Risks of inland flood caused by compounding storm tide and precipitation events

In addition to storm tides, coastal low-lying areas are under an increasing thread of inland flooding due to intense rainfall. In particular, the coincidence of both types of events poses challenges to the regional water boards since their technical drainage capacities are limited.

In this study we first look at the historical compounding events and associated drainage system loads on the example of the Emden water board at the North Sea coast of Lower Saxony, Germany. The water board has an area of 465 km², one third of which is located below sea level. It is protected by dikes from the sea impact and an access of rainwater accumulated inland and unable to flow out naturally is removed via a system of pumps and sluices. To assess a development of potential inland floodings in the future, a set of four future projection scenarios was constructed. It is based on the RCP2.6 and RCP8.5 pathways and two realizations with different climate models from the CMIP5 projections, which were used to force the hydrological (SIMULAT) and hydrodynamic (TRIM-NP) models to obtain 480 years of run-off timeseries and sea water levels (the later available via coastDat). Additionally, corresponding estimates of the future mean sea level rise were used.

An analysis showed that mainly intense rainfall poses a big challenge for the drainage system, however the largest loads were associated with a combination of heavy rainfall and a series of moderate storm tides. The results for future projection scenarios suggest that the intensity of compounding storm tide and rainfall events will continuously increase, chiefly due to the rising mean sea level as well as projected increase in winter precipitations. Comparable to the past, future compound events will cause more potential damage than single extreme events. The model results indicate an increasing frequency and intensity of inland drainage system overloads along the North Sea coast if timely adaptation measures are not taken.

Bormann et al., 2024