Compound Flooding From Storm Surges, Precipitation, Rivers, and Groundwater—Hydrodynamic Modeling in a Coastal Catchment

Several climatic and state variables may drive the occurrence of coastal zone flooding, including, for example, storm surges, sea level rise, heavy rainfall, and high river and groundwater levels. The co-occurrence of such events, that is, compound or cascading events, has been shown to escalate flooding impacts. The contribution from groundwater is, however, routinely overlooked. Here, we apply an integrated hydrological/hydrodynamic/groundwater model to investigate underlying causes and compound effects in a Danish Wadden sea catchment. Two models were developed: a long-term model and a short-term overbank-spilling model. The long-term model was calibrated and used to simulate a 30-year historical period. Extreme value analyses were carried out for observed time series of sea levels and precipitation, together with simulated river water stages and groundwater levels. The co-occurrence of extremes was used to identify compound effects on high river-stage incidents (as flood proxy). The overbank-spilling model was then used for simulating flooding for a subset of the largest river stage events identified from the long-term model. The analysis showed that the river-stage events were closely correlated to the sea level extremes, but that the largest river-stage events were almost exclusively additionally compounded by precipitation, groundwater, or both. High groundwater tables seem to correlate with the flooding events with the largest spatial extent, and prolonged extreme events where either precipitation or sea level was elevated during long periods. Thus, this study shows that there is a general need to acknowledge the potential effect of groundwater levels on the resulting flooding on terrain in coastal zones.