Reliability analysis of dam spillway capacity under extreme flood uncertainty: Boostan Dam, Iran

This study focuses on the Boostan Dam, located in the Gorganrud River Basin, Golestan Province, Iran, a region prone to severe flooding. The dam plays a key role in flood control and water supply. Accurate flood risk assessment and reliable spillway design are crucial to protect downstream populations and infrastructure from catastrophic damage. An integrated analytical–numerical framework is developed, combining the Frequency Factor Equation (FFE) with Generalized Extreme Value (GEV) and Gamma distributions for uncertainty analysis of PMP and flood events (QT’s and PMF). HEC-SSP is used to estimating peak discharges quantiles, then Monte Carlo–Latin Hypercube Sampling (MCS-LHS) is employed to generate flood scenarios, and HEC-HMS model simulates the corresponding hydrographs. Spillway reliability is assessed using the Load-Resistance Interference Method (LRIM). The study reveals that the Hershfield method significantly underestimates PMP by 37.5 %, and PMF by 65 %. The existing spillway, with a width of 40 m and a capacity of 785 m³ /s, offers less than 1 % reliability for PMF. To achieve 95 % reliability for floods with return periods of 1000–10,000 years, the spillway width must be increased to 50–80 m. These findings highlight the need for a paradigm shift in spillway design, advocating for probabilistic approaches that consider uncertainty and risk, ensuring greater safety and resilience in dam infrastructure. • Hybrid analytical–numerical model quantifies flood uncertainty for spillway reliability. • Load–Resistance Interference shifts spillway design from deterministic to probabilistic. • GEV-based PMP shows 37.5% underestimation by Hershfield with 95% CI = 154.3 ± 29.4 mm. • Current 40 m spillway gives < 1% reliability; needs 50–80 m width for 95% target. • Dual-spillway plan: main (50–60 m) for T = 200–1000 yr, auxiliary routes PMF excess.