DEVELOPMENT AND ASSESSMENT OF A SINGLE-EVENT DESIGN FLOOD ESTIMATION TOOL: CASE STUDY IN FOUR CLIMATOLOGICAL REGIONS OF SOUTH AFRICA

The estimation of design floods characterised by a specific magnitude-frequency relationship at a particular site is necessary for the planning, design and operation of hydraulic structures. Both the occurrence and the frequency of floods, along with the uncertainty involved in the estimation thereof, contribute to the practitioners’ dilemma to make a single, justifiable decision based on the results obtained from the various single-event design flood estimation methods available internationally. This paper presents the further development and assessment of a Design Flood Estimation Tool (DFET) in four climatologically different regions of South Africa. The DFET was developed and programmed using Microsoft Visual Basic for Applications and the functionality and application thereof were assessed by comparing the DFET-based probabilistic, deterministic and empirical design flood estimates in 12 gauged catchments. The refined and improved DFET proved to be an easy to use software tool for the rapid estimation and assessment of alternative design flood estimation methods for at-site applications in both gauged and ungauged catchments. In general, the empirical methods resulted in more acceptable design flood peaks than the deterministic methods when compared to the at-site probabilistic flood peaks. Furthermore, the use of more complex deterministic methods, e.g. Synthetic Unit Hydrograph (SUH) method, as opposed to the more simplified deterministic methods, e.g. Rational method (RM) and Soil Conservation Services (SCS) method, does not necessarily guarantee and result in more acceptable peak discharge estimates. The latter also emphasises that no single design flood estimation method could be regarded as superior to any other method when the wide range of flood magnitude-frequency problems encountered in practice, needs to be solved. Practitioners still have to apply their own experience, knowledge and skills to these particular problems until the current National Flood Study Programme (NFSP) narrows the gap between flood research and practice in South Africa.