Benefits of Real Time Control for Catchment Scale Stormwater Harvesting in Cape Town, South Africa

Abstract

South Africa is a water stressed country with average annual rainfall of about 450 mm (well below 860 mm world average) and low stream flow. In Cape Town, limits of conventional water resources i.e. from reservoirs, have effectively been reached and in 2018, the city faced the prospect of taps running dry because of the prolonged exceptional drought that commenced in 2015. With no additional major dam sites available near Cape Town, the city is exploring alternative sources e.g. groundwater, wastewater treatment and reuse, and desalination of seawater. This study assessed the potential of harvesting stormwater from the existing, largely dry, ponds situated in the 100 km2 Zeekoe catchment. Storage is critical as Cape Town has a Mediterranean-type climate with short, wet winters and long dry summers. Since the potential storage capacity of the existing ponds is somewhat limited and their physical increase not practical due to land limitations in urban areas, an investigation was carried out into storage enhancement using Real-Time-Control (RTC) techniques that would safeguard the original purpose of the ponds which was largely flood-control. The envisaged RTC would dynamically manage pond outlets based on water levels and the expected inflow volume from rainfall so that ponds are pre-emptively drained just sufficiently to maintain their flood-control function before a storm event and end up full afterwards. An assessment was undertaken to compare demand met with and without RTC. It was determined that application of RTC on ponds and vleis would increase their effective yield by 113–290%.