Saltwater Intrusion and Storm Surge Processes in Coastal Areas under Climate Change: A Modelling Study in Northern Germany

Climate change will affect coastal groundwater resources due to a rising sea level and an increase in storm intensity and frequency. Increasing saltwater intrusion into the groundwater zone as well as the intrusion of saltwater, supplied by overtopping waves, into the subsurface through the unsaturated zone can be expected. Using Hydrogeosphere, these scenarios are investigated here in a two-dimensional cross-sectional surface-subsurface model of a region at the North-German coast that is heavily influenced by anthropogenic activities, and where groundwater resources are essential for future generations. It was necessary to develop a new approach to model variable density and variably saturated flow and transport for this coupled surface-subsurface system. We investigated the effect of tidal fluctuations on the pressure head and on the concentration of seawater in the subsurface. The results show that pressure head is affected only in areas where hydraulic conductivity is high and that an impact on concentration of seawater is relevant only within a very localised zone close to the sea. We found additionally that an increase in sea level by 60 cm causes the saltwater wedge to intrude up to 160 m further into the subsurface. The simulation of overtopping waves which lead to the formation of a pond behind the dyke showed that the infiltration depth of this water as well as the saltwater concentration which could reach potential drinking water wells is highly dependent on the hydraulic conductivity of the subsurface. Results of a 3D model of the catchment comprising the cross section show that certain processes such as e.g. submarine groundwater discharge and effects of coastline civil works and protection measures are better studied in 3D. 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.