Improving the water quality in the Zandvlei Estuary, Cape Town, by retrofitting sustainable drainage systems in the Diep River catchment

Abstract

The Zandvlei Estuary is the only functioning estuary along the False Bay coastline of Cape Town and is therefore of extreme local  ecological importance. The most significant problems are eutrophication and siltation caused by the increased total inorganic nitrogen  (TIN) and soluble ready phosphorus (SRP) levels due to urban development and the associated increased impervious surface area in the  catchment that drains into it. In South Africa, stormwater drainage systems conventionally channel everything they collect into receiving  water bodies without significant treatment. Sustainable drainage systems (SuDS) provide an alternative approach to managing  stormwater runoff. They are designed to manage both stormwater quality and quantity while potentially improving biodiversity and  amenity. This project modelled the potential improvement in the quality of the water entering Zandvlei Estuary resulting from the  implementation of selected SuDS control measures in Zandvlei’s Diep River catchment using the software program, PCSWMM. SRP, TIN,  total phosphorus (TP) and total suspended solids (TSS) were selected as pollutant indicators. Treatment trains that included a large,  constructed wetland at the bottom of the catchment will likely provide the greatest improvements to the water quality entering Zandvlei  – potentially reducing SRP, TIN, TP and TSS by approximately 59%, 53%, 53%, and 66%, respectively – as well as potentially reducing the  runoff by 48%.