Salt River Estuary Restoration Part 2

Part 2

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

3.3 The National Water Act

The National Water Act, provides the legal framework for the effective and sustainable management of the South African water resources. Published in 1998 the aim of the NWA was to reform the past discriminatory laws relating to water resources. Central to the National Water Act is a recognition that water is a scarce and precious resource that belongs to all the people of South Africa. The NWA recognises water resource management to achieve the sustainable use of water. The act aims to protect, use, develop, conserve, manage and control water resources as a whole, promoting the integrated management of water resources with the participation of stakeholders. Not addressed in the act is the vision on how to rejuvenate impaired river systems. Reference is made to the Catchment agencies as the body “that manages water resources within its defined watermanagement area according to its catchment management strategy” (Department of Water affairs and Forestry, no date).

No mention or vision of the restoration of impaired water bodies was found within the act. The Salt River falls under the Berg River catchment, which includes all Cape Town’s catchments. During this study it was not possible to receive information about future visions or strategies for the catchment, therefore a analysis of relevant Cape Town strategies, which address the upgrading of urban rivers was done in the following section.

3.4 Strategies of the City of Cape Town supporting river upgrade

The City of Cape Town developed in recent years a package of strategies for development with the aim of sustainable development in a resilient city. The overarching strategy being the Integrated Development Plan, which sets the scene in its implementation plan for the environmental, the resilience and the water strategies as well as the Coastal management framework.

3.4.1 Cape Town IDP

The Integrated Development Plan (IDP) gives full effect to the Organisational Development and Transformation Plan and sets out the cities development priorities over five years (2017-2022).

The last years with extreme weather events have deepened city challenges, resilience has emerged as an important urban concept. The cities location on the Atlantic ocean with most of the development low lying or close to the sea, sea level rise poses a major threat to the city. The changing rainfall patterns, temperature and wind extremes have alsready affected the city negatively. These changes can be seen as a result of climate change.

“The City is making a pledge to a concerted effort to improve its resource efficiency and security, as well as to address factors that affect climate change. this includes climate change mitigation initiatives to improve air quality, the diversification of its energy mix, as well as adaptation measures such as conserving biodiversity” (CoCT, 2017).

“Resource efficiency and security of Cape town’s environment, including its natural resources, landscapes, ecosystems and green infrastructure, forms the basis of the city’s economy and plays a crucial role in building resilience. Natural resources include the provision of basic resources such as water and renewable energy (sun and wind), but also ecological services such as air and water purification, flood prevention and mitigation, coastal buffers, the recharge of aquifers, soil production, absorption of waste and pollution, pollination, and carbon sequestration” (CoCT, 2017).

Out of the implementation plan, the following prgrammes are relevant to either the site or the concept of river and wetland rehabilitation in Cape Town.

• Climate change programme, “adapting climate change project due to the considerable risks that the effects of climate change pose in an urban environment, particularly to vulnerable people and communities, the City needs to strengthen Cape town’s capacity to adapt and build resilience to the economic, social, physical and environmental impacts of climate change. The City will aim to reduce Cape town’s carbon footprint in order to contribute to the global reduction of greenhouse gas emissions” (CoCT, 2017).

• Invasive species management “project of the city will be rolled out on all City land across the metro. in line with the regulations of the national environmental management: Biodiversity act (NemBa), act10 of 2004, this will see the identification, control and management of existing

as well as new and emerging invasive species, preventing them from spreading and building viable populations” (CoCT, 2017).

• Integrated coastal management “is managing the sensitive dune systems along Cape town’s coastline. as they provide natural buffers against storm surges, the retention of functional coastal dune cordons is a priority for the City in reducing the impacts of climate change” (CoCT, 2017). This programme is described in more detail further down.

• Green infrastructure project, the main focus will be the development of a green infrastructure plan covering the entire City of Cape town area. this plan, scheduled for development in 2017/18, will serve as a planning and management tool for natural open spaces and natural systems in Cape town, including nature reserves and the Biodiversity network, parks, public open space, rivers, wetlands and the coast. A specific focus will be the ecosystem services that these natural assets provide, such as flood attenuation, waste absorption, air and water purification, resource provision, and recreational and cultural benefits” (CoCT, 2017). Thgreen infrastructure plan could not be accessed during this study.

• City resilience programme, “in 2016, the City was selected as a member of the 100 Resilient Cities initiative. This initiative assists cities with their strategic planning to proactively and sustainably address their physical, social and economic challenges. Urbanisation, globalisation, rapid technological advancement and climate change

mean that Cape Town’s key systems are increasingly interdependent with other parts of the region, the country and the rest of the world, and are also potentially more vulnerable to disruption. A holistic approach to risk is required – not simply preparing for shocks, but understanding how stresses impact on the ability of our city to thrive and respond in moments of shock” (CoCT, 2017). This programme resulted in the Resilience strategy, which is introduced below.

3.4.2 Resilience Strategy

The Resilience Strategy introduces shocks and stress factors that the city has to prepare for in future. Prioritised shocks that are relevant to this study:

• Rainfall flooding “affecting many Capetonians because of its geographic position. Due to climate change, Cape Town is expected to have more frequent and intense flood events in the future, which can impact large tracts of the city” (CoCT, 2019).

• Drought,”from 2015 to 2018, Cape Town experienced the worst drought in its recorded history. Climate change has increased the likelihood of more frequent and intense droughts in the future” (CoCT, 2019).

• Heat wave, “increased incidences of extreme heat events as a result of climate change are a distinct possibility for Cape Town. The impact of heat waves on vulnerable people, particularly the elderly and young children, can be particularly severe” (CoCT, 2019).

The prioritised stress factor relevant to the study is:

• Climate change, “impacts Cape Town in a number of ways and exacerbates the occurrence and severity of extreme weather events such as droughts, heat waves and storms. Cape Town also has large coastal areas and low-lying residential areas that may be impacted by future sea level rise” (CoCT, 2019).

“Shocks, including climatic events, can strike at any time. There are opportunities to ramp up ambition in supporting actions that build resilience” (CoCT, 2017).

The Resilience Strategy recognises the following climatic changes Cape Town will face in the future:

• “a decreased annual average rainfall and changes in the seasonality of rainfall.

