Kees Nederhoff

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The most densely populated areas in the Netherlands lie below \ac

. These areas are protected by the coastal dune system.  The importance of this dune system is reflected by the extensive collection of Dutch regulations that ensure the safety level of the dunes \citep

. The federal water act states for example that every six year the safety of all primary water defenses need to be assessed. Currently however it is not precisely known to what extent hard structures have impact on this level of safety.
On top of the assessment of safety of the current system, the Delta Commissioner suggested the idea of multifunctional coastal systems. \cite

explored the possibilities of applying several functions to the coastal area and stated that multifunctional use of this system could be possible, but for that there is more fundamental knowledge needed about the behavior of hard structures in these coastal areas.
In order to increase our knowledge of the effects of hard structures, measurements are necessary. In the Netherlands these measurements are however scarce. Other options to obtain cases for validation are numerical desk studies, large scale flume experiments or by looking abroad. 
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Researchers have been studied the response of 'soft' coastal system during storm conditions for the last three decades. The DUROS+ model \citep

is the prescribed model in the Netherlands for the safety assessment of the dunes. In situations where the safety assessment with the DUROS-model are not adequate, administrators can decide to use more generic models. Several comparisons between process-based models like XBeach \citep

or DUROSTA \citep

have been carried out \citep

. It is therefore fair to say that the processes involved with the soft response of the dune are relativity well understood.
This level of knowledge is not present for the impact of 'hard' elements during storm surges or the influence of hard-soft transitions on the amount of observed erosion. The effects that have been observed:
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In order to understand, validate and evaluate the relations described, measurements are necessary.   
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The objective of this thesis is to get a better understanding of the effects of hard element on erosion process during storm surges. This will be done by validating the existing knowledge, calculations rules and models against a case study.
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\item Cross-shore. Experiments \citep

have shown that scour holes, both in front as behind hard elements, develop during storm surges.  These effects have been reproduced in the process-based model XBeach \citep

.

\item Longshore. Flume experiments \citep

have shown that at the sides of hard elements an increase in erosion can be distinguished. These effects have been reproduced in the process-based model XBeach \citep

. In order to describe this effect calculation rules are derived to determine the increase in dune retreat \citep

.

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