Poor water quality in the Keysers River is stimulating eutrophication and siltation in Zandvlei Estuary near Cape Town, South Africa. The catchment is representative of many developing urban watersheds which share the global challenge of diverse pollutant transport in stormwater runoff from agricultural, industrial, commercial and low-income residential land-use. Green infrastructure presents local governments with innovative, lower-cost opportunities to build water resilience, enhance liveability and create biodiverse urban ecosystems which contrast hard stormwater solutions that are expensive in regions where limited budgets are constrained by expenditure on basic service provision. This study aimed to determine whether the implementation of Sustainable drainage systems (SuDS) in degraded urban catchments could enhance natural drainage conditions. A hydrological model of the catchment was developed in the stormwater modelling software PCSWMM (CHI, 2018). Three SuDS scenarios were modelled using low impact development (LID) controls and storages which mimic natural drainage areas. Pollutant loads were compared in a 1 in 6-month storm event to determine reductions in Total Kjeldahl Nitrogen (TKN), Total Phosphorus (TP) and Total Suspended Solids (TSS). The first scenario included a variety of SuDS with swales and infiltration trenches for the vineyards and major roadways upstream, and bioretention areas and constructed wetlands for the low-lying urban areas. The second scenario used a combination of a retention pond and constructed wetland which focused on enhancing connectivity between the diverted Keysers River, an adjacent farm dam and wetlands downstream. Lastly, a wetland scenario was investigated and involved re-establishing wetlands and bioretention areas throughout the lower, urbanized reaches of the catchment. The wetland scenario’s model results showed the most effective removal of nutrients and sediment from the system. Furthermore, the study recommended that bioretention areas be developed in the low-lying urban areas and that effective sediment management strategies are implemented in the vineyards and major roadways.

Figure 1. The current scenario of the Keysers River catchment modelled in PCSWMM (CHI, 2018)

Figure 2. Pollutant load reductions for the three scenarios implemented in PCSWMM