Prehistoric storm surges during the Holocene – triggers for long-term changes of the NW German coast?

The development of the NW German coast is characterised by the sea level rise since the beginning of the Holocene and a resulting general retrogradational shift of the coastline. Extreme events, i.e. storm surges, have also contributed to these changes and should not be underestimated.

The palaeo-coast was un-diked and thus the morphological conditions and sedimentary processes differed fundamentally from those observed at the present-day coast. It can be assumed, that prehistoric storm surges therefore had different effects on the coastal environment at the time. Sedimentological records of such events are documented along the East Frisian North Sea coast and the Ems-Dollard estuary. However, the short-term and long-term effects of storm surges on the palaeo-coast remain poorly understood. How far and how strong did the water inundate the hinterland? Did they trigger transgressions, which have been originally correlated to increased sea level rise? Are there regions particularly vulnerable to erosion and if yes, what are the geological preconditions?

In the scope of the multidisciplinary project “CoastAdapt”, we approach these questions by focusing on mm to cm thick clastic seams within peat horizons of the Holocene coastal sediments. Such deposits can only be formed during severe storms when the water reaches the fens of the hinterland or in a peat cliff situation. In the latter case the upper part of the peat is being uplifted, fine-grained clastic material is deposited, and eventually preserved within the peat when the upper part is settling again after the storm. Therefore, these clastic seams are a well detectable archive of storm surge history along the coast.

Here, we present first results of the spatial distribution of these storm surge layers for different time segments based on comprehensive analyses of borehole archive data together with updated sea level rise data for the NW German coast. Furthermore, we present preliminary considerations regarding the potential influence of geological conditions that may have facilitated the inundation by storm surges.