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.

Hydraulic modelling and floodplain mapping

The design discharge can then be converted into an estimate of flood water level and velocity, using a hydraulic model. The Hydrologic Engineering Center’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.

Key outputs of the HECRAS model are the flood depth and velocity – which are used in generating a flood hazard assessment (FHA). The flood hazard along a river can then be categorised as high, medium and low, considering the water’s velocity and depth, the flood levels and extent of flooding, and the impact of excess flow on physical developments.

Benefits of assessment

For municipalities, understanding flood risk allows effective early warning systems to be developed and implemented, to alert communities and other relevant agencies such as emergency responders. An FHA can also identify areas where relocation of vulnerable infrastructure and communities might be necessary.

In the permitting of building or infrastructural developments, an FHA can be used to determine which areas are within the flood line, and where flood remedial measures may need to be performed.