FLOODPLAIN INUNDATION MODELING AND MAPPING FOR GILGEL ABAY RIVER: NILE BASIN, ETHIOPIA

Flood is an overflowing of water over land surface when the volume of water raises higher than the banks of the water body. Ethiopia is one of the east African countries that is severely affected by it due to the countries rainfall characteristics in which, more than 80% of its annual precipitation falls in four wet month. The objective of this research was to model the flow characteristics of Gilgel-Abay River and to develop flood map for the area. Integration of HEC-HMS and HEC-RAS with ARC-GIS was used to create a regional model for floodplain determination, modeling analysis representation. HEC-HMS was used for hydrologic model calibration with the use of daily time series data for return periods of 2, 10, 25, 50 and 100 years and the model was compared with different frequency analysis methods. HEC-RAS with HEC-GEORAS interface in coordination with ARC-GIS was used to carry out the two dimensional hydraulic modeling and generation of flood maps. Daily discharge and rainfall data were used for HEC-HMS Calibration and Validation with both results showing good match between measured and simulated precipitation data, with acceptable range of coefficient of determination R2 (between 0.7 and 1, Calibration value being 0.72 and Validation 0.73) and Nash-Sutcliffe (NSE) (between 0.5 and 0.75, Calibration value being 0.51 and Validation 0.54). The result of hydrologic model demonstrated peak discharges of 223.9 m3/s, 367.5 m3/s, 438.4 m3/s, 440.5 m3/s and 501.6m3/s for return periods of 2, 10, 25, 50 and 100 years respectively