Surface water resources for potable water supply in Cape Town, South Africa are constrained and the city is threatened with future water scarcity. Additionally, the rivers receive poor quality water from urban drainage affecting biodiversity and amenity. In this study, it is postulated that a design shift of stormwater ponds from conventional, i.e. flood control, to multi-functional where issues of: water supply, water quality management, biodiversity preservation and amenity provision are adopted could contribute towards addressing these challenges. In particular, the viability of stormwater harvesting from ponds as a source of supplementary water for the City will be investigated. Further, additional benefits that may be offered by stormwater ponds including: water quality improvement, increased amenity for local residents, the preservation of biodiversity and possible value capture from increased property values will be assessed. Consideration will also be given to the operation and maintenance requirements of enhanced ponds.

The Lotus River catchment, one of the 16 catchments in Cape Town with ponds, has been selected as a case study for detailed modeling to assess the viability of stormwater harvesting. Water demand and stormwater water resources are being assessed to identify areas and sectors which could be economically supplied through stormwater harvesting. The stormwater could either be used employing the 'fitness-for-purpose' principle, i.e. restricted to certain water demands e.g. irrigation of residential gardens and open parks, toilet flushing, car washing, street and car park washing, selected industrial uses such as cooling and cleaning, and urban agriculture; alternatively it could be blended into the raw water supply to one of the City’s bulk potable water treatment works. The detailed modelling of the Lotus River catchment is being done using the Water Evaluation And Planning (WEAP) – a water resources allocation model; and Storm Water Management Model (SWMM) – a detailed hydrological/hydraulic model. Stormwater pond storage options for water supply being assessed in the models include open surface water storage and Managed Aquifer Recharge (MAR) where appropriate. Open surface water storage will potentially be enhanced through the use of 'Real Time Control' (RTC), i.e. releases from ponds based on forecasted future storms so that the ponds are as empty as possible before a storm event and as full as possible afterwards. Water quality improvement is being estimated as the aggregated reduction of pollution by the ponds. An economic analysis with water supply, biodiversity preservation and amenity provision as some of the benefits will be carried out to assess the viability of the option.

In general, the outcomes from this study will include identification of areas where stormwater ponds can be adapted to function as water resources; identification of the potential benefits and costs of stormwater harvesting from stormwater ponds in these areas; an economic analysis to assess whether water supply from stormwater ponds is viable; a strategy for water supply from stormwater ponds in Cape Town; and preliminary guidelines for the design of multi-functional stormwater ponds.