Mitigating flood vulnerabilities through retarding basin systems: A case study in Bengawan Solo River, Indonesia

Flooding is one of the most significant global disasters, causing severe social, economic, and environmental impacts, particularly in riverine regions. Retarding basins are effective flood mitigation measures that temporarily store excess water during peak flows, thereby reducing downstream flood risks. This study investigates the application of this concept by analyzing the Jabung Retarding Basin in Lamongan Regency, part of the Bengawan Solo River, Indonesia. The objective is to evaluate the basin's effectiveness in reducing flood extent, depth, and associated economic losses before and after implementation. Using HEC-RAS 6.5 and ArcGIS 10.3, flood inundation mapping was conducted for return periods of 10, 20, 25, and 50 years. The results show that the Jabung Retarding Basin reduced the flood-affected area by 39.72 %–42.69 %, with the most significant reductions observed in areas where inundation depths exceeded 1.50 m. However, its effect on average flood depth was limited, decreasing slightly from 1.036 to 1.046 m to 0.995–1.031 m after operation. In addition to reducing flood extent, the basin significantly lowered economic losses. For instance, a flood with a 50-year return period previously caused an estimated loss of approximately $9.55 million, which decreased to $5.89 million post-operation. Although this study is site-specific, the findings demonstrate that retarding basins can contribute to integrated flood risk management in other flood-prone or deltaic regions. The methodology employed can be adapted to similar hydrological and socio-economic contexts. Future mitigation efforts should also consider downstream impacts by enhancing floodway capacity, optimizing drainage systems, and implementing early warning systems.