Model based assessment of climate change impact on inland flood risk in coastal areas caused by compounding storm tide and precipitation events

In addition to storm surges, inland flooding due to intense rainfall becomes an increasing threat at coastal lowlands. In particular, the coincidence of both types of events poses great challenges to regional water boards since their technical drainage capacities are limited. The evaluation of historical inland flood events at the German North Sea coast at the gauge Knock near Emden shows that in the past mainly moderate storm series in combination with large-scale, heavy precipitation led to an overload of inland drainage systems, whereas storm tides and precipitation alone could be handled well. Evaluation of the drivers of inland flood events simulated for the control period of two climate models confirms that a combination of storm tides and precipitation leads to highest drainage system overloads. Moderate system overload is also caused by heavy precipitation events alone rather than by storm tides without precipitation. Scenario projections based on a set of combinations of two highly resolved climate models and two emission scenarios suggest that the intensity of compound events of rainfall and storm tides will increase consistently against the background of mean sea level rise for all investigated climate change projections, while simulated system overload is higher for RCP8.5 compared to RCP2.6 scenario. Comparable to the past, future compound events will cause more potential damage compared to single extreme events. Such behaviour can be expected to induce an increasing frequency and intensity of inland drainage system overloads along the North Sea coast if timely adaptation measures will not be taken.