Salt River Estuary Restoration Part 4

Part 4

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

6. Scenario discussion

6.1 Status quo

"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"(CoCT, 2019).

“Cities need to dramatically redefine and restructure their relationships with water, particularly in regions where climate change challenges their reliance on engineered hydrologies. The early settlement patterns of cities often took advantage of natural hydrogeographical positions –river and lake fronts, natural harbors, and so on – either for security, navigation, or commerce. However, these strategic land–water interfaces were often aggressively manipulated, dividing water and land, forcing them into submission, and compromising ecological, social, and functional relationships with the very waters they depended upon. Now, cities need to unpack these histories, reimagine themselves and redesign with water through more nuanced and interdependent nature–city relationships” (Topos, 111).

6.1.1 Limits and deficits

The limits and deficits of the current canalized system are many, the disruption of the natural processes within the stream and the catchment are great. The decision to look at the bottom end of the system was to unravel the chang-

es and problems of the greater system in the most neglected area of the catchment. The challenge was to find a way of reconnection between the terrestrial, aquatic and marine systems within an urban setting.

The current hydrological system comprises of the following components, see diagram across:

• Salt River Canal

• Old Salt River Canal

• Zoarvlei wetland

• Table Bay

• Stormwater pipes

Only the Salt River canal and Table Bay have an above ground visible connection. All the others are only connected through the stormwater system underground. Because of the concrete lining of the canals a below ground groundwater connection is non existent. The fragmentation of the system resulted, that all fresh water bodies belong to three different catchments. All these facts make the system unnecessary complicated and results in confusion with loss of control.

All flood studies of the system didn’t acknowledge the drastic change the system went through during the development of Paarden Eiland and the loss of the estuary.

Filling of marsh land and drainage of an estuary into a canal can’t be returned. The estuary was under threat from two sides, the sea with the harbour development, the change of the coastline and its dynamics as well as from the land first through agriculture and infrastructure and later through urban development.

In an metropolitan city with a growing population and one of the biggest harbours in the country, these changes need to be accepted and a solution needs to be found within its restrictions.

6.1.2 Ecological deficits

The remnants of vegetation and with that associated habitats are so small and isolated, that they have a very low ecological value.

Fluvial processes within the canals and the wetland are interrupted or restricted. Estuarine processes are restricted to the canal sections.

The ecological deficits of the area are widely accepted including the rather negative view on a river as a risk factor, then as an amenity.

With the loss of the salt marsh, the Table Bay lost a feeding and breeding habitat for its diadromous species. The canal discharges without cleansing processes the polluted water, together with the floating water hyacinth and all submerged and movable rubbish into the sea. The concrete lined canals are disconnected from their floodplains and lost their riparian zones. The land next to the canals is terrestrial. With no riparian zones breeding and feeding habitats are missing within the canal. The lateral and interstitial fragmentation resulted in the need of irrigation next to a watercourse. Air quality is compromised by high traffic rates due to the container transport from and to the harbour. The high rate of impermeable surfaces without vegetation results in a heat island effect during summer.

6.1.3 Sources of flooding

Sources of flooding in the study area are identified as:

• Tidal flooding

• Fluvial flooding

• Pluvial flooding

Currently there are no visible permanent flood protection systems in case of river flooding installed within the area. The flood predictions of the before mentioned flood models would have a devastating impact on the area. The percentage of impermeable or sealed land is very high. The harbour was build on fill. High density of industrial land use comes with maximum property use and sealed surfaces, which results in a high run off rate.

Space for attenuation was eradicated with the decision of the canal construction, with the wrong assumption, that a canal would resolve flood problems. The lateral fragmentation resulted in a decoupling of the canal from its floodplain.

The many storm water outflows into the Table Bay result in unrestricted inflows during storms at high tide.

Bridges crossing the canals and the wetland are not only for transport infrastructure like roads and rail, but also for service pipes or combinations of the two. The area needs river crossings for accessibility and supply, its close proximity to the harbour and the CBD result in a high density of transport routes. Two bridges are lower then a bankful canal and are causes of flooding with overtopping banks.

6.1.4 Old Flood mitigation concept

The City of Cape Town’s approach to flood mitigation on the Salt River canal is still the widening of the canal.

As discussed in the flood studies, “In 1974, the City’s Executive Committee approved a recommendation by the Utilities and Works Committee that the Salt River Canal (canal downstream of the Railway Bridges) be widened (by 15m) and that land adjacent to the canal, that was required to effect the widening, be acquired by the City” (Aurecon, 2019). This hasn’t happened yet.

Before the scenarios, it was researched, what the impact on the adjacent land to the Salt River would be if:

• the canal was widened on both sides by 7,5m, red line

• the canal was widened 15m, yellow line on either side

• the canal was widened 30m, blue line on either side. This is illustrated on the drawing on the left. A 15m widening could be achieved, if the sides would interchange where necessary or the 7,5m widening on both sides. This would not allow for riparian embankments, but be just a strict canal widening.

