Lost in the North Sea—Geophysical and geoarchaeological prospection of the Rungholt medieval dyke system (North Frisia, Germany)

Abstract

We performed geophysical and geoarchaeological investigations in the Wadden Sea off North Frisia (Schleswig-Holstein, Germany) to map the remains and to determine the state of preservation of the medieval settlement of Rungholt, especially its southern dyke segment, called the Niedam dyke. Based on archaeological finds and historical maps, Rungholt is assumed to be located in the wadden sea area around the island Hallig Südfall. During medieval and early modern times, extreme storm events caused major land losses, turning cultivated marshland into tidal flats. Especially the 1st Grote Mandrenke (or St. Marcellus’ flood), an extreme storm surge event in 1362 AD, is addressed as the major event that flooded and destroyed most of the Rungholt cultural landscape. Cultural traces like remains of dykes, drainage ditches, tidal gates, dwelling mounds or even plough marks were randomly surveyed and mapped in the tidal flats by several authors at the beginning of the 20th century. Due to the tidal flat dynamics with frequently shifting tidal creeks and sand bars, the distribution of cultural remains visible at the surface is rapidly changing, making it hard to create a comprehensive map of the cultural landscape by surveying. Today, the Niedam dyke area is fully covered by tidal flat sediments, depriving any remains from further archaeological investigation. Since little is known about the precise location or state of preservation of these remains, our investigation aimed at the rediscovery of the medieval dyke system and associated structure with modern and accurate geophysical, geodetical and geoarchaeological methods. Magnetic gradiometry revealed a large part of the medieval dyke, confirming two tidal gates and several terps connected inland with the dyke, providing a detailed example of a Frisian medieval dyke system. Based on our results, the so far inaccurate and incomplete maps of this part of Rungholt can now be specified and completed. Beyond that, seismic reflection profiles give a first depth resolving insight in the remains of the dyke system, revealing a severe threat to the medieval remains by erosion. The site is exemplary for the entire North Frisian coast, that was influenced by multiple flood events in the middle ages to modern times.

Seismic imaging of medieval dike remains in the Wadden Sea (North Frisia, Germany)

“…In extension to the course of the previously recorded dike, all SES profiles show equally comparable dike imprints, describing a course towards the west with a bend to the northwest. In the upper layers, the maximum dimensions of the symmetrical, rounded imprint are 37 m in width with a depth of 27 cm, which is comparable to the initial study (Wilken et al 2022).…”

“…The SES profiles measured in the tidal channel Dwarsloch lie parallel in extension to the measurements of the initial study (Fig. 2) and show a stratigraphy that can be matched with corings from the adjacent area (Wilken et al 2022). The transition between young tidal flat sediments and pre-medieval, laminar strata is distinctive.…”

“…In the initial study, we used a high resolution two-channel SES with a center frequency of 4 kHz and a theoretical vertical resolution of about 0.1 m, positioned by RTK-DGNSS (Wilken et al 2022) The characteristic dike imprints derive from the pressure distribution underneath the original dike, leading to compression and deformation of the underlying sediment. The width of the imprint increases from shallow to deeper layers, while the imprints' height (of the deformation in regard to the overall course of the reflecting layer) decreases, therefore the dike imprints are only detectable in the upper 10 m of the stratigraphic sequence.…”

“…Research of the 1920s discovered occasionally exposed remains of a dike with tidal gates, drainage systems and terps in the area of Hallig Südfall (Busch 1923). An initial study (Wilken et al 2022) revisited this site to test geophysical prospection methods in the tidal flats. Although the dike remains recorded in 1921 were eroded thereafter, it was possible to trace the dike by magnetic gradiometry and image its characteristic imprint in the underlying sediments through marine reflection seismics.…”

Barge or tjalk? A wreck investigation in the tidal flats of North Frisia

“…In the present case more silty subsoil and water running down the tidal creek recommends a combination of gradiometry and EMI. A geophysical prospection and a feasible recording methodology of such archaeological features are of high interest in tidal flat areas, as erosion by tidal creeks and storm surges endangers the heritage of the area (Wilken et al 2022b).…”