Using a surface water hydrodynamic model to understand how spatial variability in ocean surge affects groundwater salinization in Delaware Inland Bays

Coastal communities around the world face an increasing risk from surge-driven inundation because of rising sea levels and intensifying storm conditions. During storm surges, inland propagation of ocean water drives infiltration of saltwater into the fresh groundwater, jeopardizing coastal water resources. The magnitude, shape, and duration of these ocean surges vary both between storm events and spatially during a given storm, resulting from differences in bathymetry, coastline shape, and the path and characteristics of the storm. Our study aims to understand how these temporal and spatial variabilities in ocean surge affect groundwater salinization and recovery in the Delaware (DE) Inland Bays. The DE Inland Bays are composed of two adjoining, shallow bays in Eastern Delaware connected to the Atlantic Ocean via a single inlet. The geometry of the bays results in complex surface water hydrodynamics. To study the effect of spatial variability in surge levels we used the output from a surface water hydrodynamic model, NearCOM-TVD, simulated for past storm surge events, as boundary conditions for 2D Hydrogeosphere simulations of groundwater flow and salt transport. During Hurricane Sandy (2012), the largest surge event in the past decade, there was a 0.7 m range in the maximum surge height within the bays. Corresponding groundwater simulations showed that for this range, there was 33% more surge-induced aquifer salinization for the highest surge level within the bays relative to the lowest. This elevated salt mass persisted over the ten year recovery period. Results from Hurricane Sandy will be compared with more moderate storm surge events. The resulting salinized volumes and recovery times from all the storm simulations were used to develop a salinization vulnerability metric for the Delaware Inland Bays. The goal of these simulations is to identify the surge conditions that present the greatest salinization risk and the locations in the Inland Bays that have the highest salinization vulnerability, as well as to improve understanding of the feedbacks between ocean and groundwater hydrodynamics.