Ines Bruns

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.

The anthropogenic impact in the German Exclusive Economic Zone (EEZ) is high due to the presence of manifold industries (e.g., wind farms, shipping, and fishery). Therefore, it is of great importance to evaluate the different impacts of such industries, in order to enable reasonable and sustainable decisions on environmental issues (e.g., nature conservation). Bottom trawling has a significant impact on benthic habitats worldwide. Fishing gear penetrates the seabed and the resulting furrows temporarily remain in the sediment known as trawl marks (TM), which can be recognized in the acoustic signal of side-scan sonars (SSS) and multibeam echo sounders (MBES). However, extensive mapping and precise descriptions of TM from commercial fisheries at far offshore fishing grounds in the German EEZ are not available. To get an insight into the spatial patterns and characteristics of TM, approximately 4800 km² of high-resolution (1 m) SSS data from three different study sites in the German EEZ were analyzed for changes in TM density as well as for the geometry of individual TM. TM were manually digitalized and their density per square kilometer was calculated. In general, TM density was highest in August and October. Moreover, different gear types could be identified from investigating individual TM in SSS data. Beam trawl marks were observed to have widths of up to 22 m whereas otter board marks showed widths up to 6 m. The persistence of TM was estimated to 2–7 days minimum for all three sites based on the SSS data from 2015–2019. A maximum persistence could be defined at one site (Dogger Bank) and it was five months for the investigation period 2016–2017. Besides the main factors driving TM degradation (wave-base impact, sediment-type), different methods for TM detection (SSS, MBES, under-water video) are discussed. The study provides valuable information on the physical impact of bottom trawling on the seabed and can support existing monitoring strategies.