Work package 6

Storm surges as a controlling factor in prehistoric coastal development – geological conditions and natural adaptation mechanisms

Subproject 6 focuses on the investigation of prehistoric storm surges and their impact on the undiked North Sea coast. Before the construction of dikes, a wide transitional zone existed between the open Wadden Sea and the mainland (fig. 1), shaped by fundamentally different morphological processes than today. Consequently, storm surges in prehistoric times had significantly different effects than those of later events, which are typically linked to dike breaches. The extent and consequences of those events are documented relatively well in historical sources. In contrast, the impacts of prehistoric storm surges remain largely unexplored. This raises key questions: How far did the water reach inland? And to which extent did short-term storm surge events contribute to long-term coastal changes, especially in the context of Holocene sea-level rise? To address these questions, storm surge deposits are being mapped and combined with existing paleo-landscape reconstructions. The goal is to identify geological conditions that either facilitated or impeded the inland penetration of storm surges — for example, reactivated tidal channels or elevated landforms such as peatlands or geest areas.

The North Sea coast as it might have looked before the construction of dikes and extensive human activity (AI-generated with ChatGPT Plus GPT-4). The tidal flats gradually transition into salt marshes, which are bordered landward by peatlands. Further inland, the geest areas are forested.

The mapping is based on stratigraphic records from the archives of the State Authority for Mining, Energy and Geology, as well as newly collected field data. Sediment samples from the coastal Holocene are extracted using core drilling methods and analysed for typical storm surge deposits (fig. 2). These are clastic sediments embedded in peat, referred to as Klappklei (fig. 3). Radiometric and luminescence dating methods provide chronological context and help to reconstruct the coastal development. The resulting sedimentological data are then correlated with each other and with geophysical data, such as geomagnetics and ground-penetrating radar. This integrated approach allows for the creation of detailed paleo-landscape reconstructions, offering deeper insights into natural coastal dynamics — an essential foundation for future sustainable and nature-based coastal protection strategies.

A 17 meters long sediment core drilled near Bremerhaven reveals the complete Holocene sediment sequence at this location. Marine tidal flat deposits or brackish-lagoonal sediments (grey) alternate with terrestrial peat layers (dark brown), reflecting coastal changes throughout the Holocene. Within the peat horizons, several layers of “Klappklei” (see detail section) are also present. (Figure: AG Naturrisikoforschung und Geoarchäologie, Geographisches Institut, JGU Mainz in cooperation with NIhK)

Main aspects:

  • Sedimentological analysis of prehistoric storm surge deposits based on archive data and sediment cores
  • Dating of prehistoric storm surge events
  • Geological preconditions influencing the impact of storm surges prior to the construction of dikes
  • Landscape reconstructions and coastal development during the Holocene
Formation of "Klappklei" illustrated by the modern example of the Sehestedter Außendeichsmoor (“outer-dike mire” of Sehestedt) on the eastern Jade Bay. Due to its location, the bog can come into contact with seawater during high water levels. During storm surges, the upper part of the bog is being uplifted, allowing seawater and suspended sediment to enter the space beneath. After the storm surge, the peat settles back into its original position, trapping the previously introduced sediment within the peat layers. (Figure: Modified after Behre & Kuҫan, 1999)