A dam breach has terrible effects on the economy and people's lives. One of nature's most destructive occurrences is flooding, and some of the worst flash floods are caused by dam failures. The goal of the present study is to analyze the impact of a possible failure– collapse on a potentially affected area downstream of the under-construction Ajima Chacha dam in the Amhara region, North Shewa Zone, and also assessed the flood hazard areas. The HEC-RAS 5.0.7 hydraulic analysis software was used to study the dam breach, and the flood wave propagation, estimate the extent, depth, and velocity of floods, and prepare the flood inundation map in ARC GIS 10.7. The analysis was performed using two different relief datasets of the same area: a digital elevation model (DEM) was utilized to create the geographic information system (GIS) layers. The best-fitted regression equation for this study is (Froehlich 2008) for both scenarios, and the peak flow at overtopping 43,595.34 m3/s, and 24,182.74 m3/s at piping flow. Different dam breach and flood scenarios, where the water flows over man-made structures, settlements, and agricultural lands, were also examined. The study area is dominated mainly by two geological formations with different hydrogeological characteristics that dictated the positioning and structure of the dam and determine the processes that shape the geomorphology and surface roughness of the floodplain, affecting flow conditions. The agricultural and village settlements were mixed and inundated, ranging from 2640– 3161 hectares. They will be at risk in general. In the case of Chacha town, from its total settlement area of 347.5 hectares, 77.33 ha will be inundated or 22.25% of its total area. In terms of hazard indicating values, 17.73ha is classified as high risk, 6.80ha as medium risk, and 47.05ha as low risk. The results show that the impact of a potential dam break at Ajima Chacha dam is serious, and appropriate management measures should be taken to reduce the possible risk. Keywords: HEC-RAS; ARC GIS; dam break; unsteady; flood mapping; Hazard assessment; Ajima Chacha.