Dams are constructed for specific purposes such as irrigation, flood control, navigation, sediment control, and hydropower generation. Gidabo dam is one of such dams and provides flood control and irrigation water for sugarcane and rice. It is an earth fill dam having 25.8m height and 335m crest length with side ogee spillway to pass 10,000 years’ flood. There were different economic developments downstream of the dam including irrigation command area, irrigation structures and settlement areas. Since Nature is full of uncertainties, it is likely that Gidabo dam would subjected to sudden breaches due to the probable maximum flood (PMF).Therefore dam breach analysis and flood inundation map preparation were conducted. The Hydrologic Engineering Center’s River Analysis System new version (HECRAS 5.0.3) was used to analyze the dam breach for overtopping failure. RAS Mapper, which is, GIS tools of HEC-RAS and ARC-GIS were used to develop flood inundation map. Dam breach parameters were calculated within the HEC-RAS model by using HEC-RAS breach parameter calculator tab. A two dimensional unsteady flow simulation of the Gidabo dam breach was performed by using the inflow hydrograph as upstream boundary condition. From the rainfall data analysis the probable maximum precipitation (PMP) resulting probable maximum flood (PMF) was found 273.32mm that results a peak inflow of 6387.1m3/s. The resulting breach parameters were 143m breach bottom width, 1.4 breach side slope (H: V) and 2.57hr breach formation time. The peak breach outflow was found to be 15848.85m3/s which covers 2046 hectares with maximum depth of 12.14 m. From total area, the inundated area coverage 72.8% accounts for the commands areas and the population settlement area are at the risk from the left part of the canal.