A Modelling Study of Saltwater Intrusion and Storm Surge Processes in Coastal Areas under Climate Change

In coastal areas, climate change will cause a rising sea level and more intense, frequent storms. As a consequence, an increased intrusion of dense saltwater into the groundwater zone, both by displacement of the saltwater-freshwater interface and through the unsaturated zone by overtopping waves, can be expected. Using HydroGeoSphere, both intrusion mechanisms are investigated in a two-dimensional cross-sectional surface-subsurface model of a region of the North-German coast, where groundwater resources are essential for future generations. It was necessary to develop a new approach to model coupled variable-density and variably-saturated flow and transport for this setting. Simulations indicate that tidal fluctuations only affect pressure head in areas where hydraulic conductivity is high, and the impact on the concentration of seawater is relevant only within a very localised zone close to the sea. For example, an increase in sea level by 60 cm causes the saltwater wedge to intrude up to 150 m further into the subsurface. The simulation of overtopping waves, which can lead to ponding behind a dyke, showed that the infiltration depth of the ponded water as well as the saltwater concentration which could reach potential drinking water wells is highly dependent on the hydraulic conductivity of the subsurface. The results highlight the potential impacts and the importance of climate change related future stresses with respect to groundwater supply in coastal areas and the necessity to model coastal systems in an integrated coupled approach.