Managing the Water Quality of the Zandvlei Estuary using Sustainable Drainage Systems in the Diep Catchment

Thewlis, Geordie

Managing the Water Quality of the Zandvlei Estuary using Sustainable Drainage Systems in the Diep Catchment

Master Thesis

2022

Abstract

The Zandvlei Estuary is the only functioning estuary-wetland river system along the False Bay coastline and is therefore of extreme ecological importance. The estuary has and continues to face threats from excessive amounts of sediments and nutrients that can destroy its functionality. Historically, Zandvlei's most significant problems are eutrophication and siltation, which have occurred in the estuary due to elevated nutrient and sediment levels. Increased Total Inorganic Nitrogen (TIN) and Soluble Ready Phosphorus (SRP) are the primary causes of eutrophication in receiving water bodies. The excessive amounts of nutrients and sediments deposited into the estuary are a result of urbanisation and the subsequent change in anthropogenic activities in and around the Zandvlei's catchments. Additionally, the increased impervious surface area associated with urbanisation has caused a significant rise in runoff volumes and runoff rates in the stormwater drainage systems. In South Africa, stormwater drainage systems conventionally channel everything they collect into receiving water bodies without significant treatment or intervention to remove harmful substances. Sustainable Drainage Systems (SuDS) provide an alternative approach to managing stormwater runoff. Unlike the conventionally used stormwater systems designed with the singular goal of removing runoff, SuDS can provide both stormwater quantity and quality management while also allowing for the development of biodiversity and amenity. This project investigated the viability of improving Zandvlei Estuary's water quality by implementing SuDS in Zandvlei's Diep Catchment. PCSWMM, a modelling software developed by Computational Hydraulics International (CHI), was used to develop seven models. Included was an As-is model, developed, calibrated and verified to represent the Diep Catchment in its current state, a Pre-development model that provided an estimate of the Diep Catchment's runoff volumes and stormwater constituent loads before urban development altered the catchment, and five SuDS scenarios to test various treatment train designs. Pollutant indicators that were modelled included SRP, TIN, Total Phosphorus (TP) and Total Suspended Solids (TSS). SRP and TIN were included as they cause eutrophication, while TP and TSS were modelled as they are good measures of pollution. Scenario 1 utilised Source Control SuDS to capture and treat runoff as close to its source as possible. Scenario 2 reintroduced four wetlands and two retention ponds into the main river network. The stormwater network currently only uses these wetlands and ponds as attenuation storage during large rainfall events. Scenario 3 proposed a large, constructed wetland at the confluent of the two major stormwater systems in the Diep Catchment, Langevlei Canal and the Diep/Sand River. Scenarios 4 and 5 utilised combinations of SuDS controls from Scenarios 1, 2 and 3 to create more holistic treatment train systems. Scenario 4 employed the Source Controls from Scenario 1 and the large, developed wetland in Scenario 3. Scenario 5 utilised two wetlands and a retention pond from Scenario 2 and the large wetland from Scenario 3. The SuDS scenario results were compared to those obtained from the As-is Scenario to provide percentage reductions of indicator loads and runoff volumes. Additionally, the Pre-development results, which indicated the likely natural indicator loads and runoff volumes and thus probably best represented sustainable conditions, served as a benchmark against which all the modelling results could be compared. Scenario 5 provided the most significant mean reduction in indicator loads (57.3%), while Scenario 4 provided the largest drop in runoff volume, approximately 52%. As the primary goal of this project was the reduction in indicator loads, Scenario 5 would provide the most significant improvement in Zandvlei Estuary's water quality.

Keywords

Civil Engineering

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