Hydrologic–Hydraulic Modelling for Flood Risk Mapping in the Prakasam Barrage Region, Andhra Pradesh

Dam-breach flooding poses severe risks to downstream communities and critical infrastructure, particularly during extreme hydrologic events. This study develops an integrated hydrologic–hydraulic modelling framework combining HEC-HMS and HEC-RAS 2D to evaluate potential breach-induced flooding downstream of the Prakasam Barrage on the Krishna River, India, motivated by exceptional monsoon inflows and reported operational disruptions during the 2024 flood season. Rainfall–runoff simulations using September 2024 precipitation data were routed through breach scenarios to estimate water-surface elevation, flood depth, flow velocity, and inundation extent. Results indicate peak water levels of 41.83 m at the barrage and maximum discharges of 3474.51 m3 s–1. Flood depths exceeded 13 m and velocities surpassed 1.16 m s–1 within the first 6 km downstream, identifying this reach as the most vulnerable to breach-induced flooding. Spatial overlay with land-use/land-cover data revealed that agricultural areas experienced the greatest exposure, followed by urban and wetland zones. Flood-hazard maps were produced to classify the downstream floodplain into critical, high, moderate, and low-risk categories. The findings highlight the value of two-dimensional hydraulic modelling, high-resolution topographic data, and improved gate-operation strategies for strengthening Emergency Action Plans and enhancing downstream flood preparedness and mitigation.