Compound gate–embankment failures amplify dam-break flood hazards and downstream risk

Large reservoirs upstream of rapidly growing floodplains pose low-probability but catastrophic risks, yet dam-break assessments often represent failure as a single structural breach. The Ukai Dam (Lower Tapi, India) is a composite system with gated spillway/masonry structures and earthen embankments, motivating a scenario framework that couples operational and structural failures. We develop an ensemble of dam-breach scenarios under high reservoir levels, including (i) embankment breach at full-reservoir level conditions, (ii) extreme gate release through full opening and/or gate malfunction, and (iii) compound and cascading sequences where gate malfunction coincides with, or precedes, embankment breaching. For each scenario, breach outflow hydrographs are generated using physically informed breach parameterizations and routed through a 2D hydrodynamic model of the Ukai–Lower Tapi reach to produce spatial fields of maximum inundation depth, flow velocity, and flood-wave arrival time. We translate hydraulics into impacts by intersecting hazard outputs with high-resolution population and critical-asset layers to compute population-at-risk and exposure hotspots, and we rank scenarios by consequences and warning lead time. Preliminary results reveal nonlinear amplification of downstream hazard when gate-state extremes coincide with rapid embankment breach formation, shifting both peak depths and arrival times in key settlements. By explicitly representing composite-dam cascading failures, the study provides scenario-ranked risk maps and actionable lead-time metrics to inform emergency action planning and operational decision envelopes for Ukai and similar multipurpose reservoirs.