If this study would include a flood model, this option could have been investigated in this regard, but it would not have any ecological and pluvial benefits. The water would still discharge fast or even faster. The lateral and interstitial fragmentation would persist, water quality would not change and the impact on the non canalized sections upstream could be even greater. All 10 bridges over the canal would need to be widened. This option doesn’t fulfil any other benefit, than faster discharge at a high infrastructure cost and would be a lost opportunity towards an improved quality of or added resilience to the system, it was not further considered in this study.

106 Impact of Salt River Canal widening

Source: created by author

6.2 Scenarios

6.2.1 Scenario

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 behaviour 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” (Novotny, Ahern and Brown, 2010).

What if :

• the Salt River canal could be restored into an estuarine river delta, with tidal inflow, natural river beds, where it could meander and had space for sediment processes?

• It could flow along the Salt River canal alignment, connect with the Old Salt River canal and the Zoarvlei wetland?

• three river mouth would discharge into the Table Bay.?

• the riparian embankments would hold a manifold of habitats?

• the groundwater exchange would happen interchangeably as recharge of the aquifer and recharge of the river.?

• the water would be cleaned by the riparian vegetation and the sediment processes could happen undisturbed?

• the river would be reconnected in all three dimensions to allow for habitats of macrophytes and fishes.?

• the flood risk to the adjacent communities would be reduced through wider channels with more storage capacity.?

Feasibility

• Space – the Paarden Eiland Industrial area is instrumental in supporting the adjacent harbour, densification is planned for the future, only already open spaces and undeveloped land would be available for restoration.

• River mouth Old Salt River – the old river mouth is piped underneath the last harbour extension and an opening or daylighting of the mouth could only happen with the harbour being redeveloped and reduced in size. A harbour redevelopment is not planned for the future, so a daylighting of the Old canal is not feasible.

• River mouth Zoarvlei – to flow the river from South to North, would be against the original flow direction from North to South. Draining this arm through the Diep river mouth, the Milnerton lagoon, could create hydrological problems, as the Milnerton Lagoon is a partially closed estuary and an extra amount of water could create flooding to the adjacent residential areas. There is currently a culvert crossing a railway line and the Marine Drive, connecting the lagoon and the Zoarvlei wetland, that is often clogged according to the CoCT. To open the mouth at this point would require a major transport infrastructure change.

• River mouth Salt River canal – a widening or restoration of the mouth could have a negative impact on the existing engineered sea flood protection.

• Existing bridges – with a restoration of the canal, all bridges would need to be widened, to allow for the wider river channel.

107 Study area Scenario RESTORATION, Source: created by author

• Flood protection – is not taken into account in this scenario. A higher flood risk, may arise at the diversion points, where the river is branched off into the Old Canal or the Zoarvlei wetland. At these points there are either space constraints from transport infrastructure or at the connection to the Zoarvlei from building infrastructure.

• Physical form, the morphology of the Salt River would only change on a small scale, within the river bed itself and within the urban boundaries of the existing settlements. The branching off into thee streams would create a visually more dominant river system which would benefit the experience of the surrounding landscape.

• Riparian zones would be restored and different habitats emerge and shape it. The tidal influence would determine the species returning.

• Hydrology, the flows in general would be slower, because of the roughness of the bed with its new vegetation and sediment deposits. Water will be stored in the soil and recharge the aquifer underneath. The discharge into the harbour would need to be hard engineered and couldn’t be called restored.

• Flow types are manifold, the fluvial system will shape with its deposits and erosion process different channel conditions, that create different flow regimes, within the spatial restrictions of the channel extent.

• Water quality would improve through the self-restoring ability of the natural like streams. There would be still a need to improve water quality throughout the catchment and especially from the WWTW.

• Aquatic life would slowly return, with the improvement of the connectivity of the system diadromous species can inhabit the system. Aquatic plants will succeed and macrophytes settle.

• Terrestrial life would be reconnected to aquatic live through the reconnection of the floodplain. Species home to both are able to travel between them without constructed barriers.

Conclusion

Scenario restoration - would require a dynamic self-sustaining behaviour of the Salt River. Due to the spatial constraints, significant additional land allocation to the river, the river can’t be brought back to its original functioning and its original bed. A restoration could cause additional uncontrolled flooding.

6.2.2 Scenario rehabilitation

“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”(Novotny, Ahern and Brown, 2010).

What if :

• the canal bed is broken where spatially possible and a wider river channel with riparian banks introduced?

• the Old Salt River canal has its lining removed and opens up to wide terraces that can be rehabilitated as salt marsh areas through the steady tidal inflow and succession?

• the stormwater from the surrounding area gets treated and attenuated in bioswales in streets and on bigger parking areas?

• during extreme storm events automated flood barriers stop the tidal inflow on all in/outlets between low tides so that the marsh areas can be used for stormwater attenuation?

• The Zoarvlei wetlands gets protected from poor stormwater quality with bioswales in the buffer areas?

Feasibility

• Space – the Paarden Eiland Industrial area is instrumental in supporting the adjacent Cape Town Harbour, densification is planned for the future, only already open spaces and undeveloped land would be considered for rehabilitation.

