Evaluating Flood Mitigation of Detention Reservoirs on Jeju Island from High-resolution Hydraulic Simulations on UAV-derived Topography

Flood detention reservoirs are widely adopted structural measures for flood risk mitigation; however, their post-construction hydraulic performance often deviates from design-stage expectations. This study evaluates the flood mitigation performance of the Hancheon detention reservoir system on Jeju Island by integrating unmanned aerial vehicle-derived high-resolution topography with a two-dimensional hydraulic model. The model was calibrated using observed water-level records from Typhoon Chaba (2016) and subsequently applied to simulate a 100-year design flood under multiple analytical contexts. Under the original design assumptions, Hancheon 1 was intended to serve as the primary detention reservoir, with Hancheon 2 assigned a secondary role. In contrast, simulations of the as-built configuration reveal more complex and asymmetric behavior. While Hancheon 1 contributes more effectively to instantaneous peak discharge attenuation, Hancheon 2 provides a larger contribution in terms of the cumulative storage volume, owing to its geometric configuration and staged storage cells. Furthermore, retrofit scenarios targeting hydraulic bottlenecks in Hancheon 2 substantially enhance storage activation and peak attenuation, leading to a major improvement in system-level flood mitigation performance. These findings demonstrate that the relative roles of sequential detention reservoirs depend strongly on design intent, as-built hydraulic constraints, and structural modifications, rather than on a single performance metric. This study underscores the importance of high-resolution hydraulic modeling for realistic post-construction assessment and optimization of detention reservoir systems.