Abstract
AbstractThe 1717 Christmas flood is one of the most catastrophic storm surges the Frisian coast (Netherlands and Germany) has ever experienced. With more than 13,700 casualties it is the last severe storm surge with a death toll of this order. At the same time, little is known about the hydrodynamic conditions and the morphological effects associated with this storm surge.In this study, 41 potential dyke failures in the Province of Groningen (Netherlands) associated with the 1717 Christmas flood were systematically reconstructed and mapped by using historical maps and literature and by analysing the recent topography in search of typical pothole structures and sediment fans. The dimensions of the sediment fans as derived from the topography show a good accordance with the dimensions documented by vibracore profiles, direct push tests and electrical resistivity tomography data taken at three fieldwork sites. Moreover, the fan dimensions closely agree with the dimensions as simulated using a process-based morphodynamic numerical model for one of the three sites, the village of Wierhuizen. Consequently, the recent topography is still indicative for the locations and dimensions of dyke failures and sediment fans associated with the 1717 Christmas flood. Considering the large number of detected dyke failures (41) and the large dimensions of the potholes and particularly of the sediment fans up to a few hundred metres wide and up to 0.7 m thick, this study proves significant morphological effects of the 1717 Christmas flood on the mainland of the Province of Groningen.Based on the numerical simulation approach and the comparison with field data and field observations, a maximum seaward water level of 5 m NAP for the dyke failure at Wierhuizen during the Christmas flood can be derived. A similar maximum water level is indicated for the two other fieldwork sites Vierhuizen and Kohol, which is in good agreement with the maximum storm surge level of 4.62 m NAP historically documented for the city of Emden located almost 50 km to the east of Wierhuizen.The results of the current study demonstrate that the reconstruction of historical dyke failures based on (i) historical sources, (ii) recent lidar/high-resolution topographical data, (iii) multi-proxy sedimentary field data and (iv) hydro- and morphodynamic numerical simulations is a highly promising approach to derive hydrodynamic conditions and the morphological onshore response of the 1717 Christmas flood in the Province of Groningen. This knowledge is essential to improve our understanding of extreme storm surge dynamics, their influence on the coastal landscape and the associated hazards for the coastal population.
Highlights
Extratropical cyclones in the zone of the Westerlies frequently cause severe storm surges in the North Sea
The observations that (i) an abrupt change towards high-energy conditions occurs from Unit 2 to Unit 3, that (ii) the uppermost layer shows its maximum thickness in profile WIE 2A, but is thinner in profiles WIE 1 and WIE 3 and is absent inside the pothole (WIE 4), and that (iii) the investigated dyke failure can directly be ascribed to the 1717 Christmas flood (Section 4.1) lead to the conclusion that the uppermost layer reflects fan sediments deposited in the zone of flow divergence during the 1717 Christmas flood when the dyke breached
With more than 13,700 casualties in the Netherlands, Germany and Denmark, it is the last severe storm surge with a death toll of this order, while, at the same time, little is known about the hydro- and morphodynamic conditions
Summary
Extratropical cyclones in the zone of the Westerlies frequently cause severe storm surges in the North Sea. Being highly exposed to northerly and westerly winds and showing low land elevations (few metres above/below sea level), the coast of Frisia (Netherlands and Germany; Fig. 1A) is vulnerable to storm surges. The Frisian coast has been repeatedly flooded in history, resulting in considerable loss of human life and vast devastation. Storm surges have caused severe changes in the coast’s morphology and thereby significantly contributed to its recent shape. This is obvious for the North Frisian
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