Storm surges and the un-diked coast: tracing prehistoric events in the Holocene sediment sequence of northwest Germany

The rising sea level has shaped the northwest German coast since the beginning of the Holocene, which resulted in a general progradational shift of the coastline. Additionally, extreme events such as storm surges affected the paleo-coast and must be considered to further complement the understanding of landscape evolution in the region.

Unlike the modern coast, the paleo-coast was un-diked. Therefore, it can be assumed, that prehistoric storm surges had substantial different effects on the coastal environment at the time. While storm surge deposits have been identified along the northwest German coast, they have not yet been systematically studied.

The interdisciplinary project CoastAdapt aims to close this gap by investigating the inland reach of prehistoric storm surges, the geological conditions that may have favoured inundation, and whether some transgressions attributed to sea-level rise may have actually been storm-driven. Focus lies on cm-thick clastic layers (known as Klappklei) found within Holocene peat. These layers form when storm surges temporarily disturb coastal peat horizons, allowing fine sediments to be deposited and preserved within the peat horizon. As Klappklei only forms during high water levels, it is considered to be a valuable marker of prehistoric storm surge activity.

We present a comprehensive mapping of these deposits for different time intervals of the Holocene, alongside estimates of associated inundation extent. The results are based on borehole archive data, integrated with updated sea-level reconstructions. A preliminary regional case study from the East Frisian coast provides new insights into landscape development and effects of storm surge events.