Modelling the Impact of Estuarine and Coastal Morphological Changes on Tidal Dynamics in the German Bight

Tides of the North Sea’s coastal waters in the German Bight are well documented by numerous tide gauges. At several stations the operation started in the 19^th or early 20^th century, thus making it possible to determine long-term developments of the tidal regimes at these stations. Trend analysis of such long time records indicate a general increase of Mean tidal high and low waters (MHW / MLW) with rates of several centimeters up to a few decimeters for the past century. In most cases, MHW-rise is faster than MLW-rise, hence causing also the mean tidal range (MTR) to rise.

In addition to large-scale influences like Sea Level Rise (SLR), the observed trends can also reflect local effects, especially within the shallow Wadden Sea, a very depth-variant and highly dynamic region of intertidal flats and tideways in the southeastern German Bight.

This study investigates morphological changes as one potential factor with focus on the Lower Saxony coast of Germany and the estuaries of Ems and Weser. Both rivers are important navigational routes and have repeatedly been subject to heavy anthropogenic modifications.

The tidal dynamics are compared under historic and present morphological conditions using the semi-implicit numerical model SCHSIM on unstructured grids. The cross-scale model domain highly resolves the investigation area and additionally covers the Greater North Sea and parts of the Northeast Atlantic, enabling a physically consistent propagation of the tidal wave. A reconstruction of the estuarine bathymetry prior to the major interventions was realized through comprehensive digitization of historic navigational charts.

Besides anthropogenic interventions, the study aims to consider the natural adaption of tidal flats and marshland to SLR. As long as the rates of SLR remain moderate, the Wadden Sea can compensate this trend with growth rates similar or even higher because the enlarged tidal volumes transport more sediment into the basins that can then settle during slack water.

With a simplified application of this process, namely just vertically shifting parts of the Wadden Sea bathymetry, a sensitivity analysis on different SLR-scenarios and their impact on tidal characteristics in the coastal zone is carried out. While this approach does not consider the individual change of morphological entities, as for example the shift of tidal flats, it can reproduce the general trend of rising MTR due to faster rising MHW than MLW, which is not achieved without the adaption.

(funded by German Federal Ministry of Education and Research (ALADYN-C, reference03F0756C))