Flood risk assessments are critical for authorities to gather the data needed to develop and implement effective flood mitigation strategies, according to Tshilidzi Netshitangani, Principal Scientist at SRK Consulting (South Africa).
“Such mitigation strategies could include building levees, improving drainage systems and implementing flood control measures”, said Netshitangani. “But good practice is that the municipality develop flood hazards and identify potential impacts along the main rivers and stormwater conduits”.
Flooding in South Africa is destructive and often leads to municipalities issuing disaster conditions, he noted, causing significant damage and disrupting livelihoods. Managing the risk of floods requires careful consideration of a number of 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 the number, location and types of buildings and other assets that could be damaged.
Modelling of Rainfall Run-Off
“Lack of hazard and risk assessments can lead to poor risk management decisions, from insufficient protection to the wasting of scarce financial resources on unnecessary protection”, he said.
“In many situations, discharge measurements are either non-existent or of insufficient quantity or quality to be able to conduct a discharge frequency analysis”.
In such situations, he explained, 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.
Estimating the Design Discharge
Rainfall runoff models are used to estimate peak discharge, that passes a certain point during a flood.
