Modelling the impact of SuDS on stormwater quality management in the Bongani River catchment, Knysna, South Africa

Abstract

The Bongani River is a primary source of polluted stormwater runoff discharging into the shallow Ashmead Channel, a portion of the Knysna Estuary situated on the southern coast of South Africa. One of the ways to improve the quality of stormwater in the Bongani River is to introduce sustainable drainage systems (SuDS) into the catchment area to improve stormwater management. The feasibility of reducing nutrient loads using SuDS was investigated using a continuous hydrological model of the Bongani River and its catchment. Besides the current situation (Current Scenario), various scenarios were developed in PCSWMM (Personal Computer Stormwater Management Model). The total phosphorus reduction objective for SuDS set by the City of Cape Town (used in the absence of Knysna-specific stormwater quality objectives) is 45%. All the scenarios modelled showed pollutant load reductions of between 47% and 78%, exceeding the 45% target, but none approached the pre-development baseline which indicated some 89% and 90% lower concentrations of total nitrogen and total phosphorus, respectively, compared to current conditions. This performance gap highlights the extent of nutrient enrichment in the Bongani River catchment and suggests that, while SuDS can provide improvements, additional watershed-scale interventions are necessary to restore water quality conditions.