Salt River estuary restoration
Restoration methods for urban rivers on the example of the Salt River in Cape Town, South Africa
Master thesis by Kathrin Krause in Water and Environment, Department of Civil engineering, at the Bauhaus University, Weimar, Germany, 2020
Salt River Estuary restoration
Restoration methods for urban rivers on the example of the Salt River in Cape Town, South Africa
Master Thesis by Kathrin Krause in Water and Environment Department of Civil engineering, at the Bauhaus University, Weimar, Germany, 2020
Supervisors: Roy Holzey (BUW)
Dr. Kevin Winter (UCT)
Affidavit
I, Kathrin Krause hereby declare, that this thesis is my own unaided work, both in conception and execution, and that apart from the normal guidance of my supervisors, I have received no assistance. All sources of information are declared and true.
Kathrin KrauseCape Town, 20th July 2020
„We don’t accomplish anything in this world alone . . . and whatever happens is the result of the whole tapestry of one’s life and all the weavings of individual threads from one to another that creates something.”
Sandra Day O‘ConnorAcknowledgements
I would like to thank my family, Michael, Elsa and Emil for their unwavering support and trust in me. Giving me the time and space to explore a new terrain or better a new horizon. Thank you to my parents Birgit and Heinz for bringing me up in the believe, that anything is possible and their continued support throughout my life.
Thank you Tina, Sabina, Eva and Marion for your inspiration, the important talks, your understanding and believing in me.
The person who taught me to find treasure in drawing, if you interpret things while drawing them and at the same time make them accessible for everybody else, thank you Tarna.
A big thank you to Willem Hutten from the Cape Town Heritage and Environmental resources department for finding all these beautiful maps for me.
Thank you to my two supervisors Roy Holzey and Dr. Kevin Winter for agreeing on the topic and supporting me.
Urban estuaries are under threat all over the world, they are drained, cut off or piped. Open water bodies especially close to the sea are seen as a flood risk to properties and human life’s. The role of estuaries as transition zones between land and sea is undermined. Extreme flood events and sea level rise give estuaries a new importance of buffering floods. Urban rivers and streams in South African are often piped or canalized. The Salt River canal in Cape Town, a former lagoon, is currently concrete lined with no lateral or interstitial connection.
The hypothesis of this study is, that a reinstated estuary can protect from tidal and fluvial flooding, improve water quality, and be a biodiversity asset for the city. The historical analysis of the evolvement from an estuary to a canal, which is presented as a graphical analysis with historical maps and aerials, sets the scene in understanding the current system.
Research of methodologies of river and estuary restoration methods are discussed together with a review of international best practice projects. City of Cape Town strategies towards climate change on the case of rivers are reviewed.
Further a analysis of the catchment and the study area and findings from site visits, describe the status quo of the Salt River canal. This was constrained by the fact that critical information could not be collected due to Covid-19 restrictions, which also affected and closed CoCT operations.
Different approaches exist to describe the methods of returning an engineered system to a close to natural system. Three of these terminologies were used to develop and discuss scenarios for the canal. Detailing and evaluation of the rehabilitation and reclaimation scenarios gives a informed feed back of the restoration options and their limits for the study area..
1. Introduction
1.1 Problem area and objective
In response to a changing climate, the restoration of urban rivers plays a vital role in the protection and improvement of the urban environment. The Salt River is a man made canal in Cape Town, that drains the Salt River catchment into the Table Bay. Its short reach is situated between industrial areas, railway yards, container depots and mixeduse areas. It is crossed by several rail, highway and road bridges, that feed the Cape Town city centre. More than 300 years ago the estuary was destroyed to gain agricultural land. Today the Salt River is Cape Town’s river with the poorest water quality and is out of sight in the “backyard of the city”. With its place so close to the city, a restored river could have a positive effect on the urban micro climate in the immediate surrounding areas, the water quality in the Table Bay, the recharge of the groundwater, the fish population of the Liesbeek and Black River and the bird population of the Raapenberg and Zoarvlei wetlands. Currently the sole function of the canal comprises of directing the flood water from the catchment as fast as possible into the sea. It is understood that a restored river has a much greater environmental and hydrological potential.
1.2 Investigation framework
The Hypothesis of this study is, that a reinstated estuary can protect from tidal and fluvial flooding, improve water quality and be a biodiversity asset for the city.
A great part of the study focuses on the historical evolve-
ment of the canal from an estuary. For that research historical maps and books discussing the history of the Cape Town rivers were used and compared.
Due to the Covid-19 pandemic it was not possible to receive any current flow or water quality data, latest aerials or recent survey data for the study area and the catchments. The research requests were done on time, but were delayed and in the end fell through because of the lock down of all administrations. Initially it was planned to model scenarios for the study, but without the necessary data input the modelling was not possible. In preparation for that a student licence for the program PSWMM used by the CoCT was acquired. As a response to that the study is based on available old reports and site visits before lock down.
The main research method was to collect, combine, overlay and simplify information in a graphical way to highlight major problems. It emphasizes the current situation of a once fluvial systems now in an urban context. Water bodies are negated and regarded as flood risks only.
The scenario method was used to discuss different techniques and evaluate them. Three scenarios were developed. In case of the Salt River canal the interchange of river, sea and urban environment comes into focus. In times of protecting our cities from the effects of climate change, a balance needs to be found for natural processes to happen within the urban context and the protection of land, lives and goods.