• River mouth – all river mouths are staying the same in order to protect infrastructure like the harbour and Marine Drive as well as the doloss protected coast line.

• Existing bridges – most bridges are conserved only bridges 4,5,6 and 10 need to be re constructed or replaced.

• Flood protection - is together with habitat recreation the biggest advantage for this scenario, With the additional storage capacities flood protection from fluvial and pluvial flooding is greater then currently. The automated flood barriers will hold back tidal flooding during storm events.

• Physical form, the morphology of the Salt River would only change in places on a small scale, within the river bed itself and within the urban boundaries of the existing constraints. Banks will be formed by flow dynamics within the wider sections of the channel.

• Riparian zones would be restored in places and different habitats emerge and shape these. The tidal influence would determine the species returning.

• Hydrology, the flows in general would be slower, because of the roughness of the bed with its new vegetation and sediment deposits. Water will be stored in the soil and

108 Study area Scenario RHABILITATION, Source: created by author

recharge the aquifer underneath. The new wetland zones can be flooded during storms and through tidal inflow. Freshwater inflow into the Old canal system would be reduce, because of alternative use of the stream water from Devil’s peak.

• Flow types are manifold, the fluvial system will shape with its deposits and erosion process different channel conditions, that create different flow regimes. Especially in the “softened areas”.

• Water quality would improve through the self-restoring ability of natural like stream sections. There would be still a need to improve water quality throughout the catchment and especially from the WWTW. Stormwater inflows are cleaned through filtering bioswales and other filters. A concern is the waterquality from the harbour which might need to be monitored for chemical substances.

• Aquatic life would slowly return to the rehabilitated reaches, with the improvement of the connectivity of the system diadromous species can inhabit the system in the suitable places. Aquatic plants will succeed and macrophytes settle.

• Terrestrial life would be reconnected to aquatic live through the reconnection of the floodplain in the places where possible. Species home to both are able to travel between them without constructed barriers.

Conclusion

Scenario rehabilitation would be a hybrid between conventional and green engineering. Due to spatial and infrastructure constraints a rehabilitation is only viable in places, which would result in a compromised system with bottle necks. The existing situation of three systems that are not connected, the Zoarvlei, the Old canal and the new Salt River canal, is difficult to understand and to monitor. All three are situated in the same low-lying plain and share the same flood risk at the same time. They can’t give immediate flood relief to the other two systems, because of non-connection. Holding on to a stormwater system, that was developed in phases and is therefore fragmented, doesn’t help to solve the bigger problems of biodiversity loss and flooding.

Although this scenario is a compromise, it is starting to address the issues of the canal. From a monitoring and long term financial impact, the flood barriers are high maintenance and costly.

“Tidal marshes are muddy areas that are alternately drowned and exposed due to low and high tides. Most tidal flats are dissected with many channels flowing a meandering path. At high tide seawater invades the tidal flay from the ocean. At low tide much of this sediment is deposited at the bottom of the salt marsh” (J.J. Bhatt, 1978).

6.2.3 Scenario Reclaimation

“An estuary demands gradients not walls, fluid occupancies not defined land uses, negotiated moments not hard edges. In short, it demands the accommodation of the sea not a war against it. Accommodate uncertainty through resilience, not overcome it with predictions. The transition from sea to land is not a logical one. There is little if any common ground between time and space, depth and surface, horizon and boundary. These are qualitatively different measures that sit uncomfortably with each other. Instead of a common ground there is negotiated unease, an analogical tension that keeps land and sea alive through practices that respect their difference. Design in an estuary must begin with a new visualisation, with the appreciation that design in an estuary solves the problem of flood not by flood-control measures, but by making a place that is absorbent and resilient” (A. Mathur,D.da Cunha, 2009).

What if :

• the rail yard has to give back the land to the salt marsh?

• the canal lining in most places gets removed and a fluvial system is established in this part of the catchment.

• the pipes from the former mouth and the power station into the harbour are disconnected and only the Salt River with its marsh/lagoon exists?

• stormwater from the adjacent Paarden Eiland and Salt River suburb gets discharged into the marsh/lagoon and filtered there?

• WSUD practices are introduced in Paarden Eiland and Salt River (suburb) to reduce the runoff rate, clean stormwater and recharge the groundwater?

• the catchment boundary moves further west to include the two streams from Devils peak and their connected stormwater system.

• the clean water from the stream is monitored and would be stored or used locally?

110 Study area Scenario RECLAIMATION, Source: created by author

Feasibility

• Space – the Paarden Eiland Industrial area is instrumental in supporting the adjacent harbour, densification is planned for the future. If the former Salt marsh area, now a train depot, gets rehabilitated the train depot needs to be consolidated and moved into a different area. The Ysterplaat station and the connecting tracks need to be either lifted onto a bridge or moved next to the N1 highway (which would make pedestrian access shorter).

• River mouth – only the Salt canal mouth stays as it is, the pipes into the harbour will be disconnected. The harbour water, which is contaminated with heavy metals won’t enter the system anymore directly.

