Revised Regional Maximum Flood (RMF) method and regionalisation

Abstract

South Africa receives an average annual rainfall of about 450 mm. Hydraulic structures are typically constructed to either store or manage the excess water resulting from runoff. These hydraulic structures are designed and evaluated to withstand a particular flood peak that can occur in its catchment area. Adequate flow or rainfall records may often not be available to enable a reliable flood estimation. In South Africa an empirical estimation method (the Regional Maximum Flood (RMF)) that utilises regional envelope curves to estimate the maximum observed flood peaks that can be expected in a region, is available. The RMF method adopted by Kovács in 1980, and revised in 1988, is robust and simple to use. The current research revisits the method as applicable to South Africa, and presents an update of the method, taking more than 30 years of additional data and a revised regionalisation approach into consideration. Numerous previous researchers evaluated the RMF method and concluded that the method needs to be updated. It was identified that recently observed flood peaks exceeded the existing RMF envelopes. It was further identified that the Kovács regionalisation process is inconsistent, and a revised regionalisation approach was proposed. The revised regionalisation resulted in 15 RMF K regions and their associated envelope curves. The new RMF K regions are smaller, with the highest K value equal to 5.8 and the lowest value 2.8. The recommended envelope curves were drawn 15% above the maximum observed flood peaks for each region, allowing for possible future climate impacts. The revised RMF envelope curves are considered to adequately represent the RMFs in South Africa and are therefore applicable for determining the expected maximum regional flood at any site in South Africa.