1.3 Structure of the study
The analysis of the current situation starts with a research of the historical evolvement from an estuary to a canal, which is represented as a graphical analysis with historical maps and aerials. Some of the findings in this research are contradictory to the commonly known history of the Salt River, found in books and articles. The gradual degradation of the salt marsh and the Salt River is summarized in a juxtaposition of all the diagrams traced from the historical maps.
A review of contemporary methodologies of river and estuary restoration methods was done and completed with a discussion of international best practice projects. City of Cape Town strategies towards climate change on the case of rivers are reviewed as a way forward from current conservation thinking to a pro active restoration.
A catchment and study area analysis investigates the system and its influences correlating information from site visit and publicly available. It was not possible to obtain detailed information on flow data, water quality data, visions and restrictions for the canal, the harbour and the catchment, due to the closure of all administration during lock down.
Different approaches exist to describe the methods of returning an engineered system to a close to natural system. Three of these terminologies were used to develop and discuss scenarios for the canal. Detailing and evaluation of the rehabilitation and reclaimation scenarios gives a informed feed back of the restoration options and their limits for the study area..
Study area with catchments
1.4 The study area
“Today, the Salt River is no more than a canal that receives water from the Liesbeek River and the Black River. Although flow in the canal is still tidal, the once great Salt River Lagoon and its estuary are no more”(Cate Brown and Rembu Magoba, 2009).
The Salt River canal is a drainage canal, that was built in 1950’s to drain the Salt River marsh for urban development and the establishing of the Culemborg rail way yard. The mouth needed to move further north for the expansion of the Cape Town harbour. Today the canal is 2,4km long and has 14 bridges of various depth and heights crossing it. It is the downstream reach of the Salt River catchment, which comprises of the Vygekraal River, Elsieskraal River, Black River, Liesbeek River, Jakkalskraal, Blomvlei River, Kalksteinfontein, Bokmakierie and Kromboom River. The canal meets the Atlantic Ocean in the Table Bay in close proximity of the harbour. As an urban canal, it serves the function of transporting stormwater fast towards the sea. One inspiration for this thesis was the Book “When the Rivers �un �ry - The global water crises and how to solve it”, (Pearce,F. 2018) that I picked up at an airport on my way to a reference week in Weimar. “The rivers had been straightened and the land had been built on so much that there was nowhere for the water to go. However, you raise the levees, a river in flood will find the weakest spot and burst through. Rivers were to be allowed back onto the floodplains, and overspill areas of low-lying land would
“The liquid state is very rare in the universe, water is the only liquid that forms naturally on the earth’s surface” (Rutherford Platt,1971).
be set aside to lessen the impact of major floods”(Pearce, 2018). The book explains in drastic and simple ways, how we humans, by understanding how nature works exploited it for our own good, not thinking of our broader communities or generations to come.
The Salt River canal must have been such a project, optimizing the value of the land in terms of development and ignoring natural processes and habitat loses. Draining the wetlands and the salt marsh, changing the outflow to sea and building a sterile canal, that has no connection to its surrounding, but draining the newly created catchment straight out to sea, without riverine processes like groundwater recharge, bed formation, cleaning and pooling of water.
The chosen study area comprises of the “Old” Salt River, that stretches now over three catchments, namely the Diep River catchment, the Salt River catchment and the City catchment. It includes the Zoarvlei wetland area, that is an abandoned meander of the former Salt River and the Old Salt River canal, where the old river mouth was once situated. The Salt River marsh, that is now mostly developed as railway yards and the existing canal are all part of the study area, see illustration 5.
Today the area is turning its back side on all waterbodies and is mostly developed as an industrial area that is affiliated with the container harbour.
Above ground, the land sparkled with water. A lazy river meandered through; tinted the colour of strong tea by the ample nutrients it picked up along on its way to meet the sea. The river’s water was full of fish, and along its shores tiny insects and boisterous amphibians flourished. Overhead, birds swooped low over the water, and their chatter and song cut through the crisp air from before dawn. (Kotze, 2019)
2. Historical research
2.1 The history of the Salt River Canal until now
The history of the Salt River known widely and often referred to as in the “Rivers and Wetlands of Cape Town”, was used as base information and is described below.
“The Salt River originally comprised of the Salt River Lagoon, which received water from the Liesbeek and the Black Rivers, and possibly at some point, Platteklip Stream” (Cate Brown and Rembu Magoba, 2009). The lagoon meaning the water body behind the mouth gets referred to in the document as the salt marsh. Platteklip stream never entered the Salt River system, it was sometimes confused with the streams from Devil's Peak, which have no name. Platteklip stream flows through the CBD. “The area around the Salt River Lagoon was probably widely used by San hunter- gatherers and early Khoi nomadic pastoralists” (Cate Brown and Rembu Magoba, 2009). The richness in vegetation and different stream conditions mad it the perfect destination for fishing, hunting and plant collection. “Certainly, Jan van Riebeek’s diaries make reference to hunting game in the wilderness around the mouths of the Salt River, Black River and Diep River – with the caution to beware of hippos” (Cate Brown and Rembu Magoba, 2009). Today it is hard to imagine, that hipos lived in this part of the city.
