The hydro-meteorological estimation of probable maximum precipitation under varying scenarios in Sri Lanka

Abstract

Probable maximum precipitation (PMP) is widely used as an input in rainfall-runoff modelling to estimate probable maximum flood (PMF), which is required for the design of spillways of large dams. In relation to dam safety studies at 32 dam sites, an analysis was done of extreme rainfalls in Sri Lanka, which is influenced by the Asian monsoon system and cyclonic effects. Accordingly, the Hershfield’s statistical method, which is one of the recommended procedures of the World Meteorological Organization (WMO), was used for estimating PMP. It is also a technique that can be readily used in design office practice. However, it is of significant value and scientific interest to explore what the outcome would have been if the physically based hydro-meteorological procedure had been adopted. For this purpose, long series of annual maximum daily rainfall data from seven stations were analysed. The research shows that the maximum moisture and corresponding wind run or the use of maximum moisture alone are two scenarios, which yield results compatible with the statistical PMP. Although globally there is a tendency to exclude wind maximization, this does not automatically apply to a country like Sri Lanka, which experiences cyclonic rainfall and special attention should be paid to precipitation efficiency. These results are further confirmed by a frequency analysis based on the generalized extreme value (GEV) distribution. An approximate value of PMP could be in the order of 2.0–3.0 times the maximum observed daily rainfall. However, this aspect needs further investigation when observed maxima are near PMP storms.