FLOOD FORECASTING AND EARLY WARNING SYSTEM IN LOWER AWASH SUBBASIN

Abstract

Flooding in Lower Awash sub-basin of the Awash River Basin, alike the other eight basins in Ethiopia, are attributed to rivers that overflow or burst their banks to inundate low lying areas. Logiya and Mille rivers are the two rivers in Lower Awash sub-basin that flow through the flat fields of flood plain areas. The objective of this study was to develop early flood warning system for the Lower Awash Sub basin in order to reduce the impacts of flood problem in the sub basin using the hydrological, meteorological and spatial data. Rainfall runoff models were processed for both catchments, Logiya and Mille using the hydrologic engineering center hydrological modeling system and the soil moisture accounting and routing models. The predicted hydrograph was calibrated against observed one and the model parameters were optimized automatically for good simulation. HEC-HMS model was found better in both the coefficient of determination and Nash-Sutcliffe efficiency criteria and fairly applied in both catchments for supporting flood warning decision making. The calibration and verification results of the HEC-HMS rainfall-runoff model for both Mille and Logiya catchmentsrevealed good (ENS>0.65) efficiencies. The hydrologic engineering center HMS model and the soil moisture accounting and routing model were tested to estimate the possible lead time, and the former performed well. From the hydrological routing the lead time was estimated as two days for Logiya catchment and one day Mille catchment using the hydrologic engineering center HMS model. Therefore, the decision makers can use this lead time as input data what kind of measure to take and to evacuate the public and their properties.