• an increase in mean annual temperature: higher maximum temperatures, more frequent and intense heat waves.

• an increase in average wind speed and maximum wind strength.

• an increase in intensity and frequency of storms: short, high-intensity rainfall events and increased size and duration of coastal storms” (CoCT, 2019).

The goals within the resilience strategy which relate to the rehabilitation of rivers and wetlands are:

• “Rejuvenate our rivers and the spaces around them to create liveable urban waterways” (CoCT, 2019).

The aim is to tansform the urban rivers into “healthy, safe

and productive urban waterways which produce multiple resilience dividends, including flood attenuation, new work and recreation opportunities, improved water quality and crime reduction” (CoCT, 2019).

The study found that major parts of Cape Town’s rivers are restricted by urban development. They are piped, channelized, canalized. They receive pollutants from WWTW’s , roads and informal settlements. “The total length of rivers and streams in Cape Town is 1 900 km and the total length of canals and channels is 480 km” (CoCT, 2019).

• Build climate resilience

“Adaptive responses to the impacts of climate change are spread across a multitude of government, organisational, and community plans” (CoCT, 2019).

• Create multiple coastal management forums

“Empowering partnerships between coastal stakeholders to co-own risks related to impacts on the coast and surrounding infrastructure, and networks of resources able to both prepare for and respond to coastal shocks. The integrity and value of Cape Town’s 307 km coastline is dependent upon the interaction of numerous biophysical processes. Storms are drivers of rapid coastal change, often leading to abrupt erosion events and inundation of coastal areas. Historic planning decisions made without the guidance of a City- wide integrated coastal management framework have resulted in the interference with dynamic coastal processes and degraded coastal environments which now form a source of risk to human settlements located in these spaces. Risk may be physical, social

or financial and can be transferred to coastal stakeholders over periods of time and over space” (CoCT, 2019).

• Develop and implement a comprehensive citywide heat plan

“Decreased impact of heat waves when they occur through a city-wide plan, understood and owned by individuals, households, communities and businesses, allowing for the city and its economy to thrive under the circumstances, and for human life to be protected” (CoCT, 2019).

• Develop a portfolio of flood prevention capital projects

“Inclusion of a comprehensive portfolio of capital projects designed to reduce the acute risk of flooding across Cape Town in the next iteration of the City’s sector plans that can be considered for implementation over the next decade. Flooding, whether it occurs due to an overflow of water from water bodies, such as rivers or the sea, due to an accumulation of rainwater on saturated grounds, is a significant shock risk in Cape Town. Irrespective of the possibility of extreme rainfall events in our future as a consequence of climate change, the changing form of the city, including densification, new greenfield developments and the growth in informal settlements, all add complexities which can exacerbate flooding events” (CoCT, 2019).

3.4.3 Environmental strategy

Cape Town devloped a Environmental strategy with the Vision to enhance, protect and manage Cape Town’s natural and cultural resources.

The visionary long term goals related to the cities rivers and waterbodies are:

• “excellent air quality in all areas of Cape Town;

• Cape Town’s rivers and wetlands are well managed and where possible planned as cohesive corridors that are well-used recreational spaces and community assets that provide ongoing ecological services;

• Cape Town’s coastline and marine environment are of excellent ecological quality, free from pollution, accessible to all, provide a central role for recreation, and continue to contribute to Cape Town’s economy;

• the natural resource base, including biodiversity and the services provided by green municipal infrastructure, is restored, protected and utilised sustainably;

• the City understands and takes active steps to reduce environmental risk;

• Cape Town’s aquifers are well managed and conserved, and Sustainable Urban Drainage System (SUDS) controls and waste water treatment and recycling are optimised in a manner which promotes a Water Sensitive Urban Design philosophy and positions Cape Town as a leading example of a truly “Water Sensitive City”; (CoCT, 2017)

Principles and directives

• Resilience

The City will ensure a focus on resilience, enabling the city

to withstand and mitigate the negative impacts of environmental hazards, proactively reduce Cape Town’s vulnerability, and protect the city’s economy.

The City will:

• “take steps to prevent and minimise the effects of natural and man-made environmental hazards;

• recognise that natural functional ecosystems provide the most efficient and cost-effective buffers to natural environmental hazards;

• ensure that the City has a good understanding of environmental risk, particularly those risks associated with climate change, and develops appropriate plans and tools accordingly;

• prioritise environmental management and infrastructure development and maintenance approaches that emphasise soft engineering, and the restoration and rehabilitation of natural systems;

• where natural defences to environmental hazards do not exist, or have been negatively impacted and thus have reduced effectiveness, proactively work towards rehabilitation of these defences, with the aim of restoring the defensive function;

• ensure that invasive plant and animal species are controlled and/or eradicated as required by national legislation and to minimise the impacts of fires on the city;

• ensure that the city’s natural resources and natural/ semi-natural open spaces are managed according to best practice in order to improve resilience and optimal functioning;

• ensure an appropriate urban-natural interface that protects communities from natural hazards; and

• ensure that climate change risk is taken into account in the management of natural resources and in the approval and implementation of developments” (CoCT, 2019).

• Ecosystems Approach

Ecological infrastructure and ecosystem goods and services will be recognised, protected, and, where possible, pro actively restored.

The City will:

• “protect, invest in and proactively work towards rehabilitation of ecosystems and ecological infrastructure, with the aim of restoring the functioning of these systems and the ecosystem goods and services they provide;

• clearly define, map, manage and rehabilitate ecological infrastructure that provides ecosystem goods and services to the communities of Cape Town;

• recognise the interconnectedness and interdependence of ecosystems and their associated goods and services, and ensure that negative cumulative and downstream impacts are prevented, or where they cannot be prevented, minimised or mitigated” (CoCT, 2019).

• Protected Natural Heritage

Cape Town’s natural heritage is a significant economic and social asset, and contributes significantly to the unique sense of place, strong global identity, and distinctive landscapes that are characteristic of the city.

Principle In taking decisions, operating, and planning for the future the City will ensure that the value of the city’s natural heritage is recognised, protected and promoted, and that the benefits and opportunities it provides to communities are realised.

