The Bongani Stream is a primary pollutant source of surface water runoff discharging into the Ashmead Channel, a shallow portion of the Knysna Estuary situated on the south coast of South Africa (Harvey, 2019). The Ashmead Channel is experiencing persistent macroalgal blooms which threaten the ecosystem’s biodiversity.
A hydrological model of the Bongani Stream was developed with the objective of investigating the feasibility of reducing pollutant loads using Low Impact Development (LID). It was developed in PCSWMM (Personal Computer Stormwater Management Model) that was calibrated using flow data collected during the study. Water quality was indicated through the use of event mean concentration (EMC) wash-off parameters for Total Nitrogen (TN), Total Phosphorus (TP), and Total Settleable Solids (TSS) which were estimated from an analysis of grab samples. The calibrated model was designated as the Current Scenario and was compared to the following additional scenarios:
The Settlement Upgrade Scenario suggested that the TN and TP loads would be reduced by 79-82% and 73-80% respectively, performing better than the other LID intervention scenarios. The Bongani Wetland Scenario indicated the greatest reduction of TSS (by 49-50%) and total flow volumes (by 77%).
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