Evaluation of the use of flood attenuation controls for the management of urban stormwater impacts in Cape Town, South Africa

[摘要] ENGLISH ABSTRACT: In the context of rapidly expanding cities, it is imperative that urban planning in South Africa hassufficient guidance regarding stormwater and river corridor management, in order to providesolutions that address issues of flood risk and the environmental health of river systems.Attenuation of stormwater runoff, the focus of this study, is one of the most important structuralmechanisms used for the mitigation of many of the negative impacts caused by uncontrolled urbanrunoff. Typically, it involves the use of attenuation ponds or wetlands, which temporarily storerunoff during a storm and release flow downstream at a reduced rate so as to mimic natural flowpatterns.The focus of urban stormwater management and flood control has historically been on theprotection of human life and property. However, in recent decades, through growing environmentalawareness and the advancement of the concept of sustainable development, urban stormwatermanagement has become a growing field of research worldwide, with a broader focus whichconsiders not only flood control, but also water quality, aquatic biodiversity and the amenity value ofurban drainage systems. Flood attenuation controls are becoming more widely used within SouthAfrican urban areas, primarily due to policies or legislation brought into effect by local authorities.However, there is often little understanding regarding the positive and perhaps negative effects thatthese attenuation controls are having on receiving watercourses downstream.Three case studies were assessed by means of stormwater modelling simulations to evaluate variousflood attenuation practices which are currently in use in South Africa. Two of the study areas, theMosselbank River Catchment and the Bayside Canal Catchment, were selected in areas of CapeTown where future development has been proposed by spatial planners. The third study area, theUpper Kuils River Catchment, was evaluated in terms of the performance of existing attenuationfacilities in an area which is already almost completely developed. The study found that attenuationfacilities constructed with a single culvert-type outlet structure, designed to reduce flows duringlarge storm events, do not mitigate the impact of post-development runoff occurring during lowerrecurrence interval storm events. Attenuation facilities with multi-stage outlet structures werefound to be much more effective at mimicking pre-development flow during a range of stormevents. It was also found that because attenuation does not reduce post-development runoffvolumes to pre-development levels, but merely reduces peak flow rates, the cumulative runoff frommultiple attenuation controls across a large (>30 km2) urban catchment resulted in higher runoffpeaks in downstream watercourses.The study concluded that more widespread use of stormwater Best Management Practices (BMPs)and Sustainable Drainage System (SuDS) controls allows a greater portion of runoff to infiltrate,resulting in less runoff volume and therefore reduced peak flows downstream, especially during lowrecurrence interval storm events. In addition, the study recommended the use of detailedcatchment-wide stormwater modelling to understand specific catchment dynamics holistically, thusincreasing the potential for designing effective attenuation controls in urban stormwater systems.

[发布机构] Stellenbosch University