“There is some doubt as to whether it was in fact an island at this point, as some sources suggest that the Salt River only had one mouth to the sea, and that a second mouth, ‘tweede mond’, north of the first was then dug, encircling
the land strip with water” (Cate Brown and Rembu Magoba, 2009). The research in this study confirms that doubt. The second mouth gets first mentioned on the 1860 map. “Geological evidence, however, suggests that it was highly likely that the Salt River formed a delta, and that the location of the river mouths constantly changed, shifting up and down the Table Bay coastline” (Cate Brown and Rembu Magoba, 2009). The first mouth changes location regular as shown in the juxtaposition later in the chapter. “The Company had stabilised the dunes, at great cost and trouble, to prevent the sand blowing into the town. (Cate Brown and Rembu Magoba, 2009) Dunes along the coastline were shown on the maps until 1882. After that they disappeared as a feature on the maps.
The close proximity to Cape Town and its flat topography made the Salt marsh land perfect for development except, that it was wet, with an unpredictable riverine system. Today’s main land use of industry was early established on Paarden Eiland. “Lime was produced from mussel shells washed up on the beach, and the first lime kilns were fuelled with the timber of ships wrecked on the beach. Paarden Eiland was also used variously as a leper colony and as a waste dump“(Cate Brown and Rembu Magoba, 2009). The obstruction of river crossings made it the perfect spot for isolation.
“Today, the Salt River enters the sea via a canal that follows an entirely artificial route to the sea. All that remains
of the lagoon is a small wetland area, Zoarvlei, between Paarden Eiland and the residential suburb of Brooklyn, and the wetlands at the confluence(s) of the Liesbeek and Black River” (Cate Brown and Rembu Magoba, 2009). Part of the research for this study comprised off a review of historical maps to understand the history and influences of the change from an estuary to a canal. The findings in this process are new and were not mentioned before in this detail in the known publications on the Salt River. The emergence of the Salt River Canal in maps was produced with the available maps from 1660 until 1958. 1958 marks the completion of the transformation from estuary to canal. The canal hasn’t changed since then.
All maps were scaled to the same scale and the visible aquatic features of the study area were traced for the study area, to achieve a diagram for comparison. The overlay maps are explained in terms of the river name, morphology, its crossings and other relevant information, The diagrams of the aquatic features are juxtaposed on a spread in historical order.
2.2 The emergence of the Salt River Canal in maps
Historical scaled map overlay within study area
Description of
• Name on the map
• Morphology
• Crossings
• Tributaries
• Other charackteristics
Original map source
1660 (drawn in 1920)
• Name Sout river and Sout vlei
• Wide river bed with an almost continuous width along the delta.
• No visible crossings.
• Liesbeek and Black River are separate tributaries.
• One farm along the banks and a look out point.
1780
• No name mentioned
• Meandering river with sand banks and marshy areas, that are temporarily flooded, unnatural delta form at the mouth.
• No crossings, multiple pathways next to river.
• Liesbeek and Black River are separate tributaries.
• One big building close to Salt marsh.
• Stream from Devils Peak tributary to Salt marsh.
1787
• No name mentioned
• Meandering river with sand banks and marshy areas, that are temporarily flooded.
• One dotted crossing, multiple pathways next to river.
• Liesbeek and Black River are separate, not clear.
• One big building close to Salt marsh.
• Stream from Devils Peak reaches the sea directly.
1789
• Name Soute river. Olede mond mentionend but not visible.
• Differentiated river bed marsh areas and sand banks are shown.
• No visible crossing over the Salt River, but over the tributary from Black and Liesbeek River.
• Dune landscape along the shore.
• Stream from Devils Peak reaches the sea.
1794
• Name from now on Salt River.
• Differentiated river bed with sand banks.
• Small crossings over the Salt River itself and pathways next to it. Road to Drakenstein highlighted.
• Liesbeek and Black River are one tributary with an old arm from the Liesbeek.
• Stream from Devils Peak not clear.
Source:
1806
• Differentiated river bed Salt marsh area and other sand banks.
• Many visible crossings.
• Liesbeek and Black River are one tributary.
• Stream from Devils Peak not on map.
Source:
1860
• Differentiated river bed with Salt marsh area and oth er sand banks. First time the second mouth (at Milner ton) is shown.
• No visible crossings over the Salt River itself.
• Liesbeek and Black River are one tributary and con strained by the Vortrekker road crossing.
• Stream from Devils Peak disappears above main road.
Source:
Source:
1865
• River bed with big Salt marsh area and other sand banks. Second mouth shown. Salt marsh restricted to the south with Vortreeker road and Rail way line. Small inundation north of the mouth.
• Crossings over the Salt River. Several farms established along the river.
• Another Liesbeek arm ends in a wetland south of Vortrekker road/ Railway line
1901
• River bed separated from Salt marsh area. New rail way line crossing the river and restricting the western banks. Second mouth clearly shown.
• Liesbeek and Black River are one tributary and con strained by Vortrekker road crossing.
• Salt marsh restricted by further development to the south.
• This must be not a situational map, but a future development plan.
1902
• Differentiated river bed with salt marsh area and other sand banks. Second mouth not shown or closed.
• The new railway crosses a big part of the salt marsh with a wooden bridge, the south of the second mouth and constraints the western banks. Another road crosses the marsh north to south.
• Liesbeek and Black River are one tributary and constrained by Vortrekker road crossing.
• This plan compared to the one before, shows the actual situation.
1934
• River bed cut off south of second mouth with spliced Salt marsh area. New earth channel for drainage of Liesbeek and Black River.
• New Marine Drive crossings over the Salt mouth itself.
• Wetlands appear next to the cut off river.
• Railway built over part of salt marsh.
• Liesbeek and Black River are one tributary.
1937
• River bed cut off south of second mouth. Salt marsh area disappeared completely. New earth channel for drainage of Liesbeek and Black River.
