Evaluation of critical storm duration rainfall estimates used in flood hydrology in South Africa

Abstract

Design rainfall comprises of a depth and duration associated with a given probability of exceedance or return period. The purpose of the study was to evaluate and compare the methods used in flood hydrology to estimate depth-duration-fre quency (DDF) relationships of design rainfall in South Africa based on the critical storm duration or time of concentration (TC) of a catchment. The influence of the type of rainfall, areal and temporal distribution of rainfall were also investigated to establish if a relationship exists between the catchment area, T Cand areal reduction factors (ARFs). The DDF relationships based on the least-square regression analyses of Log-Extreme Value Type 1 distributions, the modified Hershfield equation, the regionalised South African Weather Service (SAWS) n-day design rainfall data and the Regional Linear Moment Algorithm and Scale Invariance (RLMA&SI) approach were compared in 3 distinctive T C-ranges. The results showed that the RMLA&SI approach can be considered as the preferred DDF relationship in future design flood estimations.

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