Foreshores in the Wadden Sea

Foreshores in the Wadden Sea

Van Osselen, K.R.

Kok, M. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2016-12-09

Dutch flood protections are assessed every five to six years in a nationwide safety assessment round. During the last assessment round large dike stretches in the Dutch Wadden Sea could not meet the safety requirements, and were assessed with failure. Dike strengthening is therefore needed in these areas. However, in front of these dike salt marshes are present, which act as shallow foreshores. These could reduce the load in these dikes, which could lead to less or no dike strengthening. The dikes is assessed in the last official assessment round with a probabilistic program Hydra-NL. In this program the safety of a dike can determined on the basis of a dike profile and Hydraulic Boundary Conditions. The derivation of these conditions is done with the wave model SWAN. For the derivation of the hydraulic conditions a bottom profile is needed, in which the shallow foreshore are already included to some extent. Typically, the foreshores have already been included until 50-100m in front of the dike toe. Here the hydraulic conditions have been given in a so called output point. In the program Hydra-NL a foreshore module is present. With this module the final stretch of foreshore can be taken into account. With this module the design conditions in the output point are taken and transformed to the location of the dike toe. This transformation leads to lower wave heights, which leads to lower loads on the dike. The calculation method used in the foreshore module of Hydra-NL is the ENDEC method. This module is first compared on theoretical basis to other available transformation methods as SWAN and DaF. This are all modules used in other official calculation tools. Outcome of the comparison was that the ENDEC module can, in some situations, give a significant deviation from the other modules. To determine the extent to which the effect of the foreshores have been included in the official safety assessment a different database of hydraulic conditions has been derived. Instead of near the dike, these conditions are defined at locations in front of the foreshore. These can therefore be seen as the deep water conditions. The effects of the foreshore are assessed on to failure mechanisms. A height assessment, using the maximum overtopping as criteria and a cover stability assessment, for which the wave height notably plays a big role. For the height assessment it was found that including the final foreshore stretch does not lead to a significant increase of the minimum required crest height. The assessment on the dike cover however shows that including the last part of the foreshore could lead to a substantial reduction of wave load. Particularly the lower dike parts experience lower wave loading. As most of the dike stretches in the Wadden Sea have failed on the cover slope of the outer dike, including the foreshore in this manner could lead to a reduction of failure probability.

Foreshores
Wadden Sea
Flood Risk
Hydra-NL

http://resolver.tudelft.nl/uuid:397fd2b9-1ec6-470c-b10e-c07562f197b8

53.43994, 6.59125

Student theses

master thesis

(c) 2016 Osselen, K.R. van