Abstract:
Design flood estimations display relatively wide confidence bands of uncertainty around all
estimates of flood magnitude-frequency relationships. Taking cognisance of this, and the fact
that most of the available design flood estimation methods in South Africa were developed in
the 1970s and have not been updated since, led to the development of the Standard Design
Flood (SDF) method (Alexander 2002a; 2002b; 2003). In this study, the SDF method was evaluated
by establishing the accuracy of the regionalised SDF runoff coefficients, taking both the areal
extent and homogeneous hydrological catchment responses into consideration. The SDF
runoff coefficients were evaluated, calibrated and verified at a quaternary catchment level in
SDF basin 9 (primary study area) and in 19 of the other 29 SDF basins in South Africa (secondary
study areas) by establishing catchment parameters and evaluating the ratios between the results
obtained through the SDF method and probabilistic analysis. The results showed that the original
SDF method overestimated the magnitude and frequency (return period) of flood peaks in all the
basins under consideration, while the verification results confirmed that the calibrated/verified
SDF method, based on quaternary runoff coefficients, significantly improves the accuracy in
comparison with the probabilistic analysis results. The result confirmed that the probabilisticbased
approach of the original SDF method does not have the ability to overcome the deficiencies
evident in the other design flood estimation techniques used in South Africa. Revision of the
runoff coefficients at a quaternary catchment level is proposed.
