A review of the regional maximum flood and rational formula using geomorphological information and observed floods

Abstract

Flood estimation methods in South Africa are based on three general approaches: empirical, deterministic and probabilistic. The "quick" methods often used as checks are the regional maximum flood (RMF) and the rational formula (RF), which form part of the empirical and deterministic methods respectively. A database of annual flood peaks was used in a probabilistic approach to review these methods and to provide preliminary insight into their estimates of flood peaks. This paper examines the following: the relationship between floods and landscape; the estimation of the return period of the RMF; the use of ratios in scaling RMF flood peak estimates to flow rates of shorter return periods; the applicability of the modified rational formula (MRF); the examination of the relationship between scaling parameters and regional parameters. It turns out that the RMF is the best of all methods examined in this preliminary study (other than statistical) in estimating the 200-year flood peak at an ungauged location.