ESTIMATION OF PROBABLE MAXIMUM PRECIPITATION FOR PROBABLE MAXIMUM FLOOD DEVELOPMENT AND APPLICATION IN DESIGN FLOOD HYDROGRAPH ( A CASE STUDY ON BLUE NILE BASIN(BNB))

Abstract

Probable maximum precipitation (PMP) is a characteristic of rainfall at a particular location that can be used in designing water impounding structures the resulted extreme floods (PMF) have long been used to design the flood­ handling facilities of major dams whose failure might cause loss of life or extensive property damage. So far the PMP for rainfall stations in Ethiopia have been estimated by Hershfield statistical method with frequency factor (Km) determined by the Hershfield's chart. But the chart was originally developed by using over 2,600 metrological stations ninety percent of which were taken from USA which might not give reliable estimates for a country like Ethiopia having different climatic conditions with that of USA. Recent research outputs of different countries like China, Romania (Desa et al. (2001) and M.N. Desa et al. (2003)) also showed that the frequency factor (Km) resulting from the Hershfield's chart is not reliable world over. The objective of this study therefore was to evaluate the frequency factor obtained from the Hershfield's chart and to provide fresh and reliable estimates of PMP in Ethiopia using historical rainfall data. In this updating study, annual maximum 1-day, 2-days and 3-days rainfall data from 64 rainfall stations with records longer than 15 years found in Blue Nile Basin, were analyzed to obtain estimates of point PMP using an appropriate frequency factor calculated. Results of the 24-hours, 48-hours and 72-hours PMP are presented in the mapped form. In Ethiopia also comparison of the PMP value using the new Km