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.