• New Marine Drive crossings over the Salt mouth itself.
• Wetlands disappear next to cut off river.
• Railway built over part of Salt marsh.
• Liesbeek and Black River are one tributary.
1945
• River bed cut off south of second mouth. Salt marsh area disappeared. New earth channel for drainage of Liesbeek and Black River.
• New Marine Drive crossings over the Salt mouth itself.
• Wetlands disappear next to the cut off river.
• Railway built over part of Salt marsh.
• Liesbeek and Black River are one tributary.
1953
• Name Salt River Canal.
• New canal built for drainage of Liesbeek and Black river, creating a third mouth. Earth channel to first mouth is cut off. Former river bed even more reduced.
• 13 bridges built over the new canal.
• All remnants of the Salt marsh disappeared.
• Liesbeek and Black River are one tributary.
1958
• Name Salt River Canal.
• New canal creating a third mouth. Earth channel to first mouth is cut off, reduced and transformed into a concrete canal.
• 13 bridges built over the new canal.
• All remnants of the Salt marsh disappeared.
• Liesbeek and Black River are one tributary.
2.3 Juxtaposition: Salt River becomes Salt River CANAL
2.4 Findings from the maps
The Salt River estuary formed the downstream end of the Diep river catchment. Liesbeek and Black River were only tributaries into the estuary. Due to sandy soil conditions and the flat topography, the river had space to braid between sand banks. A small mouth to the Table Bay let in the tides. The bottom end of the estuary was an extensive salt marsh area that shifted, shaped and fed by fresh- and salt water. The river meandered close to the shore line coming from the North. At one point that is today the mouth of the Diep river the river was very close to the sea. This bend is shaped in a way that makes it seem possible, that during high floods the water found its way to the sea at this point and was later at low flow closed again by tidal sedimentation from the ocean. This would explain, why it’s shown in some maps as the “Oude mouth”, the old mouth. Development along the river must have started early, with the fluvial land rich in plant and animal species. The first colonists tried to make the river navigable to travel inland and found it impossible as it was simply too shallow, with too little water during the dry season. On the 1794 map we can see the remnants of human intervention close to the mouth. Channels in rectangular shapes were built to change the flow in or out..
During the 18th and 19th century farms and pathways developed along the river bed with the occasional crossing. The first big restriction for the delta, was the formalisation of the inland road, now called Vortrekker road and the parallel railway line. They both formed a dyke south of the Salt marsh. The dyke restricted the Black and Lies-
beek river to one channel or to one bridge crossing. With the estuarine land being so flat and close to the city, the pressure of developing this land became very high in the beginning of the 20th century. The layout map from 1901 indicates the development of Paarden Eiland.
The development of the harbour and the railway in the 1930’s was a big development threat for the salt marsh land. One map states “Salt River Marsh - All land between the Railway, Salt River and the Sea within Woodstock Municipal limits are reseved for Railways.”. The land needed to be drained and the flow somehow regulated.
Engineering solution was, to make the temporary second mouth the regular mouth for the Diep river and drain the Black and Liesbeek river through the first mouth. The Salt River was cut off south of the second mouth and a new earth channel was built for the Black and Liesbeek river. As a result the catchment was divided into the Salt River catchment and the Diep river catchment.
The piece of river in between, now known as Zoarvlei wetland, was abandoned and drained only stormwater from the surrounding new developments.
The first river mouth was in the way for the extension of the harbour. In the 1950’s a new concrete canal was built. This canal we know now as the Salt River Canal. The Salt River canal formed the third mouth. With the building of the harbour extension in the 1970-1980’s the first mouth became piped from the harbour as a result. A concrete canal was built next to the highway to drain the surrounding land and the stream from Devil's Peak. Zoarvlei, the
abandoned river segment, now a protected wetland, gets more and more restricted by development and is today the only remnant of the old Salt River. It drains currently on the southern end through a pipe into Table Bay and on the northern end into the Diep River. This connection to the Diep River results in the Zoarvlei being part of the Diep River catchment.
A second pipe that drains into in the harbour is the former outlet from the Salt River Power Station. The coal power station was situated next to the river mouth until it was demolished in 1980’s. Today stormwater from the industrial area drains through this pipe. The construction of both pipes can be seen on the aerial view of the Ben Schoeman dock construction in 1973, see next page.
2.5 Conclusion
• The Salt River was the down stream part of the Diep River. Liesbeek and Black River were tributaries.
• Historically the Diep River catchment combined the Salt river catchment with parts of the City catchment.
• Presently the Salt River constitutes its own catchment.
• The first Salt River mouth exists as a pipe into the harbour, allowing for a tidal influence on the Old Salt River Canal.
• The second mouth was most likely man made, or it resulted from a major storm event, that breached the dunes into the ocean.
• Seasonal changes of high and low flow are captured in the maps. The exact dates of the the maps are not known. In the maps the seasonal changes are represented by a bigger water body without sand banks for winter condition with high flow. A braided river with many arms and sandbanks indicates a low flow situation like in summer.
• The Salt River was a river dominated estuary with a changing mouth position.
• The urbanization of Paarden Eiland and the harbour expansion caused the modification of the Salt River to a canal.
1 South from Diep River mouth;
2 Salt River mouth;
3 South from Salt River mouth;
4 Salt River power station from the sea;
5 Salt River Canal and Zoarvlei;
6 Otto Du Plessis after construction;
7 Salt River mouth;
Source: UCT Digital collection, Special collections, Maps
3.1 Methodologies of river restoration
3. Methodologies
3.1 Methodologies of river restoration
3.1.1 Restoration definition
River rehabilitation, restoration or reclaimation, how do we call it? Different terminologies exist to describe the methods of returning a engineered to a close to natural system. Four methods explained here are Restoration, Reclaimation, Rehabilitation and Daylighting, they are often confused and interpreted as restoration.