• Existing bridges – most bridges 4,5,6,7,8,9,10 need to be re constructed or replaced. Bridges 1,2,3,11,12,13 can stay as they are.

• Flood protection - is together with habitat recreation the biggest advantage for this scenario, With the additional storage capacity of the new Salt marsh flood protection is ensured. Within the catchment flood attenuation still needs to be upgraded.

• Physical form, the morphology of the Salt River would change in places on a small scale, within the river bed itself and within the urban boundaries of the existing constraints. The biggest change would be on the Salt marsh area, where the river could shift its bed all the time, during tidal in and outflow, during floods and depending of the flow from upstream sometimes more and sometimes less.

• Riparian zones would be restored and different habitats emerge and shape these. The tidal influence would determine the species returning.

• Hydrology, the flows in general would be slower, because of the roughness of the bed with its new vegetation and sediment deposits. Water will be stored in the soil and recharge the aquifer underneath. The new wetland zones can be flooded during storms and through tidal inflow. The old Canal will be either filled or combined with the Salt River. The stormwater draining into the old canal now flows directly into the Salt marsh.

• Flow types are manifold, the fluvial system will shape with its deposit and erosion process different channel conditions, that create different flow regimes. The Salt marsh area will be an area of constant shifting, creating areas of different flows.

• Water quality would improve through the self-restoring ability of natural streams and wetlands. There would be still a need to improve water quality throughout the catchment and especially from the WWTW’s. Stormwater inflows are cleaned through filtering bioswales and other filters, like litter traps.

• Aquatic life would slowly return to the rehabilitated reaches and the Salt marsh, with the improvement of the connectivity of the system. Diadromous species can inhabit the system in the suitable places. Aquatic plants will succeed and macrophytes settle.

Concept RECLAIMATION, Source: created by author

• Terrestrial life would be reconnected to aquatic live through the reconnection of the floodplain and the riparian edges in the places where possible. Species home to both are able to travel between them without constructed barriers.

Conclusion

Scenario reclaimation reintroduces the salt marsh, which restores the biodiversity connection and mitigates the flood risk. The spatial impact and the need for land is the in half way in between of the three scenarios. The simplicity of the system makes it more robust and easier to manage. It would bring the river back into the city as an entity and a visible system of constant change. Almost the full length of the canal can be rehabilitated. Incoming diadromous species can find breeding, food and refuge habitats. The marsh could become a destination for migrating birds. The size of wetland/marsh can make an improvement on the air quality and heat island risk of Paarden Eiland. The marsh can clean the water from upstream and reduce pollution of Table Bay. Initially a higher investment would be necessary in obtaining the necessary land. This scenario is most likely the most cost effective in the long term due to lower maintenance and operating costs. It would open up new development opportunities along Vortrekker Road and could become a destination for recreational activities and therefore contribute towards the cities vision. It will open up long term new development opportunities and strengthen the resilience of the city.

112 Concept Reclaimation flows at high tide, Source: created by author

113 Concept Reclaimation flows at low tide, Source: created by author

6.3 Sustainable urban drainage

For the scenarios Rehabilitation and Reclaimation it was envisaged to reduce the run off from the study area and in particular from the industrial properties. The diagrams show the proportion of industrial used properties in the area in red.

All parking areas were traced from the aerial to understand were sealed areas could be made permeable to absorb storm water. There are bigger areas that could be consolidated in the west and south. The central area has almost no suitable space as well as the area between Zoarvlei and Marine Drive.

The harbour was left out, because it is a filled area with no information on the type of fill, concentrated stormwater recharge could contaminate the groundwater, if the fill consists of rubbish or rubble.

In total 14% of the industrial area is dedicate to parking and could be retrofitted with permeable paving or bio swales to reduce the run off. Another reduction could be achieved by harvesting rainwater from the roof areas for sanitary and cleaning uses. The runoff coefficient would change from 0.6775 to 0.5008 , which would result in less peak flow. This would prove that already WSUDS interventions in this area would reduce flooding.

This concept was not further investigated within this study.

114 Study area parking lots and other sealed surfaces on industrial properties (red), Source: created by author

7. Evaluation

Hydro-morphological assessment categories

Channel geometry

Substrates

Channel

Riverbanks/ riparian zone

Channel vegetation and organic debris

Flow

Longitudinal continuity as affected by artificial structures

Bank structure and modifications

Vegetation type/ structure on banks and adjacent land

Adjacent land use and associated features

Floodplain

Degree of

(a) lateral connectivity of river and floodplain

(b) lateral movement of river channel

Scenario

REHABILITATION RECLAIMATION

Varies cross sections

Natural substrate coarse to fine sand

Indigienous riparian vegetation

Seasonal with less effluent discharge

No artificial structures affecting continuity

Banks get shaped by dynamic flow

Indigenous riparian vegetation

Urban development with sustainable urban drainage systems

Varies cross sections

Natural substrate coarse to fine sand

Indigienous riparian vand salt marsh egetation

Seasonal with less effluent discharge

No artificial structures affecting continuity

Banks get shaped by dynamic flow

Indigenous riparian vegetation

Urban development with sustainable urban drainage systems

Floodplain restricted by urban development

Channel movement within wider channel section

Salt marsh becomes floodplain

Channel movement greatest changes within salt marsh area

L Comparison of Hydro-morphological assessment for the scenarios: Source: adapted from by author from (Schmutz,S., 2018)

A comparison of the restoration measures doesn’t show a major difference between the scenarios, the difference is in the reclaimation of the salt marsh area. The assessment of the hydro-morphological indicators also shows no big difference between the scenarios. The biggest differences

are expected to be in flood protection and habitat, which can't be proven in this study. The flood storage capacity of the reclaimation scenario is three times bigger then in the rehabilitation scenario.