The City will:

• “ensure that the city’s natural and semi-natural open spaces that protect indigenous biodiversity and landscapes and promote sustainable economic and recreational activities - including nature reserves, critical biodiversity areas, river corridors, wetlands, estuaries, beaches, and the coastline - are appropriately protected and managed; and

• work towards implementation of the Biodiversity Network in order to protect a representative sample of biodiversity pattern, process and natural vegetation to meet national biodiversity targets” (CoCT, 2019).

3.4.4 Water Strategy

“The three-year drought in Cape Town was a 1-in-590-year event based on historical rainfall records. The additional uncertainties associated with climate change now need to be included in future planning, including changes in rainfall, temperature and wind, and a likely increase in the intensity and frequency of extreme weather events” (CoCT, 2019). After the shock of the three=year drought the cities priority lies in water supply, although the water strategy also recognises flood risks and water quality in environmental waters.

The future vision for Cape Town is to become a water sensitive city by 2040. “The City will actively facilitate the transition of Cape Town over time into a water-sensitive city with diverse water resources, diversified infrastructure and one that makes optimal use of stormwater and urban waterways for the purposes of flood control, aquifer recharge, water reuse and recreation, and is based on sound ecological principles. This will be done through new incentives and regulatory mechanisms as well as through the way the City invests in new infrastructure.” (CoCT, 2019)

“In its broadest context, water-sensitive urban design encompasses all aspects of integrated urban water cycle management, including water supply, sewerage and stormwater management. It represents a significant shift in the way in which water and related environmental resources and water infrastructure are considered in the planning and design of cities and towns, at all scales and

densities. Increasingly, the terms “water-sensitive urban design” and “water-sensitive cities” are used interchangeably. However, there is a subtle yet important distinction between the two: While “water-sensitive urban design” refers to the process, “water-sensitive city” refers to the destination” cited in (CoCT, 2019). Water sensitive urban design in form of Sustainable Urban drainage is already part of planning new developments. The challenge within the city lies in areas developed without SUDS. Can they be retrofitted?

The Challenges

“Stormwater and urban waterways are often considered a costly problem – water is polluted, and the adjoining areas are often unsafe and remain unused. Cape Town is home to an extensive network of rivers and wetlands. These freshwater systems fulfil a dual function, serving as havens for plant and animal life, as well as ecological infrastructure networks for the management, treatment and conveyance of stormwater and treated effluent” (CoCT, 2019).

“The “built” stormwater infrastructure – roadside gutters, kerb inlets and pipes – interfaces directly with Cape Town’s receiving freshwater and coastal environments. An integrated understanding of, and approach to, the management of these connected systems are essential for the protection of the receiving environment. The ongoing organic and inorganic pollution and littering of Cape Town’s stormwater and freshwater systems poses a threat to both biodiversity and human health” (CoCT, 2019).

“Improved stormwater management is vital for protecting

the citizens of Cape Town from localised and more widespread flooding. This also presents an opportunity for the capture and storage of stormwater for productive use” (CoCT, 2019).

“The health of our urban water ecosystems tells part of the story of rivers and wetlands. They are generally not suitable for recreation, and unsightly in places, making them undesirable as public gathering spaces. Another part of the story of our rivers and wetlands is that many are regarded as unsafe due to criminality” (CoCT, 2019).

“Recently, the City significantly increased its ten-year investment programme to upgrade wastewater treatment works in order to achieve an even higher effluent quality standard and to provide capacity upgrades to support city development. In addition, the extension of the treated wastewater effluent distribution system is proceeding annually, steadily increasing the diversity of the City’s water resources by replacing potable water use and reducing the volume of discharge into Cape Town’s rivers and urban waterways” (CoCT, 2019).

Supporting the transition to a water-sensitive city

• Water management

All forms of water in the city – rain, stormwater, groundwater, greywater and blackwater, canals and rivers – need to be managed in an integrated way that makes the best, sustainable use of this scarce resource and reduces the risk and impact of flooding.

• Waterquality

For environmental waters, especially those associated

with wastewater discharge, toxicity tests will also be conducted to assess the impact of chemicals on aquatic ecosystems. The City will ensure that wastewater treatment processes are adequately managed, and that effective and credible water treatment technologies are applied to ensure that the final effluent is safe for both humans and the environment.

Currently, the City monitors the quality of river water associated with wastewater discharge to ascertain the impact on receiving water bodies, as required by wastewater treatment plant operating licence conditions.

• Rainfall uncertainty

“It is wise to consider the possibility of a step change in rainfall for Cape Town. Although the impact of climate change is uncertain, it is prudent for Cape Town to develop plans that take this uncertainty into account. This requires a scenario-based planning approach, since the climate is beyond the City’s control” (CoCT, 2019).

3.4.5 Coastal Management Framework

Coastal and sea defence decision framework

Much of the City of Cape Town’s extensive coastline has been developed with fixed infrastructure, thereby significantly restricting natural coastal processes from taking place unhindered. This coastal change may be caused by either erosion or accretion processes, as well as through climate change effects. These climate change effects are manifested through an increase in mean sea level and an increase in the intensity and frequency of coastal storms.

The City will therefore need to consider the implementation of coastal and sea defenses in order to protect vulnerable infrastructure. Ill-informed decision making in responding to pressures caused by coastal processes may exacerbate risk and also lead to the permanent loss of existing beaches and coastal environments. The principles which guide this framework include:

• risk averse decision making

• decisions centred on the common good

• application of multi-criteria assessment for decision making. The City recognises three broad categories of coastal and sea defence options, including:

• engineered responses, i.e. sea walls

• ecosystem-based responses, i.e. use of dunes as buffers

• socio-institutional responses, i.e. development of setback lines. (CoCT, 2008)

3.4.6 Floodplain and River Corridor Management Policy

“In order to ensure sustainable development and associated activities within or adjacent to natural and built stormwater systems, and that there is a balanced consideration of potential flood risk, environmental impacts and socio-economic needs, all developments within these areas shall be planned and designed in accordance with best practice and the requirements and conditions laid down in this policy.”It furthermore ensures administrative actions with respect to land use planning applications that are lawful,reasonable and procedurally fair.

Scope and Application of the policy" (CoCT, 2009).

