Kees Nederhoff

Worldwide, many of the most densely populated areas are located near the coast. Climate change and population growth put more and more pressure on these coastal areas. This leaves coastal zone managers with a difficult task, as free space is becoming sparse, coastal disaster risk reduction plans need to be spatially efficient. In this thesis we address a sandy coast with hard structures, such as buildings or dune revetments. These structures can either provide additional protection, or enhance local erosion by developing a scour hole in cross-shore direction or result in additional erosion in adjacent locations. Field measurements and experimental data featuring these phenomena are scarce, but the measurements of the devastating impact of hurricane Sandy on the New Jersey shore provide new model validation possibilities by introducing a new field case. Figure below shows the devastating impact of hurricane Sandy (October 2012) around a hard structure at Camp Osborne, Brick, NJ. 

Researchers have been studied the response of 'soft' coastal system during storm conditions for the last three decades. The DUROS+ model (van Gent et al., 2008) 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, such as XBeach (Roelvink et al., 2009). It is 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:

• Cross-shore. Experiments (Steetzel, 1987) 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 (Van Thiel de Vries, 2009)
• Longshore. Flume experiments (Boers, 2008) 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 (van Geer, 2012). In order to describe this effect calculation rules are derived to determine the increase in dune retreat (Deltares and Arcadis, 2013) .

In order to understand, validate and evaluate the relations described, measurements are necessary. 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 in New Jersey were in an increase erosion is observed next to a condo during Hurricane Sandy.

Oblique photo material from the USGS at Camp Osborne, Brick. Pictures are taken on 21/05/2009 and 05/11/2012.

References

Roelvink, J. A., Reniers, A. J. H. M., van Dongeren, A. R., van Thiel de Vries, J. S. M., McCall, R. T., and Lescinski, J. (2009). Modelling storm impacts on beaches, dunes and barrier islands. Coastal Engineering, 56(11--12):1133 – 1152.

Van Gent, M., van Thiel de Vries, J. S. M., Coeveld, E. M., de Vroeg, J. H., & van de Graaff, J. (2008). Large-scale dune erosion tests to study the influence of wave periods. Coastal Engineering, 55(12), 1041--1051.

Steetzel, H. J. (1993). Effect of objects on dune erosion (Dutch: Effect bebouwing duinafslag). Delft.

Van Thiel de Vries, J. S. M. (2009). Dune erosion during storm surges. PhD. Delft University of Technology, Delft.

Boers, M., van Geer, P., & van Gent, M. (2008). Dike and dune revetment impact on dune erosion.

Van Geer, P., de Vries, B., van Dongeren, A., & van Thiel de Vries, J. S. M. (2012). Dune erosion near sea walls: model-data comparison. Coastal Engineering, 1--9.

Deltares, & Arcadis. (2013). Calculation rules objects in dunes (Dutch: Handreiking NWO in duinen). Delft: Deltares.