7.1 Scenario Rehabilitation

“A functioning stream clearly needs to handle a wide range of flow. Of special importance is the bank-full flow, generally considered to be the channel-forming flow, but the stream must also be resilient to both floods and droughts. The stream must be competent to transport sediment load both from the watershed and the channel. A functioning stream is stable, existing in a state of dynamic equilibrium where erosion balances deposition. A functioning stream should simulate natural stream geometry, including plan, profile, and cross section. Finally, the stream should provide a variety of habitats to support the full range of aquatic and riparian biodiversity” (Matlock and Morgan, 2011).

Objective: Flood protection

• Acknowledging the floodplain

The downstream floodplain reflects on the area that is needed, if a catchment is flooding. The Salt River was originally the downstream reach with the alluvial floodplain of the Diep and Salt River catchment. The low valley close to the sea used to flood and shrink depending on the season. The anthropogenic development in this area forced the river to drain fast through a canal without fluvial interactions. The former wet marshland is drained completely and lost its ecological value. The widening of the channels into wetland zones gives the tidal flooding space to accumulate, settle and shape. It would give species room for new habitat creation and would on the Salt River reduce the upstream influence of the tidal flooding.

• Reduction of runoff

Increase permeability in adjacent sub-catchment, a natural estuary is usually a space with little spatial restrictions, so that the floods from the sea and the river can expand and shrink. It is a permeable space, where the runoff from the surrounding areas is small compared to the discharge of the river and the tidal inflow. In the case of the study a high amount of runoff from the adjacent properties is draining into the canals. Because of the relatively flat surface of the area and the limited amount of space in the canals, it runs slowly and comes to localized flooding. To reduce localized flooding measures like rainwater storage on site in rainwater tanks and infiltration measures like bioswales and bio retention are proposed to help flood

attenuation, these measures are not further explored in this study.

• Attenuation

Retention in adjacent areas is only limited possible due to a lack of open spaces suitable for that.

It has to be taken into account that in the whole catchment measures need to put in place to reduce flooding in the downstream area. The flood problem cant be solved here alone.

•

Increase in flow capacity

The widening of the channel in suitable places with the removal of the lining will allow for riparian zones with vegetation and different flow regimes. The connectivity in all 3 dimensions is important for riverine processes. In this scenario certain areas are not widened due to spatial or infrastructure constraints. This will need special attention in the threshold zones between widened channel and remaining canal. A lateral contraction of the channel will form between the widened sections, a bed reinforcement before and after the contraction is necessary to avoid soil erosion to the bed. It has to be on both sides of the contracted channel because of the tidal inflow.

•

Increase in storage capacity

Reconnecting the Salt River lateral and with the interstitial through the removal of the lining in places will allow groundwater recharge and storage within the soil during slow flows. The widend channels and their floodplains will also alow a increase capacity.

• Reduction in flow obstruction

Low bridges and debris jams on bridge columns are obstructing the flow during floods and can be the cause for over topping banks, damage to bridge structures and erosion of the river bed.

These bridges were identified earlier and the proposal is to either remove or replace them. Bridge 10 over Vortekker road has to be lifted and stretched to allow for the wider channel.

• Prevention of tidal flooding during storm events

Controlled flood barriers on all outlet/inlets

The study site has 5 outlets into Table Bay, that become inlets at high tide. Only the level of the Salt River canal outlet is known, the information for the other outlets couldn’t be obtained during the Covid-19 pandemic.

It is proposed in this scenario to install tidal barriers to stop tidal inflow after low tide and use the storage capacity of the system for 4-5 hours before the freshwater volume from upstream can be released into the ocean. These barriers can be equipped with pumps to reduce flooding from upstream and pump the water out at a higher level during the closure of the inlets. This would require a major technical and monitoring system to function efficiently. Floodgates need to be opened rather sooner then later to allow for connectivity. Examples of flood barriers can be found in the Netherlands and in the UK. The design of these barriers is not part of this study.

• Flood reduction

Wetlands and floodplains are spaces to naturally store and save water in times of high rainfall and long periods of

SCENARIO REHABILITATION

precipitation. (Haase, 2017) The risk of flooding is reduced through the bigger capacity of the wider channels, with their reconnected floodplain and the additional storage capacity of the porous soil. Run off from the adjacent area is reduced through WSUD measures.

