The prospects for stormwater harvesting in Cape Town: Part 1 – transforming existing stormwater ponds into reservoirs using real time control

Abstract

In 2018, the City of Cape Town (CCT) in South Africa came close to ‘Day Zero’ – the day taps would run dry due to an extreme drought that began in 2015. With severe droughts likely to be common in future, this study investigated the potential for catchment-scale stormwater harvesting facilitated by the transformation of stormwater ponds into reservoirs. In this new approach, water levels in the ponds would be dynamically managed using real-time control (RTC) to ensure continued use as flood control infrastructure. The study was restricted to the 89 km² Zeekoe Catchment situated on the Cape Flats in the southern part of the city. Assuming the water would be used for outdoor non-potable uses such as agriculture and residential gardens, the temporal mismatch between the seasonal demand and winter rainfall meant limited supply. Stormwater could meet a limited percentage of the demand, but with most lost to the sea as overflow. To minimise the effect of the mismatch, it was determined that at least 4 Mm³ balancing storage i.e., 20–30% of the mean annual stormwater volume estimated at 18 Mm³, was required. The available 1 Mm³ storage (5.5% of the mean annual stormwater volume) in the catchment was found to be inadequate as the stormwater supplied from the storage would only meet 44–60% of the demand, with a spill (water lost as overflow) of 35–51%. Dynamic management of the ponds with RTC was investigated to provide the required storage. This involved continuous adjustment of stormwater flow rates with a set of rules to optimise storage capacity. With this management approach, it was possible to achieve the required 4 Mm³ to meet the identified demand in the study area and minimise the loss through spill.