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-frequency (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, TC and 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 TC-ranges. The results showed that the RMLA&SI approach can be considered as the preferred DDF relationship in future design flood estimations. The results also showed that a direct relationship exists between the catchment area and TC, thus ARFs can be explicitly expressed in terms of only the catchment area. Keywords: Rainfall, depth-duration-frequency, time of concentration, areal reduction factors, design flood
Highlights
Design rainfall comprises of a depth and duration associated with a given probability of exceedance or return period
The studies focusing on long durations based on daily point rainfall data included studies done by the SAWB (South African Weather Bureau) (1956), Schulze (1980), Adamson (1981), Pegram and Adamson (1988) and Smithers and Schulze (2000b)
The purpose of the study was to evaluate the methods used in flood hydrology to estimate DDF relationships of design rainfall in South Africa, based on the critical storm duration or time of concentration (TC) of a catchment, in 3 distinctive TC-ranges
Summary
Design rainfall comprises of a depth and duration associated with a given probability of exceedance or return period. Short and long duration design rainfall estimations can either be based on point or regionalised data. Rainfall durations less than 24 h are generally classified as short, while long durations typically range from 1 to 7 days (Smithers and Schulze, 2004). The studies focusing on long durations based on daily point rainfall data included studies done by the SAWB (South African Weather Bureau) (1956), Schulze (1980), Adamson (1981), Pegram and Adamson (1988) and Smithers and Schulze (2000b). Smithers and Schulze (2000a; 2000b) used a regionalised approach in an attempt to increase the reliability of the design values at gauged sites, as well as for the estimation of design values at ungauged sites (Smithers and Schulze, 2003)
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