Objective: Enhancing biodiversity through encouraging estuarine processes

• Acknowledging the tidal change

An estuarine habitat is dependent on the tidal change of submergence and exposure. Currently with the canal being out of sight tidal changes are not visible and don’t impact on their surrounds. Birds could be observed during site visits using the low tide in the canal for resting and search of food. The constant water level and salinity change creates a specific habitat for species

• Introduction of different flow types

Allowing fluvial process

In a fully functioning stream, the natural processes of erosion and deposition are roughly balanced. Erosion on the outside bend of a bank is matched by deposition on the inside bend. The stream only slowly moves acr oss its floodplain. Erosion and deposition being an in stream fluvial processes, that create constantly new channel forms, depending on the force of the flow.

• Riparian edges

Vegetated riparian edges provide roughness to the channel, that creates different flow regimes in the different zones of the stream, low flow between the vegetation.

With the more protected low flow regimes these areas give refuge to smaller and young species.They will be partially submerged or exposed during the tidal change.

• Re-establishing salt marshes

Giving space back for tidal change and new habitats, allowing the tide to spread and bring in sediments again. Although the tidal inlets will stay as they are as pipes or concrete channel, they are currently letting sediments in as seen at low tide. The wide area at the Old Salt River mouth has space for a small tidal wetland.

Objective: Reinstate connectivities

• Reconnection of the floodplain

“Ecosystem functions of the riparian corridor include habitat, barrier, conduit, filter, source, and sink functions. A stream in dynamic equilibrium provides many more ecosystem services than transport of water and sediment. The riparian corridor is the vegetated zone along streams that forms the transition from aquatic to terrestrial habitat” (Matlock and Morgan, 2011). The flood plain reconnection is only possible in small spaces due to the spatial restriction of the urban fabric and the infrastructure of the surrounding areas. Overland flow during flooding is happening in the low-lying areas, like the rail yard and between the Zoarvlei and the Salt River.

• Reconnection of interstitial

Rehabilitating the canal to a channel

The reconnection of the interstitial will have a big impact on the groundwater system of the area, which is currently

dominantly fed by the seawater, due to the high percentage of impermeable space in the surrounding area. Existing vegetation might show the change first. The interstitial connection can provide the opportunity for benthic organisms to settle.

Objective: Other socio-economic benefits

• Biodiversity connection between coast and mountain Rivers and their riparian edges also act as aquatic and tereestrial corridors that connect different biodiversity areas. The rehabilitated river corridor has the potential to connect Table Bay with Table Mountain.

• Heat island reduction

A reduction of hard reflective surfaces will reduce the heat island generation. The bigger waterbodies will bring cooler air to the area.

• Air pollution reduction

Wetlands and marsh areas are carbon sinks and absorb nutrients and chemicals. Fluvisoils the typical soil type for wetlands are able to bind many organic and inorganic compunds dut to their high contents of fine grain material. (Haase, 2017) The “greening” of the area will also alow for a reduction of CO2 due to plant processes.

• Attractive landscape in a changing industrial environment

Rehabilitating the riverine landscape within the urban area will enhance the quality of life for the working and living there, with an attractive natural inspired landscape, that reintroduces tidal and seasonal change.

SCENARIO REHABILITATION

119 Scenario Rehabilitation illustrated Section B - design, with high water level, Source: created by author

SCENARIO REHABILITATION

121 Scenario Rehabilitation illustrated Section C - design, with high water level, Source: created by author

7.2 Solutions Scenario Reclaimation

“Waters in a sectional world do not flow on a surface as much as they rise and fall, evaporate and condense. They do not flood, they soak. Waters that surveyors find too fluid on the surface to draw meaningfully on maps, waters that are yet vital in depth” (Mathur, Da Cunha, 2009).

“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. ” (CoCT, 2019).

Scenario Reclaimation is the preferred solution to restoring the estuary and achieving a good ecological status. It has similarities with the scenario rehabilitation, but has a better chance for success, as the space available for fluvial processes is much greater and not so fragmented.

Objective: Flood protection

• Salt Marsh

With the reclaimation of 500 000sqm of railyard for an estuary, this scenario relies on the logic of simplicity and land availability for riverine and estuarine processes. The marsh area would combine the Salt River and the two streams from Devils Peak and would change the catchment boundary, including the two into the Salt River catchment. The capacity of the marsh is 15 times the capacity of the current system within the study area. This would still require more attenuation facilities upstream in the catchment. Salt marshes also act as sponges and store water in their soil.

• Reduction of runoff

Increase in permeability in adjacent sub-catchment, a natural estuary is usually a space with little spatial restrictions, so that the floods from the sea and the river can expand and shrink. It is a permeable space, where the runoff from the surrounding areas is small compared to the discharge of the river and the tidal inflow. In the case of the study a high amount of runoff from the adjacent properties is draining into the canals. Because of the relatively flat surface of the area and the limited amount of space in the canals, it runs slowly and comes to localized flooding. To reduce localized flooding measures like rainwater storage on site in rainwater tanks and infiltration measures like bioswales and bio retention are proposed to help flood attenuation, these measures are not further explored in this study.

