Rapid urbanisation, population growth and climate change will continue to have adverse effects on catchment areas in South African cities. The two major problems that have historically occurred in the Zandvlei Estuary, located in Cape Town, South Africa, are elevated nutrient and sediment concentrations which have led to eutrophication and siltation of the waterbody and is problematic for water quality, biodiversity, and the tourism industry. Sustainable drainage systems (SuDS) aim to holistically manage surface water drainage in a manner that alleviates the negative environmental impacts of urban stormwater runoff.

Conventional engineering solutions have historically failed to address stormwater quality and ecosystem goods and services in urban catchments. The primary objectives of this report included the hydraulic and hydrologic modelling of the Keysers Catchment in the computer software PCSWMM and developing diverse sets of SuDS treatment train scenarios aimed at reducing nutrients and sediments from being transported to the outlet of the river system.

The treatment trains were implemented into the PCSWMM scenarios using low impact development (LID) controls and storage areas which mimic natural drainage systems such as wetlands. The three SuDS simulations developed in the PCSWMM model were compared to the current scenario in a 1 in 6-month storm event by calculating the percentage reduction of nutrients as total Kjeldahl Nitrogen (TKN) and total Phosphorous, as well as sediments as total settleable solids (TSS) at the outfall of the river system.

The SuDS scenario included several swales in the Cape vineyards, a swale-infiltration trench combination along the M3 Freeway, three bio-retention areas in Steenberg, as well as two wetland areas in the low-lying urban areas of the Keysers Catchment. The percentage reductions in this scenario for TKN, TP, and TSS were 69%, 65%, and 63%, respectively.

The combination scenario focussed on improved connectivity of the currently diverted Keysers River with the Dreyersdal Farm dam and the Dreyersdal wetlands. This involves grading away the berm that separates the water bodies and creating the following: development of a sediment forebay at the outlet of the Keysers River from beneath the M3 Freeway which flows into the Dreyersdal Farm dam, which releases into the Dreyersdal wetlands which conveys stormwater to where the Keysers River continues downstream. The percentage reductions in this scenario for TKN, TP, and TSS were 68%, 57%, and 50%, respectively.

The wetlands scenario involved re-establishing wetlands in the lower reaches of the Keysers Catchment. The Dreyersdal wetlands, the area between Honeywell Road and Military Road, as well as the area upstream of the mouth of the Keysers River, were considered for the scenario. The percentage contaminant reductions in this scenario for TKN, TP, and TSS were 78%, 71%, and 67%, respectively, and was deemed the most effective solution to improving water quality.

In conclusion, the wetlands scenario was the most effective at removal of both nutrients and sediments, however it is recommended to use the bio-retention areas presented as they address amenity benefits for the community. The SA SuDS Guidelines stresses that effective household and source control measures need to be taken up by communities and private land-owners to fully reap the benefits of the green infrastructure provided.

Figure 1: A map of the Zandvlei catchment, showing the location of the Keysers River.

Figure 2: Land use types in the Keysers River catchment.

Figure 3: The current scenario of the Keysers Catchment modeled in PCSWMM.

Figure 4: Rainfall-runoff relationship for the three storm events used in the current scenario.

Table 1: Pollutant loads for the current and pre-development scenarios for three storm events.

Figure 5: The comparison between pre-development (green curve) and post-development (blue curve) runoff volumes.

Figure 6: Comparison of the percentage reductions for the three scenarios implemented in PCSWMM.