Downstream Flood Inundation Assessment Due to Dam Breach of Dudhkoshi Storage Hydroelectric Project Using HEC‐RAS 2D

Dam breach is rare but catastrophic event that occurs when a dam fails and releases impounded water downstream causing significant damage. Dam breach has a low probability of occurrence but carries high risk of destruction. Dudhkoshi Storage Hydroelectric Project (DKSHEP) concrete‐faced rock‐fill dam (CFRD) was studied for dam breach under overtopping and piping failure modes. Dam breach analysis, breach flood routing, breach flood inundation and hazard mapping, and sensitivity analysis (SA) of dam breach parameters are essential in identifying and minimizing risks associated with breach floods. Shallow water equations (SWEs) were used to perform a two‐dimensional (2D) dam breach analysis using Hydrologic Engineering Center’s River Analysis System (HEC‐RAS). The dam breach model was based on two scenarios: the base‐case scenario, which used the average value of dam breach parameters, and the worst‐case scenario, which used the values of dam breach parameters that resulted in the maximum output. The dam breach parameters were estimated according to the guidelines set by the United States Army Corps of Engineers (USACE), 2007. Local and global sensitivity analyses were performed for four dam breach parameters, namely dam breach width (DBW), breach formation time (BFT), weir coefficient (WC), and trigger failure elevation (TFE) for overtopping failure, while piping coefficient (PC) was evaluated instead of TFE for piping failure. Dam breach flood routing and dam breach parameters SA were evaluated for two river profiles (R1 and R2) separated by Dudhkoshi–Sunkoshi confluence. The sensitivity of peak discharge, peak velocity, arrival time, and water surface elevation were analyzed. The values of dam breach parameters DBW, BFT, WC, TFE, and PC for the base‐case scenario were 206 m, 0.5 h, 2.8, 640.3 m, 0.55, and for the worst‐case scenario were 309 m, 0.25 h, 3.0, 640.5, and 0.5, respectively. Aeronautical Reconnaissance Coverage Geographic Information System (ArcGIS), HEC‐RAS, and OriginPro 2022 were used for dam breach analysis. Overtopping failure was more critical than piping failure mode for dam breach analysis of DKSHEP for both base‐case and worst‐case scenarios.