• Attenuation

Retention in the study area is only limited possible due to a lack of open spaces suitable for that, the open spaces close to the Zoarvlei wetland are getting flooded already. It has to be taken into account that in the whole catchment measures need to put in place to reduce flooding in the downstream area. The flood problem cant bes olved here alone.

• Increase in flow capacity

The widening of the channel in suitable places with the removal of the lining will allow for riparian zones with vegetation and different flow regimes. The connectivity in all 3 dimensions is important for riverine processes.The channel widening into the Salt marsh , will slow the flow down and spread it.

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Increase in storage capacity

Reconnecting the Salt River lateral and with the interstitial through the removal of the lining will allow groundwater recharge and storage within the soil during slow flows. The widend channel, its floodplains and the Salt marsh will also allow an increased capacity.

• Reduction in flow obstruction

Low bridges and debris jams on bridge columns are obstructing the flow during floods and can be the cause for over topping banks, damage to bridge structures and erosion of the river bed. Most bridges have to be widened and lifted to allow for the new wider channel. The railway line with the Ysterplaat station will be lifted on a long bridge crossing the marsh land.

• Prevention of tidal flooding during storm events

The new simplicity of the system only has one tidal inlet at the Salt River mouth, all other pipes, remnants of former systems get disconnected. This will prevent unexpected tidal flooding through these. The monitoring of the one visible inlet will be much easier. The salt marsh, with its area will also allow for storage before adjacent areas get flooded.

Objective: Enhancing biodiversity through encouraging estuarine processes

• Acknowledging the tidal change

An estuarine habitat is dependent on the tidal change of submergence and exposure. Currently with the canal being out of sight tidal changes are not visible and don’t impact on their surrounds. Birds could be observed during site visits using the low tide in the canal for resting and search of food. The constant water level and salinity change creates a specific habitat for species. The reclaimed salt marsh gives space to tidal changes and the new habitats emerging form that.

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Introduction of different flow types

Allowing fluvial process

Erosion and deposition being an in stream fluvial processes, that create constantly new channel forms, depending on the force of the flow. The reclaimed channel and marsh area give space and sediment for fluvial processes to happen. Meandering and anabranching will be encouraged. Riparian edges

SCENARIO RECLAIMATION

Scenario reclaimation

N Table storage capacity of reclaimation scenario, Source: created by author

All Riparian edges have a minimum 1:7 slope or shallower, where possible. The river dynamic will create new slopes with in stream fluvial processes. Vegetated riparian edges provide roughness to the channel, that creates different flow regimes in the different zones of the stream, low flow between the vegetation. With the more protected low flow regimes these areas give refuge to smaller and young species. They will be partially submerged or exposed during the tidal change. A buffer zone of at least 5m is added along all riparian edges for protection and better connection with the adjacent open spaces.

Objective: Reinstate connectivities

• Reconnection of the floodplain

Ecosystem functions of the riparian corridor include habitat, barrier, conduit, filter, source, and sink functions. A stream in dynamic equilibrium provides many more ecosystem services than transport of water and sediment. The riparian corridor is the vegetated zone along streams that forms the transition from aquatic to terrestrial habitat. (Matlock and Morgan, 2011) The flood plain reconnection is only possible in small spaces due to the spatial restriction of the urban fabric and the infrastructure of the surrounding areas.The Salt marsh will be floodplain for most of the year with the tidal change happening daily.

• Reconnection of interstitial

The reconnection of the interstitial especially within the Salt marsh will have a big impact on the groundwater

system of the area, which is currently dominantly fed by the seawater, due to the high percentage of impermeable space in the surrounding area. Existing vegetation might show the change first. The interstitial connection can provide the opportunity for benthic organisms to settle.

• Biodiversity connection between coast and mountain Rivers and their riparian edges also act as aquatic and terrestrial corridors that connect different biodiversity areas. The rehabilitated river corridor has the potential to connect Table Bay with Table Mountain.

• Water quality

The Salt River will be able to act as a buffer between land and sea and improve the water quality befor ot flows into Table Bay through the reduced flow velocity, the riparian edges, which will clean the water.

Objective: Other socio-economic benefits

• Flood reduction

Wetlands and floodplains are spaces to naturally store and save water in times of high rainfall and long periods of precipitation. (Haase, 2017) The risk of flooding is reduced through the bigger capacity of the Salt marsh, the wider channels with their reconnected floodplain and the additional storage capacity of the porous soil. Run off from the adjacent area is reduced through WSUD measures.

• Heat island reduction

A reduction of hard reflective surfaces will reduce the heat island generation. The bigger waterbodies will bring cooler air to the area.

• Air pollution reduction

Wetlands and marsh areas are carbon sinks and absorb nutrients and chemicals. Fluvisoils the typical soil type for wetlands are able to bind many organic and inorganic compunds dut to their high contents of fine grain material. (Haase, 2017) The “greening” of the area will also alow for a reduction of CO2 due to plant processes.

• Attractive landscape in a changing industrial environment

Rehabilitating the riverine landscape within the urban area will enhance the quality of life for the working and living there, with an attractive natural inspired landscape, that reintroduces tidal and seasonal change.

