ESTIMATION OF PROBABLE MAXIMUM FLOOD FOR APPLICATOIN IN DESIGN FLOOD HYDROGRAPH (A CASE STUDY ON TANA SUB-BASI

Abstract

Probable maximum Flood (PMF) is generally viewed as the flood resulting from a PMP , plus snowmelt where appropriate, applied to assumed antecedent basin conditions . It is j the flood that may be expected from the most severe combination of critical meteorological and hydrologic conditions that are reasonably possible in the region . It is J l J ] j J ] J J J J . J I ' J used in designing water impounding structures of which their failure results in extreme property damage and/or loss of life (which has no any compensation) . Procedures for selecting antecedent conditions and transforming water input to river flow vary among different agenci_ es and hydrologists , allowing significant variation in the PMF computed from given PMP and snowmelt inputs . So far there is no any common method for determination of PMF in Ethiopia. Different professionals used different approaches mostly based on the conversion of PMP estimated by Hershfield statistical method with frequency factor (Km) determined by the Hershfield's chart . The objective of th i s study therefore was to determine Probable Maximum Flood (PMF) based on the integration of probable maximum precipitation (PMP) obtained before with the help of a rainfall-runoff deterministic model in which the basin is taken as a system that has as input the rain histogram and as output the flood hydrograph . In this study , rainfall intensity of 1 - hour duration of selected self recording stations in Tana sub-basin was analyzed to obtain the conversion factor of 24-hour PMP to hourly base . HEC-HMS hydrological model of USA Army Corps of Engineer is used to generate the required PMF from the input of PMP and other basin data . Although, the analysis is with limited data sources the output of the study is within reasonable range with that of 10,000 years f lood and other previous studies .