MODELING THE IMPACT OF LEVEE REMOVAL ON GROUNDWATER RECHARGE USING ROBUST FLOODPLAIN DATA - A CASE STUDY OF THE COSUMNES RIVER

In regions with Mediterranean climates such as California, climate change is expected to bring longer, more severe dry periods and more frequent, high-intensity wet periods that threaten traditional surface water storage and release techniques. Flood managed aquifer recharge (MAR) provides a method to store high magnitude stream flows in groundwater basins where groundwater overdraft has left large storage volumes available. Levee removal and levee setback are another method that increase the available area for flood waters to spread which increases groundwater recharge both during and after flood events when inundation remains after disconnection. However, previous levee removal and setback studies focused mainly on wildlife habitat creation that improved survival and reduced flood risk and less on their impact on groundwater recharge. Levee removal and setback are costly, therefore it is imperative to adequately justify the benefits to those who are funding the project. Here we present the use of a coupled groundwater-surface water model capable of incorporating aquifer heterogeneity to quantify the impact of levee removal on groundwater recharge. The model is calibrated and validated using a robust continuous groundwater-surface water observation dataset. The model is used to quantify the effects of upstream levee setback on groundwater recharge during high flows to help inform future management decisions. By quantifying the increase in groundwater recharge due to levee setback local water managers have a basis to compare the benefits of levee setback with those of Agricultural MAR, dry wells, or dedicated recharge ponds. Proof of sufficient groundwater recharge due to levee setback may remove the need for groundwater pumping restrictions, surface water imports, or additional MAR projects while providing abundant ecosystem services and reducing flood risk.