Dam breach modeling and flood inundation mapping is a general process of predicting the dam breach parameters, routing of the outflow from the breached dam and mapping of the routed outflow to the downstream to control and minimize the hazards. The aim of this study was the modeling of a dam breach and flood inundation mapping of the Aguat-Wuha dam in upper Abbay. Meteorological and Hydrological data were collected from the National Meteorological Agency and Minister of Water, Irrigation and Electricity respectively. The rainfall data PMP, were analyzed using Hershfield statics method. During the analysis of rainfall-runoff data in HEC-HMS, the performance of the model was checked by using statistical parameters; Nash-Sutcliffe Efficiency, Coefficient of Determination and Relative Volume Error and their value are 0.6143, 0.6880 and -6.124 for calibration and 0.5724. 0.6254 and 5.293 for validation respectively. HEC-GeoRAS was used to digitize and extract topographic data from the TIN that generated from DEM and used as geometric data for HEC-RAS 5.0.6. HEC-RAS model was used to determine the maximum surface water profile, simulate outflow and route outflow hydrograph and also velocity profile for both piping and overtopping mode of failure. Froehlich 2008 method was used to calculate the breach parameters. One Dimensional unsteady analysis was performed using HEC-RAS and flood inundation map has been prepared using HECGeoRAS. The simulated result showed that peak outflow due to breach at the dam was 3254.24 and 3469.76 m3/s for both piping and overtopping failure respectively. The flood depth after the breach was in the interval of 0.00021-29.6855 m and maximum water depth was observed at 1.2 km distance from the dam to downstream. The velocity of the flood was ranging from 0.000321 - 6.84612 m/s. The total area covered by the flood was 181.78 ha. During analysis, both overtopping and piping failures were observed and overtopping failure was the worst condition since it gives the highest peak flow. Mapping was done for overtopping failure and the affected area is within 5km. In general, this study highlights the flood extent of the area to take for future developments. For this, details assessment of hazards and level of damage is required for Emergency Action Plan.