Flood inundation mapping using 2D hydrodynamic modeling for Nesa River

Severe hydro-climatic variability significantly alters flood frequencies and inundation dynamics, necessitating advanced modeling for adaptive water resource management. Accurate flood inundation mapping and dynamic risk forecasting remain critical infrastructure challenges in arid and semi-arid basins subjected to significant dam-induced hydrological alterations. This study introduces an integrated eco-hydrologic and 2D hydrodynamic modeling framework coupling HEC-HMS and HEC-RAS 2D capabilities within a Geographic Information System (GIS) environment to evaluate flood dynamics under a dual-scenario for the Nesa River in Kerman, Iran. Moving beyond standard local case studies, this framework introduces a novel, non-stationary scenario-based reservoir analysis. It evaluates integrated multi-purpose reservoir operations to optimize flood control capacity while balancing downstream ecological water requirements. The analysis also dissects severe multi-hazard failure risks at the Nesa Dam spillway during extreme return periods (25, 100, and 500 years). Hydro-meteorological calibrations utilizing the SCS curve number yielded highly resilient performance metrics (NSE = 0.77; R 2 = 0.82). Continuous boundary condition modeling for Scenario 1 (Pre-Dam Condition) versus Scenario 2 (Post-Dam Condition) reservoir control revealed severe flood storage dynamics, with a 100-year event generating 148.73 MCM of inflow volume. By incorporating backwater effects and vegetation in mountain-to-plain transitions, flood inundation dynamics under multiple simulated flood scenarios are mapped. The 2D hydrodynamic analysis captures complex spiral channel physics and upstream backwater extensions. The results show that integrated multi-purpose reservoir operations mitigate downstream inundation areas under extreme hydrological conditions, significantly cushioning downstream reaches from catastrophic failure. However, these operations simultaneously amplify upstream inundation boundaries, threatening adjacent critical rural settlements. Furthermore, the analysis reveals that accounting for upstream backwater effects prevents the structural underestimation of flood risks in sub-mountainous reaches. This integrated approach provides a robust decision-support tool for sustainable flood management and hydraulic infrastructure design under volatile climate change.