storing it underground through the process of infiltration and recharging the aquifers, finding means of reusing this stored water body post monsoon and finally draining the excess water from the sub basin at a constant and not fast pace, only when it is necessary. This is a long term process and requires strict guidelines at policy levels as well as regular maintenance at the design level. Flooding is a natural phenomena and it cannot be stopped. However the effects or impacts on the human settlements should be minimized.

Strategies which facilitate this are explored as part of this study and the past cases should act a lesson to value nature over human needs.

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Flood Management for Chennai: A case of the Adyar River