Intensity, energy and erosivity attributes of rainstorms in the KwaZulu-Natal Drakensberg, South Africa

Abstract

Rainfall intensity, kinetic energy and erosivity were analysed for 106 erosive storm events at five locations in the KwaZulu-Natal Drakensberg, from late 2001 to early in 2006. The stations cover an altitudinal range of 1060 m to 3165 m a.s.l. and provide the first detailed rainstorm data for the Drakensberg area. Erosive storm events, defined as total rainfall exceeding 12.5 mm and a maximum 5-minute intensity greater than 25mmh-1, are found to vary in duration and depth (total rainfall) with the distribution biased towards shorter, shallower storms. Erosive rainstorms are almost exclusively a summer phenomenon and the attributes of these storms (rainfall intensity, kinetic energy and erosivity) are positively correlated with rainfall depth, but not with storm duration. Inter-station similarities exist with respect to rainfall depths and mean kinetic energy from individual storm events. Altitudinal trends are, however, evident for storm maximum intensity, depths of erosive storms and cumulative kinetic energy. Together with frequency of erosive events and extent of collective erosive effects, all these rainfall attributes decrease with station altitude. Dissimilarities in cumulative kinetic energy and cumulative erosivity can be explained by the lack of erosive events during early and late summer on the escarpment and by significant erosive rains during this period at lower altitudes in the foothills.