"This policy is applicable to land use, development or building or activity proposals adjacent to watercourses or wetlands. The principles regarding flood management can also be applied to development in the vicinity of formal stormwater management systems" (CoCT, 2009).

3.4.7 Summary of Cape Town Strategies

The city recognises the risks of climate change in all five strategies. A main focus is on the protection and conservation of the natural resources. The need for restoration of urban waterways is a main goal in the Resilience Strategy. The Water strategy envisages Cape Town as a Water Sensitive City within the next 20 years. The Floodplain and River corridor management policy protects rivers and floodplains from negative impacts of new development, It doesn’t pro actively enhance and restore these.

4. Catchment parameters

4.1 Analysis of the catchments

4.1.1 Catchments

Cape Town metropolitan area is dived into 18 catchments, see adjacent diagram. The Diep River catchment being spatially the largest.

The study area spans over three catchments, the Salt River, City and Diep River catchments. Originally, the Salt River estuary spanned over all three of these catchments and with this combined the Diep and Salt River catchment into one. “A catchment being a three-dimensional land system or drainage basin, which converts precipitation and groundwater inputs to stream flow and whose components are assessed in terms of their influence on these processes.”(Smithson, The Physical Environment, 2002). For the status quo assessment of the catchment only the Salt River catchment is described as it has influence on the Salt River canal. The other 2 catchments are influencing the Old Salt River canal and the Zoarvlei wetland. It was not possible to gain the same depth of information for these as part of this study, due to the lock down period and the closure of government departments as part of the Covid-19 pandemic.

4.1.2 City catchment

The City catchment comprises of the streams originating on Table Mountain and Signal Hill draining north or west from it into the CBD or the Atlantic Seaboard suburbs. In these dense urban areas streams are in general piped. Table Mountain provides a platform that receives some

of the highest precipitation in the Peninsula, and is the source of countless streams and springs. “There are four main perennial streams flowing into Table Valley, namely the Platteklip Stream, Molenwater, Third Stream and Zwaartrivier. There are also several small seasonal streams draining the slopes of Devil’s Peak and the saddle to the east, and Tamboerskloof to the west. All these streams are piped at some point, where they hit the urban edge of the city” (Cate Brown and Rembu Magoba, 2009) . These mountain streams where one of the reasons for ships to stop and settlers to stay in the Cape 300 years ago . “Over the years, as polluted stormwater and untreated sewage entered the system, the waters of the stream became unfit for consumption. To reduce stenches and misuse, and to improve access over the canals, the entire system was covered over with stonework barrel arches” (Cate Brown and Rembu Magoba, 2009). These system exist until now and are in parts tourist attractions. Today the rivers coming off the mountain have been diverted into stormwater drains. “All streams coming from table mountain and Devils Peak are today no more than drains, beneath the City” (Cate Brown and Rembu Magoba, 2009). The two unnamed streams (in Save to Sea, one was called Platteklip stream) influencing the study area are emerging from Devils Peak and are piped from Philip Khosana Drive through Woodstock and Salt River. They resurface on 2 different points where they enter the Old Salt canal. Their fresh water gets polluted with stormwater along the way before it discharges into the harbour.

Rivers and catchments of Cape Town

49 Rivers and Waterbodies and their catchments of Cape Town ;

Source: created by author

4.1.3 Diep river catchment

The Diep River has its source outside of Cape Town. The river rises in the Perdeberg and Riebeeck-Kasteel Mountains to the east and north of Malmesbury and flows in a south-easterly direction for approximately 65 kilometres. This vlei is located on the Table Bay coastline between Milnerton and Table View. The Diep River is linked to the sea via the Milnerton Lagoon. Various tributaries join the Diep River on its way to the sea.

Zoarvlei wetland is the connection piece of the former Salt River that was cut off from the Diep River in the north and the Salt River canal in the south. Originally wetlands framed and buffered it, but due to development of the adjacent areas it has diminished over the years. “Development in the surrounding area destroyed the ecologically important links with both these systems. It exists today as a seasonal,freshwater coastal lake fed by groundwater intrusion and stormwater runoff, with a small outlet to the sea via a culvert leading into the Milnerton Lagoon”(Cate Brown and Rembu Magoba, 2009). As a wetland it is protected in its current state. It is the most southern entity of the Cape Wet Coast Biosphere Reserve. “Despite its poor ecological condition, Zoarvlei performs a vital role in the successful migration of birds. Approximately 125 species of birds have been recorded in the wetlands, many of them rare. In recent years”(Cate Brown and Rembu Magoba, 2009).

Before entering the sea, the Diep River flows through the Rietvlei wetland and the Milnerton lagoon, which togeth-

er cover an area of approximately 900 hectares. These two features together have generally been considered to comprise the “estuary”. Water quality problems are the major concern in the wetland. “The treated waste water from the Potsdam Waste Water Treatment Plant (WWTP) is discharged to a channel along the eastern boundary of Rietvlei which conveys the effluent stream to the head of the lagoon at the Otto du Plessis road bridge”(PEAK, 2008). The estuary type is a temporary open estuary with a bar that breaches. “Over a period of around 20 years from the early 1970’s until the construction in 1991/92 of the channel associated with the sewage works, the mouth closed on a regular basis albeit for varying periods. It was then either breached by floods, or was artificially opened by the town engineers once the water level in the lagoon reached between 1.9 and 2.0 m above MSL. Since the construction of Woodbridge Island and the channel, the mouth has again remained open” (PEAK, 2008). This mouth was called the second mouth in the historical maps since 1860. “The periodic closure of the mouth was probably due to both reduced water flows, and siltation, resulting in reduced tidal inflows that they were no longer strong enough to keep the mouth open” (PEAK, 2008). This strenghtens the opinion that the mouth was rather constructed.

Initially it was envisaged to use the Leitbild method of the EU Water Directive. The Diep river estuary could have been a reference status for the Salt River rehabilitation.

Due to the Covid-19 pandemic, it was not possible to obtain the necessary information or do site visits.

The Diep River estuary has many of similarities with the Salt River due to them belonging once to the same system. They are situated geographically at the same coastline a couple of kilometres apart. The Diep River was before the separation the upstream reach of the Salt River. Another similarity is that it receives effluent from a WWTW as well. The catchment area is much bigger and has a bigger proportion of agricultural land, whereas the industrial land is not part of the estuary itself as in the Salt River.

