On this study the effect of high urbanization rate of Addis Ababa City in general the study arearnon the Great Akaki River is assessed. The surface runoff generated from the catchment isrnestimated based on the rainfall intensity and major characteristics of the catchment area whichrnare the major factors for designing urban storm water drainage facilities and structures.rnSatellite image for 1989, 2000 and 2010 of the catchment area is taken based on the quality ofrndata and the available resolution. ArcGIS and GIS extension tools are used to extractrnhydrological characteristics of the catchment; HEC-RAS for hydraulic modeling, and HEC –rnHEC-HMS to simulate rainfall - runoff process on Great Akaki watershed which is the majorrnwatershed located at the center of Addis Ababa.rnThe hydrological and hydraulic modelings are accomplished by dividing the watershed in torndifferent sub-catchments. To compute infiltration loss SCS CN method; converting excessrnrainfall to runoff model SCS unit hydrograph, and channel flow routing accomplished by usingrnMuskingum routing method of HEC-HMS model. To evaluate the accuracy of the simulationrnmodel calibration and validation was conducted.rnThe hydrological modeling classified in to two: the first simulation shows the effect of highrnurbanization growth on the basin. Accordingly, the peak discharge for 1989, 2000 and 2010 atrnthe Bridge outlet along Addis Ababa Bishoftu Road is 131, 153.4 and 188.1m3/s respectively. Tornavoid the effect of rain fall variation on the generated peak discharge similar hourly rainfall of 18rnJuly 2010 was used for each respective year.rnThe second simulation uses frequency storm method for 10, 50 and 100 return periods and peakrndischarge of 403.80, 546.50 and 634.70m3/s for each respective return period was obtained. Thernresult found from HEC-HMS frequency storm method used for hydraulic analysis and flood maprnhazard generation. Flood inundation maps produced using ArcGIS to visualize flood depth andrnextent for each return period. Accordingly, maximum flood depth of 7.86, 9.07 and 9.82m forrn10, 50, and 100 year return periods respectively was found with flood extent of 82.34 for 10rnreturn period and 100.15 for both 50 and 100 year return periods at the middle of the final reachrnof the study area.rnKeywords: DEM, TIN, flood modeling; HEC-HMS/RAS; HEC- GeoHMS/RAS