TAFESE FONA DEBESA2026-02-052023-07-23https://etd.hu.edu.et/handle/123456789/664Due to the construction of Bisandima dams, the downstream ecosystem is highly changed in that a huge area is covered with irrigation farms, new settlements, and residence areas of inhabitants living on the farms, and fishery communities were formed due to this dam. All these investments and newly settled inhabitants are highly exposed to flooding and they are at risk from the possible failure of this dam. The objective of this study was to model the Bisandima dam breach phenomena and to develop flood inundation maps. In this study dam breach, flood routing was carried out by using unsteady flow routing in HEC-RAS 2D model using geometric data to determine flood-susceptible areas downstream of the dam for the study area. The key inputs required in the dam breach flood routing processes include Precipitation data to determine PMF, digital Elevation model, and land use data were used as input for the HEC-RAS model. Some parametric methods were also used to predict dam breach parameters for use in the model. The breach discharge resulted from the HEC-RAS model for all methods such as Thun & Gillete (1990), Froehlich (2008), Froehlich (1995), MacDonald and Langridge –Monopolis (1984), Xu & Zhang (2009) were 2079.01, 1040.21, 1332.49, 531.91, 592.22 m3 /s and 1796.52, 620.37, 572.02, 530.53, 498.46 m3 /s for overtopping failure and piping failure respectively and also breach outflow at the downstream cross-section were 969.32, 625.48, 699.46, 538.96, 592.38 m 3 /s for overtopping failure respectively. The hazard mapping due to the combined effect of flood depth and velocity showed that an area of 28ha, 9.5ha, 11ha, 3ha, and 0.5ha are under low, medium, high, very high, and extreme hazard respectively for overtopping failure and an area of 20ha, 8ha, 10ha, 2.2ha, and 0.8ha are under low, medium, high, very high and extreme hazard respectively for piping failure. As noticed from the flood inundation map almost all critical areas downstream of the dam are in immediate danger. Based on the finding of the study outcomes the following recommendations are drawn: flood early warring, watershed treatment, provide dike at both sides of the downstreamen: HEC-RASDam breach modelInundation Mapbreach parametersBisandima damThesis