# A method to calculate the probability of dike failure due to wave overtopping, including the infragravity waves and morphological changes

A method to calculate the probability of dike failure due to wave overtopping, including the infragravity waves and morphological changes

Author ContributorJonkman, S.N. (mentor)

Van der Meer, J.W. (mentor)

McCall, R. (mentor)

Vuik, V. (mentor)

Verhagen, H.J. (mentor)

2015-11-19

AbstractIn this thesis, a method was developed, with which the infragravity waves and morphological changes of a sandy foreshore are included in the calculation of the probability of dike failure due to wave overtopping. Constructing a natural foreshore in front of the dike can be an attractive and innovative method to decrease the failure probability. However, the uncertainty in the morphological development of these foreshores leads to uncertainty with respect to their contribution in protection against flooding. The morphological stability of a foreshore during extreme conditions is not well known. The current Dutch safety assessment tools do not yet include the infragravity waves and morphological changes of a foreshore during a storm. Hence, it is not yet possible to guarantee the robustness and safety of dike-foreshore system. This thesis considered hybrid defences (dike-foreshore systems), where the dike is still of importance in the protection of the hinterland. The considered hybrid defence was a schematized version of the Westkapelle sea defence, located at the coast of Walcheren in the Netherlands. The morphological changes of the foreshore calculated in this thesis, were the changes during (severe) storms. Because a single model that includes all the different relevant processes does not exist, a model framework or ‘model train’ was developed, in which different models were combined. The modelling framework best fit to solve the research questions was determined as a combination of XBeach hydrostatic, the EurOtop formulae and the probabilistic method Adaptive Directional Importance Sampling (ADIS). Including infragravity waves (and wave set-up) lead to much larger failure probabilities for the hybrid defence considered in this thesis. This difference is mainly caused by the difference in wave period at the toe of the dike. Including the morphological changes lead to a somewhat larger failure probability. The fact that this difference was not that large, was mainly caused by less wave dissipation due to erosion of the foreshore, but at the same time less transfer of energy to the low frequencies, thus a smaller wave period. The combination of a (very) shallow foreshore and dike slope of 1:8 make that the case considered here is (largely) outside the previously studied wave overtopping area. It is possible, that when the wave period becomes very large, the wave overtopping is not dependent anymore on the dike slope, but on the wave parameters only. It is therefore questionable if the EurOtop formulae calculate the right amount of wave overtopping for these types of situations, because in the formulae, the wave overtopping is dependent on the dike slope. Furthermore, the EurOtop formulae use the Tm-1,0 wave period. This wave period is very sensitive to the low frequencies. Clear guidelines should be determined on which frequency resolution and, if necessary, cut-off frequencies should be used when determining wave spectra. This thesis presented a method with which infragravity waves and morphological changes of a sandy foreshore can be included in the calculation of the probability of dike failure due to wave overtopping. Before this thesis, this was not yet possible. As shown in this thesis, it is important that the infragravity waves are included in the calculation of the dike failure probability due to wave overtopping at this dike-foreshore system, because they had a large influence on the probability of failure. The method developed in this thesis can be used at other locations without many problems, however the influence of the infragravity waves and morphological change as determined in this thesis, could be different at another location.

Subjectdikes

flood defences

wave overtopping

infragravity waves

morphology

probabilistic

foreshores

overtopping

morphological changes

soft solutions

Building with Nature

levees

dike-foreshore system

safety assessment

stochastic

XBeach

EurOtop

Adaptive Directional Importance Sampling

ADIS

hybrid defences

flood risk

BE-SAFE

hydrodynamics

probability of failure

reliability function

shallow foreshore

Westkapelle

sea defence

wave period

wave spectra

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Coordinates51.547492, 3.452201

Part of collectionStudent theses

Document typemaster thesis

Rights(c) 2015 Oosterlo, P.