Dam breach analysis and flood inundation mapping of Dire Dam, using HEC-HMS and HEC-RAS models

Pre-event study of dam breaches is required to predict the amount of outflow and downstream propagation conditions that result in dam breach issues. This study aimed at dam breach analysis and flood inundation mapping of the Dire Dam in Ethiopia. The rainfall-runoff data was analyzed using the hydrological model of HEC-HMS. The hydraulic models of HEC-RAS and HEC-GeoRAS were used in conjunction to investigate the dam breach and flood inundation mapping hazards. In HEC-RAS, the breach parameters were calculated and peak flood was routed through the cross-sections for both overtopping and piping scenarios. Dire Dam is expected to face breaching problems due to overtopping when the flood reaches 7073.02 m³s⁻¹. The peak flood hydrograph obtained for a piping mode of failure immediately downstream of the dam was obtained at 11,021.92 m³s⁻¹, while it decreased to 1418.33 m³s⁻¹ when traveling 33.2 km downstream with a time to peak of 0.4 and 2.4 h, respectively. The flood depth obtained by 1D and 2D downstream of the dam ranges between 0.0078–31.56 and 0.00195–20.45 m respectively. While the corresponding estimated flow velocity values of the 1D and 2D vary from 0.9100–12.5 and 0.0034–8.43 ms⁻¹. Thus, the 1D flood modeling overestimated the depth of the water and flow velocity when compared to the 2D flood modeling. This is because the 2D model uses an implicit finite volume algorithm, which is more precise, accurate, and stable as compared to 1D. The total area that could be at risk because of the flooding hazard downstream of the dam is 358.7 ha. Overall, the study's conclusions showed that the downstream villages face terrible circumstances as a result of the flood risk brought on by the dam break issue and it should be advised that improved flood protection infrastructure be put in place.