Numerical Assessment of Environmental Hazard Mitigation Through River Mouth Structures in Enclosed Aquatic Systems: A Case of Hypoxia Reduction

Global environmental challenges, including eutrophication and hypoxia in enclosed water bodies, require innovative solutions for sustainable water quality management. Lake Biwa, Japan’s largest freshwater lake, suffers from hypoxia in its bottom layers due to strong summer stratification that inhibits vertical mixing. To address this issue, the present study employed a three-dimensional hydrodynamic–ecosystem model to numerically evaluate the effectiveness of training walls (guiding dikes) at river mouths in enhancing vertical mixing and improving bottom-layer oxygenation. Simulations revealed that the installation of guiding dikes significantly altered horizontal advection and promoted vertical mixing, particularly during winter, when weakened stratification allowed snowmelt inflows to sink along the dikes. As a result, local increases in dissolved oxygen concentrations of up to 0.4 mg/L were observed in the bottom layer. These findings demonstrate that guiding dikes can effectively improve oxygen supply to hypoxic zones, especially during periods of low stratification, providing a promising strategy for lake management in temperate regions experiencing seasonal snowmelt.