• Understanding the system

Bringing the river and its marsh land to the forefront the system is visible and easily understandable. The hidden pipes and concrete canals are removed for a system that is dominating spatially the area as it needs to. The threshold between sea and land and river becomes a new value , which will influence property value inadjacent areas. The Salt River could become a major attraction as a recreational area.

RECLAIMATION OF THE RAIL YARD

123 Scenario Reclaimation illustrated perspective, Source: created by author

8. Executive summary

To bring our rivers back to a close to natural status requires physical space and the determination and open mind by all stakeholders to look at a system in a holistic approach. Depending on the occurring status a river can be restored, rehabilitated or reclaimed to balance out the mistakes that were done in the past. We have to find a balance between the values of land and infrastructure to the risk of losing flood storage, habitat, fluvial systems that make our environment liveable and resilient.

• The research of the history of the Salt River and the reasons of change, helped to understand the current system and its deficiencies. 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 man made, or it resulted from a major storm event, that breached the dunes into the ocean. The Salt River was a river dominated estuary with a changing mouth position. The urbanization of Paarden Eiland and the harbour expansion resulted in the modification of the Salt River to a canal.

• The Salt River is a critically modified canal, which was engineered to use the least space and shortest route to sea. It has a very low gradient and the variations in cross section are between a canal with gabion or concrete walls

and an eroded earth channel. The substrate is mainly artificial. Channel vegetation consist of alien invasive species, which are infesting the canal for decades and are an indicator for nutrient rich water. Tidal and fluvial flow are interchanging, while the fluvial flow is low in summer supplemented by treated effluent from the two upstream WWTW's. Longitudinal continuity is not interrupted by barriers, only sediment transport is disturbed. The riparian banks are artificial concrete walls or gabions with no riparian vegetation. Lateral connectivity is decoupled, the land next to the canal is terrestrialized. The former floodplain was reclaimed for urban development. All three remaining water features are decoupled from each other. Lateral connection and movement between canal and floodplain is disconnected.

• A high density of crossings over the Salt River Canal will affect any modification of the canal like widening or rehabilitating. Currently the bridges only allow for the width of the canal. Bridge number 10 - Vortrekker Road - needs to be upgraded urgently as part of flood protection. Due to its low height it is a major flood obstruction, that results in flooding further upstream.

• The main continuous finding through all reviewed flood studies is that the capacity of the Salt River canal is too small to handle floods, which results in flooding further upstream in the catchment.

• Only the Salt River canal and Table Bay have an above ground visible connection. The other waterbodies (Zoarvlei wetland and Old Salt River canal) are connected un-

derground through the stormwater system. Because of the concrete lining of the canals there is no below ground groundwater connection between Zoarvlei, Salt River canal and Old Salt River canal. The separation of the system resulted in all these fresh water bodies belonging to three different catchments.

• A 15 meter widening of the canal was envisaged by the CoCT. This would result in water being still discharged fast. The lateral and interstitial fragmentation would persist, water quality would not change and the impact on the non canalized sections upstream could be even greater. All 10 bridges over the canal would need to be widened. This option doesn’t fulfil any other benefit, than faster discharge at a high infrastructure cost and would be a lost opportunity towards an improved quality of or added resilience to the system.

• Scenario restoration - would require a dynamic self-sustaining behaviour of the Salt River. Due to the spatial constraints, significant additional land allocation to the river, the river can’t be brought back to its original functioning and its original bed. A restoration could cause additional uncontrolled flooding.

• Scenario rehabilitation would be a hybrid between conventional and green engineering. Due to spatial and infrastructure constraints a rehabilitation is only viable in places, which would result in a compromised system with bottle necks. The existing situation of three systems that are not connected, the Zoarvlei, the Old canal and the new Salt River canal, is difficult to understand and to monitor.

All three are situated in the same low-lying plain and share the same flood risk at the same time. They can’t give immediate flood relief to the other two systems, because of non-connection. Holding on to a stormwater system, that was developed in phases and is therefore fragmented, doesn’t help to solve the bigger problems of biodiversity loss and flooding.

• Scenario reclaimation reintroduces the salt marsh, which restores the biodiversity connection and mitigates the flood risk. The spatial impact and the need for land is the in half way in between of the three scenarios. The simplicity of the system makes it more robust and easier to manage. It would bring the river back into the city as an entity and a visible system of constant change. Almost the full length of the canal can be rehabilitated. Incoming diadromous species can find breeding, food and refuge habitats. The marsh could become a destination for migrating birds. The size of wetland/marsh can make an improvement on the air quality and heat island risk of Paarden Eiland. The marsh can clean the water from upstream and reduce pollution of Table Bay. Initially a higher investment would be necessary in obtaining the necessary land. This scenario is most likely the most cost effective in the long term due to lower maintenance and operating costs. It would open up new development opportunities along Vortrekker Road and could become a destination for recreational activities and therefore contribute towards the cities vision. It will open up long term new development opportunities and strengthen the resilience of the city.