“Restoration is the process of returning an ecosystem to a close approximation of its former condition. The restoration process reestablishes the general structure, function, and dynamic self-sustaining behavior of the system. However, it may not be possible to recreate the system exactly because the surroundings and stresses may not be the same as in the predisturbance period.
Reclaimation is a process designed to adapt a resource to serve a new or altered use. This could mean a process to convert a resource into a productive use. For example, a restored stream can provide water for irrigation or other nonpotable uses, including wading and other types of recreation. Other examples are reclaiming urban brown-
fields, removing freeways and reclaiming surfaces covered by pavements for ecological uses, and converting a mining excavation pit into a lake.
Rehabilitation puts a severely disturbed and/or partially irreversibly modified and damaged resource back into good working order. It is often used to indicate improvements primarily of a visual nature or to an ecological status less than that of a natural system. The ecologic potential of rehabilitation must be determined by a study.
Daylighting restores to the open air some or all of the flow of a previously covered or converted and buried river, creek, or natural stormwater drainage. Daylighting reestablishes a waterway in its old channel wherever feasible, or in a new channel threaded between the buildings, streets and roads, parking lots, or playing fields” (COMMITTEE ON RESTORATION OF AQUATIC ECOSYSTEMS, 1992; PINKHAM, 2000) cited in (Novotny, Ahern and Brown, 2010).
“Water is not a commercial product like any other but, rather, a heritage which must be protected, defended and treated as such” (European Parliament, 2000).
3.1.2 The “Leitbild” method
The European Water Framework Directive (WFD) introduced 2000 a new focus in river management by putting the protection and restoration of the aquatic environment as a key issue on the water policy agenda. “The objective of achieving good water status should be pursued for each river basin, so that measures in respect of surface water and groundwaters belonging to the same ecological, hydrological and hydrogeological system are coordinated”
(WFD,2000)
The “Leitbild” method mainly applied in Germany as a river status’ assessment as well as for restoration planning relates to the “natural potential” of a river ecosystem in the absence of “human disturbance”. In Germany a set of maps is categorising each river basin and reflects on the parts with a very good ecological status, which can be used as references for the restoration objective.
The WFD promotes integrated river basin planning in order to achieve ecological objectives, and large-scale restoration activities can play a key part in enhancing the quality of river systems from the source to the sea. The characterisation of river basins (including analysis of pressures, impacts and economic analysis) is done in each country. The ecological and chemical status of surface waterbodies are assessed in:
Ecological Status:
• Biological quality (fish, benthic invertebrates, aquatic flora)
• Hydro morphological quality such as river bank struc-
ture, river continuity or substrate of the river bed
• Physical-chemical quality such as temperature, oxygenation and nutrient conditions
Chemical Status:
• Chemical quality that refers to environmental quality standards for river basin specific pollutants. These standards specify maximum concentrations for specific water pollutants. If even one such concentration is exceeded, the water body will not be classed as having a “good ecological status”
Deciding which particular ecological status or potential class is assigned to a water body depends on whether the quality element worst affected by anthropogenic alterations matches its normative definition for that class. The Directive’s strong emphasis on ecology as opposed to the chemical water quality, does not mean that ecological status was introduced as an alternative to measuring water quality or that it can be applied with the same thinking. An artificial or heavily modified waterbody can only achieve a good ecological potential instead of a good ecological status.
This method relies on the categorization of the river basins and the reference status as a guiding tool. South Africa has no similar system yet, therefore could this method not been used for this study.
3.1.3 Urban sustainability and River restoration
“Water is at stake especially in cities where population growth and urban development cause density, lower permeability and increase flooding risk. In modern cities, water infrastructure is often designed as a linear system, a collector to take rainwater and waste out of urban environments as fast as possible. This results in extreme impoverishment of its ecological vital functions” (Sabbion, 2017).
“During the last century, almost everywhere, waterways have been heavily regimented and artificialized to maximise space for urban growth. In some cases, waterways have been completely fragmented, and have become mere collectors of city waste, which transect deprived and residual urban areas. Channelization and culverts target river flow control, while width restriction seeks to obtain developable land for settlement and transport infra- structures with serious consequences for hydrogeological, environmental, and landscape systems. One of the fundamental functions of a healthy hydrological system is to retain water, allowing for stormwater runoff regulation and groundwater redistribution and recharge” (Sabbion, 2017).
“Water management should undergo a radical reorganisation, especially within urban environments, where an intervention is most needed to save water resources and to control the biological and chemical status of flows. The concept of city as a self-sufficient system involves new redevelopment projects, based on a more balanced, efficient and competitive use of resources. It is also crucial to improve the urban environmental and ecological herit-
age” (Sabbion, 2017)
“There are several cities, around the world, where trans-disciplinary conservation of urban ecosystem services and water management are the foundation of urban design. GBI (Green-Blue _Infrastructure) incorporation not only enhances the capacity of these cities to supply water and prevent flooding, but also provides health benefits and a better quality of life. This approach has been adopted over 20 years ago in Portland (Oregon): here catchment-scale green infrastructure, green streets, flood management, river restoration and wastewater services are integrated to deliver improved flood management and water supply” (Sabbion, 2017).
