IC Workshop Exkursion to the Wadden Sea

During the last week of June 2014 within the framework of the INTERCOAST Workshop, the second cohort of INTERCOAST PhD candidates, accompanied by Dr. Julia Mullarney, Dr. Willem de Lange and Dr. Vicki Moon (University of Waikato, NZ) have been introduced to the geology of the North Sea by Prof. Dr. Burghard Flemming (Senckenberg, DE). The program consisted of a presentation covering the geological and geographical evolution of the North Sea area with a strong focus on the German Bight, associated to a scenic flight of 90 minutes over the seven main German barrier islands bordering the Wadden Sea (Fig. 1).
Enhanced by the flat profile of the north of the Dutch and German coastal areas, the high frequency of flooding events led human communities to build their settlements on mounts (2700 BP). However extreme storm surges such as the Grote Mandrenke (1362, 10 to 200 thousand casualties), as well as land reclamation aims progressively encouraged the construction of dikes. Initially, the flux of sediments through the tidal inlets decreased within the tidal prism of the Wadden Sea and the surface of the ebb-tidal deltas decreased to the benefits of the islands.
However during the past 500 years, due to the continuing local sea level rise, fine sediments constrained towards the dikes could not find any sufficient accommodation landwards and were progressively exported. Simultaneously, the construction of dikes and dams to improve the navigability narrowed the rivers bed, increased the celerity of the flood wave and the inland tidal range (the Weser tidal range in Bremen increased from 15 cm to 4 m in the last 150 years) and eventually inversed the tidal asymmetry. As a result, the fine sediments were imported by the local rivers (Ems, Weser, Elbe). In the Wadden Sea, the consecutive deficit of sediments induced a relocation of sand from the islands shoreface to their Wadden Sea face. As an example, Juist migrated landwards over 1 km since 1650 in response to ca. 1 m sea level rise.
The plane took off from Harlesiel at around 3 p.m. corresponding to low tide. Good weather conditions allowed us to recognize different types of coastal morphologies (Fig. 2). In the intertidal zone between mainland and East Friesian Islands we observed wide-spread tidal flats crossed by numerous meanders and several large tidal channels connected to the barrier island inlets. In the upper tidal zone along islands and mainland salt marshes are common features (Fig. 3).
During the flight we noted several efforts for coastal management. At islands the installation of groins (Fig. 4) and sand replenishments are the most promising methods to stabilize the islands and to slow down their erosion which is normally accompanied by an eastward-directed migration. At the mainland’s coast, mud flat erosion is a significant issue. Therefore in order to reduce the threat of flooding but also to protect existing biological habitats, clay retention pits have been installed along the coast (Fig. 5a) helping the natural development of marshlands (Fig. 5b). At Leyhörn, near Emden, we observed a huge water storage basin (Fig. 6) which is connected to the open sea by a sluice which controls the tidal water inflow and lowers the threat of large flooding events.

Brice Blossier and Max Kluger