50 Zoarvlei wetlandr2020, Source: author

4.1.4 Climate

Cape town has a winter rainfall and dry summer climate, that is often compared to the mediterranean climate. The average annual temperature is a mild 17°C. June has the highest rainfall with cool temperatures, low wind speed and a high in humidity.

In summer blows a strong South easterly wind over the Cape flats with the highest temperatures in January/February and a low humidity, which brings a high evaporation rate.

In winter the main wind direction is North westerly, which can bring storm surges along the Atlantic coast. The precipitation rate within the metropolitan boundary varies between 300-1000mm and is influenced by the outcrops of the Tygerberg hills and the Table Mountain group.

51 Rainfall and wind average for Cape Town airport 1985-2015 Source: created by author39

52 Rainfall and temperature average for Cape Town airport 1985-2015 Source: created by author

4.1.5 Geology and Soils

The study area is situated on two geological groups the N-Qs - Quartzose sand, pelletal phosphorite, that is characterized as gravel, sandy silt, grey-black carbonaceous kaolinitic clay, peat, shelly limestone and sandstone, shelly sand and (aeolian) calcarenite, coquinite, light grey to reddish sandy soil, loamy sand the second group is the CRtPhyllite, meta greywacke, quartzite, minor volcanic rocks.

The geology of the area is characterised by Sandveld Group sands, Quaternary aeolian sands characteristic of the Cape Flats area. In the Cape Flats District, the Sandveld Group is mainly represented by the Springfontyn Formation, which was developed through the deposition of windblown sand (an aeolian deposit) and consists of reddish to grey, unconsolidated quartzose aeolian sand.

53 Geology map of Cape Town; Source: created by author

54 Soils map of Cape Town; Source: created by author

4.1.6 Ecosystem protection levels

The South African National Biodiversity Institute (SANBI) developed as part of the National Biodiversity Assessment (NBA) in 2018 national data on the threat and protection status of Marine and Freshwater systems. The NBA is the primary tool for monitoring and reporting on the state of biodiversity in South Africa. The marine and aquatic inland system are illustrated together in two diagrams, as the freshwater system impacts on the receiving marine life and vice versa. The fact, that the two systems influence each other is reason enough to protect and enhance them. While the Table Bay Marine life is moderately protected the two estuaries are only poorly protected, diagram 55. Apart from the sewage outfalls, the pollution from the harbour the Salt and Diep River with their inflowing water have a major impact on the health of the Table Bay. This is one of the reasons to help the estuaries to achieve a good ecological status. "River ecosystem types are Poorly Protected with only 13% considered Well Protected and 42% Not Protected" (SANBI,2018). The Salt River Estuary is poorly protected, while the Salt River canal is not protected.

4.1.7 Ecological state categories

"The Cool Temperate bioregion is characterised by estuaries in a heavily modified or worse state" (SANBI,2018). "Nearly 30%of salt marsh habitat has been lost as a result of poor land-use practices, flow reduction and related mouth closure, direct harvesting and overgrazing. Several

estuarine-dependent fish species are threatened by overfishing (especially gillnetting),declining water quality, and reduced flows with their concomitant influence on recruitment and marine connectivity. An estuary was assigned a condition category ranging from natural (A) to critically modified (F), which relate to decreasing levels of ecosystem function" (SANBI,2018). Both Salt River canal and estuary are critically modified.

4.1.8 Ecosystem threat status

"The estuarine realm is the most threatened of all realms in South Africa, both for the number of ecosystem types (86% threatened) and for area (99% threatened). Estuaries are under-protected in South Africa with only 18% of ecosystem types and 1% of estuarine area Well Protected" (SANBI,2018). "River ecosystem condition declined by 11% between 1999 and 2011. Of the 222 river ecosystem types assessed, 64% were found to be threatened (43% Critically Endangered, 19% Endangered and 2% Vulnerable). Ecosystems with <20% of their spatial extent in a natural/near-natural ecological condition, were considered Critically Endangered. Thresholds of 35% and 60% were used for Endangered (EN) and Vulnerable (VU) categories, whereas ecosystems with >60% in a natural or near-natural ecological condition were considered of Least Concern (LC)" (SANBI,2018). The threat status of Salt River estuary and canal is critically endangered. The Table Bay marine life is under threat to become endangered and the Diep River and Salt River estuaries are critically threatened, see diagram 57.

Ecological state categories

Ecosystem threat status

57 Cape Town Ecosystem thread status;

Source: created by author

Salt River Catchment

Topography and Morphology

4.2 Salt River catchment

4.2.1 Topography and Morphology

The fan shaped catchment form derives from the main water sources from Table Mountain and the Tygerberg Mountains. The rest of the catchment is relatively flat with the rivers spread out like rays. In the Tygerberg Mountains farm dams are diverting water for crop irrigation.

58 Salt River catchment water bodies and topography; Source: created by author

Salt River Catchment Soil

4.2.2 Soils

The soils within the catchment are classified as soils with a diagnostic ferrihumic horizon (Ga). These deeper, sandy, calcareous soils dominate the low-lying areas of the Cape Flats. They tend to be less acidic and slightly higher nutrient content than the surrounding red and yellow Capedal soil types. These soils are often subject to waterlogging during the winter months and have a subsurface accumulation of organic matter typical of wetland areas.

59 Salt River catchment soils; Source: created by author

Salt River Catchment

Waterbodies

4.2.3 Types of waterbodies

The rivers in the catchment are mainly transformed, especially in the flatter areas, where development took place. Most of the rivers are canalized, only the streams on the high slopes of Table Mountain and the Tygerberg Hills flow in their natural river bed.

The two Waste Water Treatment Works (WWTW) in the catchment are releasing treated effluent into the Kalsteinfontein and Vygekraal rivers, which changes the flow regime from an ephemeral to a steady flow. Water quality is also influenced negatively by the discharge from the WWTW’s, which can be observed, by the vigorous growth of alien water plants. The 5m contour forming the bound-

ary for the estuary goes as far inland as Pinelands and Athlone. This area simulates a flood risk for a future sea level rise.