“A very interesting aspect of urban water management is the GBI redesign in cities and surrounding areas, involving ecological corridors and suburban contexts. Restoring urban water systems can be achieved by planning new systems of infiltration, retention, evapotranspiration and water control, through the selection of vegetation and the correct application of compatible construction techniques This type of recovery addresses the specific hydrological problems of each city, without neglecting formal and spatial dimensions” (Sabbion, 2017).
Techniques and strategies for river restoration are:
• Re-meandering
• Buffer strip creation and riparian revegetation
• Flood embankment removal
• Culvert removal
• Weir removal
• Reconnecting old channels
• Bank protection removal
• Restored flood plain forests
“There is widespread agreement that urban areas should adapt to climate change, reintegrating the natural water-cycle, and creating water sensitive cities. It is globally recognised that returning rivers and catchments to a more natural state is a key strategy to improve the quality of the environment and biodiversity. River rehabilitation provides an opportunity to restore ecosystem services that have been degraded and lost. Additionally, the natural functions of watershed settlements become more balanced. River rehabilitation regards biodiversity conservation (supporting); sustainable flood management (regulating); physical habitat quality restoration (regulating); fisheries enhancement (cultural/ provisioning); pollution control (regulating); and also cultural awareness (recreation and amenity)” (Sabbion, 2017).
3.1.4 Process based river restoration
“The concept of process-based river restoration has been gaining importance. This integrated social-ecological approach adopts holistic techniques to address root causes of ecosystem degradation and establish a new balance between socio-economic needs and sustainable watershed management. In addition, it focuses on avoiding anthropogenic interference in natural processes, enhancing the resilience of river–floodplain ecosystems to future disturbances. This ensures that restoration plans and actions support sustainable recovery without requiring continual human intervention and maintenance. The aim of process-based restoration is to “re-establish normative rates and magnitudes of physical, chemical, and biological processes that create and sustain river and floodplain ecosystems. This process is based on the analysis of multiple causes and socio- economic contexts at local, regional, and national levels to maximise the restoration benefits over short, medium and long-term timescales” (Sabbion, 2017).
Four process-based principles that ensure river restoration will be guided toward sustainable actions:
(1) restoration actions should address the root causes of degradation,
(2) actions must be consistent with the physical and biological potential of the site,
(3) actions should be at a scale commensurate with environmental problems, and
(4) actions should have clearly articulated expected
outcomes for ecosystem dynamics. Examples of the processes include:
• erosion and sediment transport,
• storage and routing of water,
• plant growth and successional processes,
• input of nutrients and thermal energy, and
• nutrient cycling in the aquatic food web.
“Process-based restoration focuses on correcting anthropogenic disruptions to these processes, such that the river-floodplain ecosystem progresses along a recovery trajectory with minimal corrective intervention. Restoration of critical processes also allows the system to respond to future perturbations through natural physical and biological adjustments, enabling riverine ecosystems to evolve and continue to function in response to shifting system drivers (eg. Climate change).
Because process restoration focuses on restoring critical drivers and functions, these actions will help avoid common pitfalls of engineered solutions, such as the creation of habitats that are beyond a site’s natural potential, piecemeal stabilization of habitat features, and restored habitats that are ultimately overwhelmed by untreated system drivers” (Beechie et al., 2010).
3.1.5 The special case- Estuarine restoration
Estuary restoration is the specialized method for downstream systems.
Constructional modifications to estuary land forms have comprised:
• land reclaimation – by constructing or extending defence works and draining the land thereby isolated from tides and floods;
• making road or railway crossings of the estuaries on bridges or causeways and of adjacent land on embankments, some doubling as coastal or river flood protection banks.
Reclaimation for settlement has been achieved mainly by drainage within embankments created to protect land against tidal or river flood inundation. This has been at the expense of tidal or wetland flats.
“The tide reigning before a coast spreads upwards in rivers with identical periodicity, but the dissipation of the energy gradually reduces the amplitude and the tide ends up being imperceptible, with this being the boundary of the maritime part of the river.
In an estuary and even more so in a river, the high tide is of much shorter duration than the low tide; it may even be that the ascent of the water up-river is almost instantaneous, this is the moment of the tidal bore, breakwaters that enclose the whole bed of the river which in turn also surges quickly upwards” (Kennish, 2016).
“Landscape ecology concepts applicable to estuarine habitat restoration include minimum area, shape, and corri-
dors:
• Minimum area refers to the minimum area or size of a project required for an estuarine habitat to become fully functional” (Kennish, 2016).
• Shape refers to the shape of a patch or contiguous habitat that affects the types and number of species in the patch. Species can show preferences for edges or interiors of patches. As a patch increases in area, it usually develops a distinct interior and edge” (Kennish, 2016).
• Corridors are narrow strips of habitat that differ from the habitats on either side. Corridors can form very important protected routes of ingress and egress to habitats for species. They may also function as habitat for some species or as filters of disturbances (e.g., riparian buffer zones)” (Kennish, 2016).
Hydrologic restoration:
“Natural hydrology is necessary for restoring functional coastal marshes, and this is often accomplished by excavation of fill” (Kennish, 2016).
“Removal of anthropogenic structures that hinder the hydrodynamic and geomorphic processes in tidal marshes has been applied as a management practice throughout the United States for decades” (Kennish, 2016).
