Two-dimensional HEC-RAS modeling of flood hydrograph generated by a hypothetical Saddle Dam breach: downstream propagation toward Roseires Dam

Dam breach events represent extreme hydraulic hazards with potentially catastrophic downstream consequences. This study presents a screening-level analysis of hypothetical breach scenarios for the Saddle Dam associated with the Grand Ethiopian Renaissance Dam (GERD) on the Blue Nile River near the Ethiopia–Sudan border. The study does not estimate breach probability; rather, it evaluates the hydraulic consequences of plausible breach scenarios within the modeled domain extending downstream to Roseires Dam using two-dimensional unsteady-flow simulations in HEC-RAS. Five breach scenarios were simulated, assuming the reservoir at its maximum operating level. Breach parameters were estimated using widely accepted empirical relationships commonly applied in dam breach studies to ensure realistic and physically consistent conditions. The model integrates detailed topographic data, accurate reservoir geometry, and well-defined breach characteristics to generate outflow hydrographs and assess downstream flood behavior. The computed peak outflows range between approximately 9.6 × 10 4 and 1.27 × 10 5 m 3 /s, with shorter breach formation times producing substantially higher discharge peaks. Maximum flow velocities immediately downstream of the breach reached approximately 11 m/s, indicating highly destructive flow conditions. Flood routing results further show that the computed water surface elevation near the downstream end of the modeled reach (just before upstream boundary of Roseires Dam reservoir) could reach approximately 486.5 m+msl under the most severe scenario and under a conservative no-operation normal-depth boundary assumption. The results demonstrate a pronounced inverse relationship between breach formation time and peak discharge, emphasizing the critical sensitivity of downstream flood magnitude to breach development dynamics. The modeled inundation characteristics, including maximum water depth, velocity, and water surface elevation, highlight significant hydraulic impacts on downstream reaches up to Roseires Dam. These findings provide quantitative insight for dam safety evaluation and emergency preparedness planning within the Blue Nile basin.