60 Salt River catchment types of storm water bodiesand extent of estuary Source: created by author

Salt River Catchment Subcatchments

4.2.4 Subcatchments

The Salt River catchment extents 250km² into the central metropolitan area. The catchment drains from the Tygerberg Mountains and Table Mountain into Elsieskraal, Vygekraal, Blomvlei, Bokmakierie, Black and Liesbeek through the Salt River canal river into the Table Bay. The system receives additional flow from the two Wastwater treatment works, Borchards query and Athlone. This continuous discharge into the system made it a permanent flow from the natural ephemeral flow pattern. Ephemeral flow pattern was influenced by the winter rainfall, the very hot windy and dry summer and the porous soil conditions,

that discharge water quickly, especially in summer, when the water table is low.

The Salt River catchment is divided into 12 sub-catchments, see diagram 61.

61 Salt River Subcatchments; Source: created by author

Salt River Catchment

Land use

4.2.5 Land use

Because of the catchments urban character, the percentage of open land with permeable areas is relatively low. Part of the Liesbeek sub-catchment on Table mountain is permeable but on a steep slope. Land use in the catchment is mainly residential with some commercial and industrial areas, especially along the Salt River canal. There is no other main open space system, then the Table Mountain national park. Along the rivers and the canal are isolated open spaces, like sports fields and golf courses. The Tygerberg Mountains are mainly used for agriculture and residential uses.

“The Salt/Black/Liesbeek/Elsieskraal river catchment is

possibly in the poorest ecological condition of all of the major catchments in the city. Apart from some of the upper reaches of the tributaries of the Liesbeek River, most of the rivers are canalised or in pipes. The system also receives wastewater from Athlone and Borchards Quarry Wastewater Treatment Works, and the lower sections are seriously polluted”(Cate Brown and Rembu Magoba, 2009).

62 Salt River catchment land use Source: created by author

Salt River Catchment

Impervious erven

4.2.6 Surface permeability

Permeability of the surface area has a direct influence on the run off during rains. If the percentage of non-permeable land is high, the run off rate is high. If surfaces are sealed or soils are rocky/clayey with little water absorption the percentage of the water that gets absorbed is low.

Most of the catchment shows a high percentage of over 45% imperviousness, this is due to urban development which is the main factor for a high rate in imperviousness.

63 Salt River catchment impervious erven; Source: created by author

4.3 Marine boundary

4.3.1 Ocean

“The sub-humid to arid coast at the west coast of the Cape is characterized by the cool Benguela Current, south-westerly swells and strong north-flowing littoral drift”(Charles W. Finkl and Christopher Makowski, 2019). Table Bay is situated on the north of the Cape Town CPD. It’s shape and calmer waters made it the perfect place for a harbour 400 years ago.

The threshold between land and sea is influenced by different energies from wave and tidal movement.

“Waves are undulations formed by wind blowing over a water surface. They are caused by turbulence in air- flow generating pressure variations on the water. Once formed, waves help to disturb the airflow and are partly self-sustaining. Energy is transferred from the wind to the water within the wave-generation area. The amount of energy transfer depends upon the wind speed, the wind duration (how long the wind blows), and the fetch (the extent of water over which the wind blows). Once waves approaching a coastline ‘feel bottom’, they slow down. The waves crowd together, and their fronts steepen. Wave refraction occurs because the inshore part of a wave crest moves more slowly than the offshore part, owing to the shallow water depth, and the off- shore part swings forwards and the wave crests tend to run parallel to the depth contours” (Hugget, 2011). Waves and their energy can be observed during high tide at the downstream end of the canal. They

are the reason for the protection of the land .

“Currents are created in the near shore zone that have a different origin from ocean currents, tidal currents, and wind-induced currents. Nearshore currents are produced by waves. They include longshore currents, rip currents, and offshore currents. Longshore or littoral currents are created when waves approach a coastline obliquely. They dominate the surf zone and travel parallel to the coast”(Hugget, 2011).

“Tides are the movement of water bodies set up by the gravitational interaction between celestial bodies, mainly the Earth, the Moon, and the Sun. They cause changes of water levels along coasts. In most places, there are semi-diurnal tides – two highs and two lows in a day. Spring tides, which are higher than normal high tides, occur every 14–75 days when the Moon and the Sun are in alignment. Neap tides, which are lower than normal low tides, alternate with spring tides and occur when the Sun and the Moon are positioned at an angle of 90◦ with respect to the Earth”(Hugget, 2011) . Tidal change can be observed in the canal. During low tide a small channel flows out to sea, at high tide the canal fills with water moving “backwards” or better inland. No information could be obtained how far back the tidal influence is, but a “backward” flow could be observed by the author as far as the Liesbeek and Black River confluence.

A delta is often closely associated in time and space with an estuary, but frequently in the literature the two are not adequately separated, particularly for tide-dominated

estuaries. “For the same riverine outlet, a delta is a geomorphic and sedimentologic feature, while an estuary is a hydro chemical one where riverine freshwater flowing into a bay, a lagoon, or semi-enclosed coastal body of water mixes with seawater. To a large extent, all deltas can be estuarine in the sense that some part of them will have a freshwater-to-seawater transition, and large estuarine environments whose basin has not been filled with sediment may contain small-scale deltas along their margins or in their headwaters. Deltas within estuaries generally are relatively small sedimentary accumulations compared to the size of their estuarine setting”(Kennish, 2016). The sediment collection at the mouth of the canal is a small delta. “There are 2 concepts of delta, the delta in an estuary and the estuary in a delta, depending on the sedimentology” (Charles W. Finkl and Christopher Makowski, 2019). In the case of the Salt River the delta formed a small part of the estuary. Currently the delta is reduced to the small sediment deposit at the canal mouth. The species diversity in estuaries results from the coincidence of marine, freshwater and estuarine species. “Despite this diversity, fewer animal species are permanent residents in estuaries than in oceans or rivers, and those that are endemic to estuaries tend to be exceptionally hardy. Estuaries in general are harsh environments for aquatic life, where physiological systems must be adapted to rapidly varying salinity, temperature, light, turbidity, currents, and other physical and chemical properties of the environment” (Jordan, 2012). For seawater species the environ-

ment in estuaries is gentler, with lesser wave action and lower salinity. Diadromous species could be observed entering the canal, attracted by the outflow pressure. The question, where they go or if they turn back, once they realize that this is not a river but a sterile canal system with no habitat qualities can’t be answered in this study. “Salt marshes start to form when tidal flats are high enough to permit colonization by salt-tolerant terrestrial plants. Depending on their degree of exposure, salt marshes stretch from around the mean high-water, neap-tide level to a point between the mean and extreme high-water, spring-tide levels. Their seaward edge abuts bare intertidal flats, and their landward edge sits where salt-tolerant plants fail to compete with terrestrial plants. Salt marsh sediments are typically heavy or sandy clay, silty sand, or silty peat. Many salt marshes contain numerous shallow depressions, or pans, that are devoid of vegetation and fill with water at high spring tides” (Hugget, 2011). Currently there is no space for the formation of a salt marsh area in the Salt River system. The fragmentation of the canal doesn’t allow for it.

