Scenario-based Analysis of Flood Mitigation and Agricultural Water Security in Estuary Dam Opening: A Case Study of the Geum River

Estuary dams are vital for freshwater security and flood mitigation, yet rising demands for ecosystem restoration require a quantitative assessment of their opening. This study analyzes the hydrodynamic and hydrological impacts of various opening scenarios of the Geum River Estuary Dam on agricultural water security and flood mitigation. The SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) was configured for the study area and validated against observed data. The model demonstrated high reliability, with Nash-Sutcliffe Efficiency (NSE) values of 0.93–0.96 for tidal levels and 0.78–0.85 for water levels near the dam. Salinity transport was also accurately reproduced, showing a Percent Error (PE) of 0.51–0.66% and a Root Mean Square Error (RMSE) of 0.20–0.23 psu. The study evaluated three categories of scenarios: full opening (Scenario A), continuous partial opening (Scenario B-1), and intermittent opening (Scenarios B-2, C). Agricultural water security was assessed based on critical salinity thresholds for rice growth: 0.45 psu (no damage), 0.64 psu (initial damage), and 1.00 psu (yield reduction). Results indicated that Scenario A caused the most extensive saltwater intrusion, reaching 46.0 km upstream at the 0.45 psu threshold. Notably, while Scenario B-1 exhibited the shortest intrusion distance (15.0 km), it recorded the highest cumulative seawater inflow among the regulated opening scenarios. This discrepancy implies that relying solely on intrusion distance is insufficient for assessing agricultural water withdrawal risks. Consequently, this study suggests that a multi-faceted analytical framework, considering both intrusion distance and total inflow volume, is essential for establishing sustainable operation guidelines that balance flood mitigation with agricultural water standards.