“Tidal restoration designs often include the plans for tidal channel development. This development can occur by means of passive formation, active creation, or a combination of both. Channels are passively or voluntarily created following a particular action that restores hydraulic and sediment processes to a tidal marsh. Hydraulic geometry
and other indices provide useful guidelines for physical restoration and creation of estuarine tidal channels but do not clarify the ecological consequences of channel form” (Kennish, 2016).
“The use of seedlings continues to be an effective approach for establishing diverse tidal marsh vegetation in smaller restoration projects. Planting marsh vegetation offers managers an alternative to natural recolonization; however, this action may be cost-prohibitive for some restoration projects. Recovery and overall project success are largely dependent on-site conditions such as marsh elevation, hydrodynamics, and the presence of nearby source vegetation for natural recolonization. Vegetative recovery can be influenced by these criteria, yet it is necessary for planners and resource managers to understand that the success of natural recolonization varies at multiple spatial and temporal scales.
Because of the substantial losses and growing realization of the importance of estuarine habitats to endangered and threatened species as well as ecosystem services for humans, restoration of these habitats has been actively pursued for about the past three decades in the United States and other countries. The science of restoration has shown that restoring habitats requires the ecosystem (i.e., landscape) within which the habitats occur to be in relatively good condition. Restoration actions can range from simple restoration of hydrology to very active, complex, and expensive activities, including removal and reworking of sediments and elevations, removal of invasive species,
and remediation of contaminants. Habitats can take from a few years to centuries to fully develop depending on the type of habitat and the conditions in the landscape. (Kennish, 2016)
3.1.6 Flooding a design informant
With flooding occurring more often and with greater intensity the following points are important to address in a flood protection design.
“Integration of river and coastal flood defences with new areas of public realm;
Design defences to promote biodiversity and create wildlife corridors;
Provide additional flood storage capacity by creating new wetland areas;
Maximise visual and physical connections to rivers, coasts, wetlands etc. to increase the amenity and commercial value of new developments;
Provide safe refuge on site for the duration of a short flood by designing low carbon self sufficient schemes with their own renewable energy, water harvesting and purification
Enhance the public and private realm with high quality robust landscape designs which promote water conservation and surface water management” (RIBA, no date)
The knowledge of the last decades shows a success in designing with the flood rather then against it. Which means to give back space for flooding.
3.1.7 Water sensitive Urban Design
Water sensitive Urban Design (WSUD) should be part of any river restoration- rehabilitation or reclaimation project. ”Living with water sensitive urban design according to the Cooperative Research Centre for Water Sensitive Cities (CRCWSC) allows residents to interact with the urban water cycle in ways that enhance and protect the health of waterways and wetlands, the river basins that surround them, and the coast and bays. It mitigates flood risk and damage, and creates public spaces that collect, clean, and recycle water” (Kotze, 2019).
The importance of WSUD is recognised, but not further investigated in this thesis. In the RSA context, WSUD has the potential to: mitigate the negative effects of water scarcity; manage and reverse water pollution; develop social equity; develop intergenerational equity; increase sustainability; and develop resilience to natural disasters and climate change within water systems.
3.1.8 Conclusion
Contemporary restoration methods on urban rivers are a challenge in finding space for fluvial processes. They are focusing on an holistic approach of flood protection, developing river morphology and address biodiversity issues. In the table across is a summary of restoration measures that enable fluvial processes., Which was adapted to suit the measures necessary for the restoration of the Salt River canal.
Category Restoration measure Interventions/Techniques
Remove bed and bank stabalization
Restructure riparian zone
River morphology
Lateral connectivity
Initiate river reach specific morphology
Initiate dynamic aquatic/terrestrial transition zone
Small-scale river widening
Excavate/reestablish natural riverbed
Reconnect floodplain and natural retention areas
Initiate floodplain habitats
Longitudinal connectivity Remove migration barriers
Interstitial connectivity Remove canal lining
Flow management
Increase residual flow
Increas dynamic flow
Temperature management lower temperature
Reopen sediment sources
Lower the floodplain area
Create aquatic floodplain
Initiate floodplain vegetation
Deconstruct barriers
Decontruct canal lining
Increase residual flow
Increas dynamic flow
Increase in shading vegetation
Remove lining
Sediment management
Sediment input
Alter land use
Open riparian zones and floodplain areas
Add sediment to the river
Create buffer zones
Land use
Reclaim land for the river
3.2 Review of international case studies
3.2.1 Revitalisation of the Aire, Switzerland
In this project at the Aire in Switzerland a canal was kept in place and instead a new stream was introduced parallel to it. "In place of traditional notions that ‘stability’ was desirable in ecology, there is increasingly a recognition that disturbance (including bank erosion and deposition) is not only inevitable in many systems, but essential to their regeneration. Dynamic fluvial processes create the complex habitats needed by native species, so the most effective ecological strategy is to set aside a zone within which riverine processes can function without conflicting with human uses, often termed the espace de liberté. This approach works only where there is sufficient stream power and sediment supply, and where there is relatively little urban encroachment up to the banks. As we understand better how fluvial ecosystems function, it is increasingly clear that the physical processes of erosion, sedimentation, and channel migration do a very good job of creating high quality habitat. The most effective approach to restoring rivers will often be for us to stand aside and give the river its space" (Holzhausen, 2012).
In this project next to the canal a new meandering channel with erodible banks was created through establishing an constraint area and letting the stremflow of the water decide its shape. Two dykes reduce downstream flood risk by giving space to excess flood waters.
Concrete banks and bed were removed to revitalize dynamic stream flow processes, in parts the adjacent floodplain was lowered. A 80m wide buffer zone converted agricultural land into riparian zone and protects these.