“Environmental conditions within saltmarshes are very much determined by the tide. The tidal regime varies between sites, and estuarine saltmarshes experience the full range of tidal regimes” (Kennish, 2016).

"Salt marsh originates with the spread of vegetation onto an accretion intertidal mudflat. Fine suspended sediments (silts and clays) and organic material washed in by tides, and subsequently trapped by roots of salt marsh vege-

Plan of proposed sewage disposal scheme ; Source: UCT Digitial Collection

Plan of proposed sewage disposal scheme ,1891; Source: UCT Digitial Collection53

65 Table Bay Vessel positions, 1878; Source: UCT Digitial Collection

67 Cape Town Foreshore Plan, 1947; Source: UCT Digitial Collection

68 Table Bay Harbour, Historical Development 1880-1985; Source: CoCT Environmental and Heritage Resources

tation, generate a gently sloping depositional terrace or platform between the high spring tide level and the midtide line. Vegetation cover of a salt marsh surface does not usually follow a continuous sequence from sea-edge to land. In addition to creeks and channels, highly saline, dry, or water-filled shallow depressions often feature in the surface of the marsh. They form where vegetation has either failed to establish, or where it has died back, or where drainage channels have slumped and blocked“ (Charles W. Finkl and Christopher Makowski, 2019).

"Geomorphic effects of sea-level rise are varied. Inevitably, submerging coastlines, presently limited to areas where the land is subsiding, will become widespread and emerging coastlines will become a rarity. Broadly speaking, low-lying coastal areas will be extensively sub- merged and their high- and low-tide lines will advance landwards, covering the present intertidal zone" (Hugget, 2011).

4.3.2 Harbour

“The exposed wave beaten coastline of South Africa is graced with very few sheltered embayments, placing a premium on estuaries as sites for harbours. Cape Town is situated on a large open bay, which provides little protection for ships from winter storms” (Branch, 2019). Today the Cape Town harbour is an artificial harbour constructed and expanded between 1880-1985.

The construction and land reclaimation of the harbour forced the Salt River course to change into a different posi-

tion. The mouth was disconnected and piped into the harbour and the river tamed into a canal without connection to its floodplain. A seawall as the harbour boundary protects and disconnects the former marsh land from the sea. The influence of the ocean on the Salt River system and its surrounding land is tamed and reduced to the energy of the tidal inflow into the canal. With the construction of the harbour and the claiming of “extra” land the sea is separated by a wall from the land or vice versa.

“Seawalls are constructed to prevent inland flooding from major storms accompanied by high water levels (storm surge) and large waves. Seawalls also fix the position of the land sea boundary if the sea reaches the structure. The main functional element of a seawall is the elevation to minimize overtopping from storm surge and wave runup. A seawall is typically a massive, stone and concrete structure with its weight providing stability against sliding forces and over- turning moments”(Charles W. Finkl and Christopher Makowski, 2019). Dolosse protect the seawall further, they are 80 ton reinforced concrete blocks in the shape of chicken bones, that interlock into each other once placed. They are a south african designed coastal protection against wave action, that is used worldwide. There is no visual connection between land and sea. The only signs of being close are the sea air and a greater amount of seagulls.

“Part of the problem with the design of our urban areas is that we detached ourselves from water in the process. Often, people in cities are surrounded by water, but not able to use it” (Kotze, 2019).

5. Study area parameters

5.1 Estuary

Estuary - where the river meets the sea. SANBI declared the Salt River catchment up to the 5m contour line an estuary, see Figure 69. “An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage”(cited in EWISA, no date). “A partially or fully enclosed body of water which is open to the sea permanently or periodically. The upstream boundary of an estuary is the extent of tidal influence”. (cited in EWISA, no date)

“In South Africa an estuary is considered to be that portion of a river system which has, or can from time to time have contact with the sea. Hence, during floods an estuary can become a river mouth with no seawater entering the formerly estuarine area. Conversely, when there is little or no fluvial input an estuary can be isolated from the sandbar and become a lagoon which may become fresh, or hypersaline, or even completely dry” (CSIR 1992)(cited in Ethekwini Municipality, no date).

With the draining of the Paarden Eiland area and the canalisation of the Salt River most of the estuarine benefits are suppressed. There is no space for estuarine flora and fauna, sedimentation becomes a problem during heavy rain fall. The loss of the salt marsh to urbanisation is countered by flooding and the inability of the dry land to clean air or polluted water. Cape Town has in the Table Bay area only 2 estuaries, the Diep River estuary north of the Salt

River canal and the Salt River estuary. The Salt River canal drains a substantial urban area (250km²) into the Table Bay. With the stormwater comes the pollution from roads, waste water treatment works, informal settlements and industrial areas. This polluted stormwater is drained fast directly towards the sea.

“Many of the estuaries in the City of Cape Town have been heavily impacted by activities in their catchments. These coastal systems receive polluted water from upstream, with discharge regimes that are not natural. As a result, the City’s estuaries are in poor condition, and tend to be dominated by stands of the common reed, Phragmites australis, which thrives in these altered conditions. Estuaries are all considered to be of very high functional importance and ecological sensitivity, and are subject to a very high level of threat, due to their position in the catchment”(Snaddon and Day, 2009).

