Assessment of Flood Safety in Small Agricultural Reservoirs under Climate-Adjusted Rainfall

The intensification of short-duration rainfall associated with climate change has raised growing concerns regarding flood safety in small agricultural reservoirs. This study separates long-term rainfall records into past (before 1999) and present (after 2000) periods and conducts on-site frequency analyses using a probability-weighted moment (PWM)-based Gumbel distribution applied to annual maximum 1-hour rainfall at representative stations (Seoul, Cheongju, and Pohang). The results demonstrate increasing trends in the 50-year return period rainfall depth (P50) across all three stations, with the most pronounced increase observed at Pohang. Flood-safety assessment of the target reservoir (Yugum Reservoir) is performed by constructing 24-hour design hyetographs based on the Pohang-derived P50 values (past: 61.3 mm; present: 95.8 mm) using a fixed dimensionless temporal distribution, followed by the application of level-pool routing. Under present conditions, both the peak inflow and peak spillway outflow increase, accompanied by a rise in maximum reservoir water level, which leads to a reduction in available freeboard (maximum water level: past EL. 26.00 m; present EL. 26.32 m; freeboard: past 1.15 m; present 0.83 m). However, the maximum simulated water level remains below the dam-crest elevation (EL. 27.15 m) for both the past and present periods. Overall, this study provides quantitative baseline evidence illustrating how variations in historical rainfall characteristics influence hydrological responses and freeboard availability within a small agricultural reservoir.