"Retaining the canal as a human artefact, to serve as a trail and parkland, recreating the meandering channel to the south of the canal. The canal will be partially filled in, its north bank developed as a continuous trail, with recreation al opportunities provided in the partially-filled canal. Human use will be encouraged in the canal and trail, thereby taking pressure off the restored channel and riparian corridor" (Holzhausen, 2012). So the fluvial system was revitalized with the help of a new dynamic channel and at the same time flood control was secured through the existing canal and additional dykes.
3.2.2 Resist Delay, Store recharge
This project was chosen in a competion for the urban water strategy for Hoboken, New Jersey, after Hurricane Sandy. "This project is a multifaceted approach to managing stormwater from flooding and surge along the Hudson River. It explores using hard infrastructure and soft landscape—including permeable paving, rain gardens, and rainwater storage—for coastal defense. The project’s comprehensive approach has four integrated components: Resist: a combination of hard infrastructure (such as bulkheads, floodwalls and seawalls) and soft landscaping features (such as berms and/or levees which could be used as parks) that will act as barriers along the coast during exceptionally high tides and/or storm surge events. These measures are focused at the two main breach points of water during Hurricane Sandy.
Delay: urban green infrastructure designed to focus on slowing stormwater runoff throughout the region using a combination of public and private amenities.
Store: green and grey infrastructure improvements, such as bio-retention basins, swales, and green roofs, intended to slow down and capture stormwater, and complement the City of Hoboken’s existing Green Infrastructure Strategic Plan. Discharge: enhancements to Hoboken’s existing stormwater management system to reduce combined sewage overflow and manage flooding" (http://www.rebuildbydesign.org/our-work/all-proposals/winning-projects/ nj-hudson-river-project-resist-delay-store-discharge).
3.2.3 Restoration of Skjern River and its valley
During the period 1999–2002, 19 km of the Skjern River and 22 km2 of the cultivated river valley were restored into a meandering river, wetlands, meadows and shallow lakes. The restoration followed a channelisation of the river and an artificial draining and reclaimation of the river valley for agriculture in the 1960s. In 1987, the Danish Parliament decided to carry out the restoration to reduce the nutrient loading to the sea and enhance the re-creational value of the river valley. A comprehensive monitoring programme was initiated to follow the short-term ecological consequences of the restoration.
The river valley changed from agricultural fields into meadows with a rapid succession in plant species. The retention of nutrients in the restored area follows the extent of flooding and amounted to less than 10% of the total riverine transport. The new river was rapidly colonised with plants and invertebrates from upstream reaches, and rare species in the project area generally seem to thrive under the new conditions. The new shallow lakes and the meadows caused a minor increase in the predation of salmon and trout smolts because of the increased populations of fish-eating birds.
3.2.3 Mill River
Another project initiated after Hurricane Sandy is the Mill River project which seeks to alleviate flooding in the first place. "Like all tributaries in the region, the Mill River is a product of the glaciers that formed Long Island. For thousands of years, the Mill River flowed unimpeded into the South Shore Estuary (Back Bay), establishing a vital link between marine and upland habitats. Migratory fish moved into and out of the river, providing an important forage source for countless species and helping to drive the region's coastal ecosystem.
Beginning in Colonial times, the flow of the Mill River was harvested to power gristmills. The original dam at Smith Pond was constructed to power a mill. Later, in the late 19th Century, significant impoundments were established in the Mill River's upper reaches as part of the Brooklyn Water Works project, an elaborate effort to satisfy Brooklyn's rapidly growing water needs. These impoundments became the basis of Hempstead Lake State Park. As communities emerged, storm water and sewer systems developed with outflow pipes entering the river and roads with rail lines crossing the river.
With increasing populations and development, Mill River communities have been more susceptible to flooding. This became most evident during Superstorm Sandy, when Nassau County was hit with rain and a tidal surge of up to 18 feet. Public and private infrastructure along the river were damaged including more than 7,600 homes, as well as bridges, businesses, parks, roads, schools, and a waste-
water treatment facility at the entrance of the Bay. Inland communities regularly experience flooding due to heavy rainfall (such as experienced during Hurricane Irene and other more frequent storm events) exceeding the carrying capacity of the existing storm water infrastructure.
LWTB developed a series of projects to address a variety of flooding sources throughout the project area; in a comprehensive, practical and feasible manner.
Storm water Retrofits: The State will strategically install green infrastructure including, but not limited to: drywells, bioswales, permeable pavement, tree planting, and select bioretention and infiltration interventions throughout the project area.
Pond Drainage Improvements: will improve water quality, enhance recreation, restore the ecological system to promote native aquatic species and expand the hydraulic surge capacity of the pond, by reconfiguring the bottom of the pond. Sedimentation has reduced the hydraulic capacity of the pond to absorb storm water first-flushes and altered the ecology to favor invasive species. Project elements anticipated include shoreline stabilization, recharge basin, permeable pavement parking lot, a fish ladder, and either rehabilitating or replacing the existing weir. Dredging, wetlands restoration, landscaping (including tree planting) and construction of greenway paths will also be evaluated.
Coastal Marshland Restoration: LWTB will restore, protect and/or enhance marshlands in the Back Bay at the
mouth of the Mill River. The project will be designed in a manner to slow tidal storm surge velocity and enhance habitat for native species, including birds, fish, and benthic species" (https://patch.com/new-york/rockvillecentre/long-road-storm-resiliency-working-sandy-recovery)