Using statistical functions specifically developed for South African environments, potential flood damage in municipalities can be calculated, paving the way for a cost-benefit analysis for establishing the optimal suite of flood mitigation measures.
Such mitigation strategies could include building levees, improving drainage systems and implementing other flood control measures. The important starting point is the effective assessment of flood risk, which requires authorities to gather a range of relevant data, to identify potential flood impacts along the main rivers and stormwater conduits.
Managing the risk of floods requires careful consideration of several factors, including catchment properties such as, size, topography, land use, and the types and characteristics of storms that produce rainfall. It also requires accurate assessment of properties in a floodplain. Information such as the number, location and types of buildings and other assets that could be damaged are used in calculating the possible impact.
Rainfall Run-off and Design Discharge
Lack of hazards and risk assessment can lead to poor risk assessment decisions from insufficient protection to the wasting of scarce financial resources. In many situations discharge measurements are not readily available and up to date or of insufficient quantity or quality to be able to conduct a discharge frequency analysis.
In such situations, one of a broad class of tools known as rainfall run-off models can be used to convert estimates of extreme rainfall into design discharge estimates and design hydrographs. Rainfall runoff models are used to estimates peak discharge, which is the volume of water that passes a certain point during a flood.
Hydraulic modelling and floodplains mapping the design discharge can be converted into an estimate of flood water level and velocity, using a hydraulic model. The Hydrologic Engineering Centre’s River Analysis System (HECRAS) model is among those that SRK uses for a flood hydraulics study. HECRAS calculates high -water levels for different steady flow conditions, considering structures and controls along the watercourse.
