Climate Change Impact Assessment of Dike Safety and Flood Risk in the Vidaa River System

The impact of climate change on the flood risk and dike safety in the Vidaa River system, a cross-border catchment located in the southern part of Jutland, Denmark and northern Germany, is analysed. The river discharges to the Wadden Sea through a tidal sluice, and extreme water level conditions in the river system occur in periods of high sea water levels where the sluice is closed and increased catchment run-off take place. Climate model data from the ENSEMBLES data archive are used to assess the changes in climate variables and the resulting effect on catchment run-off. Extreme catchment run-off is expected to increase about 8 % in 2050 and 14 % in 2100. The changes in sea water level is assessed considering climate projections of mean sea level rise, isostatic changes, and changes in storm surge statistics. At the Vidaa sluice a mean sea level rise of 0.15–0.39 m in 2050 and 0.41–1.11 m in 2010, and increases in storm surge levels of up to 0.8 m in 2100 are estimated. The changes in extreme catchment run-off and sea water level have a significant effect on the flood risk in the river system. While most parts today have a low risk of dike overtopping with annual exceedance probabilities of 0.1 % or less, the worst case scenario in 2100 show annual exceedance probabilities of 5 % or more in the downstream part of the river system.