Abstract
The hydrodynamics of estuaries are forced by the tides from the open sea and the river runoff from the catchment area. The hinterland is often low-lying and densely populated and must therefore be protected by dikes. Anthropogenic climate change poses new challenges to the coastal protection. However, changes in the geometry of the estuaries can have equally severe impacts on the deformation of a storm surge wave form when it propagates through the estuary. This affects the peak water levels and hence the design water levels. This contribution focuses on the influence of retention areas or forelands seaside of the main dike lines, which are protected by summer dikes against the less severe but more frequently occurring storm surges. This is shown at the example of a retention area in the Weser estuary, which has historically been the cite of a soccer stadium and thus hosts high values which stand in sharp contrast against the low safety level against flooding. The investigation is conducted with a 3D hydrodynamic numerical model which has previously been validated for the simulation of storm surges. The results show that even very small changes in the geometry of the estuary can have effects on design levels. This is even the case when they only regard the summer dike crests heights around retention areas and not their volume. Another important finding is that the geometry changes may have their maximum impacts quite far away from the specific river reach in which they are carried out. The results underline that for designing safe and reliable storm surge infrastructure, storm events should be studied in high resolution models which are able to resolve even small scale features such as summer dike lines.
Highlights
MotivationThe Weser estuary (Fig. 1, left) can be described as meso- to macro tidal
The results show that the increase of the summer dike crests have a significant impact on the peak water levels during the event, compared to the status quo geometry: Close to the retention areas, a decrease in peak water levels can be observed
The highest increases of peak water levels can be observed 18 to 28 km from the weir and mount up to 12 cm
Summary
The Weser estuary (Fig. 1, left) can be described as meso- to macro tidal It has a tidal range of 2.8 m in the outer estuary, where salinity almost equals to seawater salinity. The tidal range amounts to a maximum value of 4.2 m This strong and almost continuous increase is due to the strong convergence of the funnel-shaped outer estuary and the narrow shape of the inner estuary which allows only little dissipation of the energy of the tidal wave. According to Thorenz (2008), an area of 6.600 km is located below mean tidal high water in the German Federal Land Lower Saxony. This area would be flooded daily twice without the coastal protection system which is in place today. The design levels are extracted from storm surge simulations of a “design event”, mimicking the design values in the outer estuary (Niemeyer et al 2003)
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