Watershed-Scale Evaluation of Flood Mitigation Benefits from Surface Water Diversion and Subsurface Injection in Coastal Louisiana

Abstract

Coastal regions are witnessing unsustainable aquifer overdrafts, leading to subsidence and increased flooding. Flooding introduces its own assortment of problems because managing substantial volumes of stormwater is often infeasible through surface storage in low-gradient areas. Aquifer recharge operations, such as aquifer storage and recovery (ASR), in conjunction with small-scale surface reservoirs, has the potential to supplement the function of flood retention basins and provide additional flood relief to local communities. The purpose of this study is to test the effectiveness of surface water capture and injection at the local watershed level in mitigating flood impacts. The study is based on using a hydrologic-hydraulic numerical modeling analysis for a pilot watershed that was found to be suitable for aquifer recharge operations in the coastal region of south central Louisiana in the US. The concept of capturing excess surface water and injecting into the subsurface was found most effective during storms with multiple flood peaks and was able to add more than 75% reduction in flood peaks compared with using a retention basin alone. A key finding is that diverting surface flows into the subsurface can be effective only when implemented in conjunction with intermittent storage. The results also showed that the benefit of such operations is realized in the case of storms that have multiple flood peaks. The results have implications on the use of subsurface injections via techniques such as ASR for flood mitigation, in addition to their original purpose for groundwater management and sustainability.