The Salt River is only recently recognised as a canal in an estuary. The National Biodiversity Assessment mapped it in its 2018 report as an estuary, using the 5m contour line as its boundary, see diagram 56. "Key drivers and pressures in the estuarine realm are freshwater flow reduction, water quality issues, fishing pressure, habitat modification, estuary mouth manipulation, biological invasion and climate change" (SANBI, 2018). There was no estuary definition found for a canal in an estuary in the literature reviewed.

Study area

Study area with Stormwater pipes, sea in and outlets, estuary overlay , Source: created by author using BGIS data for the estuary boundary

5.2 Study area characteristics

5.2.1 Extent

The study area stretches across the suburbs of Paarden Eiland, Brooklyn, Salt River and the Harbour. It is crossed and dissected by major rail lines and high ways connecting the City of Cape Town (CoCT) CBD with its surrounds. The sea forms a natural boundary. Koeberg road and the M5 are the eastern boundary. The centre is occupied by PRASA rail yards. The M5 and N1 highways are boundaries for the Salt River canal and the Old Salt River canal.

5.2.2 Topography

With the urbanisation of the area and the establishing of the harbour the topography of the area was modified. The dark turquoise areas being the lowest, show clearly the location of the water bodies. The brown areas are the highest. There is very little slope within the Salt River canal. Higher elevation in Brooklyn and Rugby created the natural boundary for the former Salt River.

The reinforced protected sea wall along the harbour and the coastline up to Milnerton is higher than some of the land behind. It has to be mentioned, that a lidar only shows the water level of water bodies and not the ground level.

70 Study area with 2015 aerial, Source: created by author

71 Study area with Lidar, Source: created by author

5.2.3 Altered topography

During the urban and harbour development substantial parts of the area were filled in to enable development. The harbour development affecting the area the most happened in the 1970-1980’s with the extension of the Ben Schoemann dock, which resulted in the piping of the original Salt River mouth, see photograph across. The salt marsh area was gradually filled in to allow the construction of the rail yards and the northern Metro line to Malmesbury with the Ysterplaat station. It is assumed that the filling was not only done with clean soil, but with any access material left, like from the Salt River Power station.

5.2.4 Wind

Because of the proximity to the sea and its low-lying character the side is exposed to strong winds. The main wind directions in Cape Town being south-easterly for summer and north-westerly for winter. The winter winds have the main influence on the wave action against the coastline.

72 Study area with filled areas, Source: created by author

73 Study area with wind directions, Source: created by author

74 Construction of Ben Schoeman dock Source: www.flickr.com

Study area Vegetation types

Study area

5.2.5 Vegetation

The Salt River was flowing through “a flat slightly undulating landscape covered by tall, evergreen, hard leaved shrubland with abundant grasses and annual herbs” (Mucina, L., Rutherford,M.,2006).

Cape Flats Dune Strandveld, the main vegetation type of the study area, occurs in the Western Cape in four discontinuous regions, the Table Bay coast is one of them. This vegetation group is endangered and under constant threat from development. Remnants of the vegetation type occur in the study area only next to the Zoarvlei wetlands, see diagram 76.

Four other vegetation types of the area are important for the riverine areas, Cape Flats Sand Fynbos, south of Vortreeker road, a Penninsula Shale Renosterveld patch west of that. Not shown in the vegetation layers of the BGIS are the Fynbos Riparian Vegetation on the immediate banks of the river and the Cape Estuarine Salt Marsh vegetation, see diagram75.

Fynbos Riparian Vegetation is part of the alluvial vegetation and occurs in the Western Cape near sea level to

1300m altitude. “Narrow, flat or slightly sloping alluvial flats supporting a complex of reed beds, dominated by tall palmiet and restios, low shrubland with moisture loving species and tall riparian thickets in places. This vegetation is in many patches well protected, but prone to infestation by alien woody plants” (Mucina, L., Rutherford,M.,2006). Fynbos Riparian Vegetation is completely lost in the study area through the modification of the river and the construction of the canal. It plays an important role in the sediment transport, in collecting sediments during floods. “The streams are mainly fed during the winter rainfall season and later through seeps carrying acidic water rich in organic compounds (tannins) and with a characteristic brownish color. Large streams carry water all year around, while smaller streams can turn into a series of disconnected pools in the upper reaches in summer. Many trees and shrubs are adapted to uprooting or heavy damage by spates. The heavy erosion also results in a soil-poor substrate and plant cover is often very patchy, located in some small depressions with some soil development or between

75 Study area with original vegetation types, Source: created by author

76 Study area with vegetation remnants, Source: created by author

Study area

boulders”(Mucina, L., Rutherford, M., 2006).

Cape Estuarine Salt Marshes must have been the main vegetation type for the Salt River estuary. It occurs on the Atlantic coast at estuaries and salt marsh plains. “Estuarine Flats and systems of low riverine terraces supporting complexes of low herb land and shrublands dominated by succulent chenopods and flood tolerant halophytes, salt marsh meadows dominated by rushes and sedges and temporary submerged sea-meadows at the lower boundary of the tidal zone”(Mucina, L., Rutherford, M., 2006).

On the Diep River mouth at the Milnerton lagoon exists a small remnant of this vegetation type, see diagram76.

5.2.6 Public Open Space

Public Open Space zoning in the study area is mostly associated with the waterbodies along the former river corridor, now next to the Zoarvlei wetland and Salt River canal north of the N1, see diagram 77.. A substantial piece between the canal and the Old canal lies on the rail yard, which can be associated with the former Salt marsh in this

Study area

area. Another open space zoning sits isolated between the Marine Drive and the rail tracks along the coast, parts of this space is used for the Milnerton market every weekend.

Parks and sports facilities are mostly situated within the residential areas of Rugby and Brooklyn. A single park exists in Paarden Eiland. The beach along the coast line can only be accessed from the north at the Diep river mouth, the area south of the Salt River mouth belongs to the harbour and can’t be accessed by the public.

77 Study area with areas zoned Public Open Space, Source: created by author

78 Study area with Parks and Sport facilities, Source: created by author

Public Open Space Zoning

Parks and Sport facilities

5.3 Salt River canal photo summary

79 Salt River Canal, Photo summary 2020, Source: author

5.4 Old Salt River canal photo summary

80 Old Salt River Canal, Photo summary 2020, Source: author

Study area

Photo

STATUS QUO