LEYENDAS DEL CANTÓN GUANO

At the start of this research three research questions were formulated. The first question is: How do the salt marshes in the eastern Wadden Sea affect the wave conditions at the dike under design conditions? The answer to this question is determined in 1D and 2D for both the measured storm of January 11th 2015 and the local design conditions. The influence of the vegetated foreshore is significant. Comparing the significant wave height at the toe of the dike at the salt marsh with locations without vegetation and a lower bed level, the difference in for 𝐻_𝑚0 near the dike is between 27% and 36% for the 1/4000 year storm. The salt marsh also lowers the wave period 𝑇_𝑚−1,0 which leads to a lower wave run-up. The 2% wave run-up is decreased by up to 41% due to the presence of the salt marsh.

The vegetation on the salt marshes is important for the amount of wave damping. Vegetation reduces the significant wave height by an additional 15% and 50% compared to a foreshore without vegetation. The bed level found in front of the dike also has a large influence on the waves for two reasons. Firstly because the reduced water depths cause the larger waves to break, limiting the height of the waves reaching the dike. Secondly, bed levels above the mean flood level are required for the type of vegetation found on the salt marshes.

The results suggest that the current extent of the vegetation at the test site in seaward direction is sufficient. The first reason for this is the fact that the significant wave height is dampened only slightly close to the dike, which means that the situation is close to an equilibrium between the wind which adds energy the waves, and the dissipative terms. The second reason is that extending the marsh further into sea by 1500 meters results in limited additional wave damping.

The second research question was: Can flotsam measurements serve as field evidence for wave run-up reduction by vegetated foreshores? To answer this, the flotsam measurements were compared with the foreshore bed level and the extent of the vegetation (or: the distance from dike to the edge or the marsh). These parameters represent the variation in the

vegetated foreshores. The correlation between the foreshore bed level and the extent of the vegetation is high, which confirms that the two are related since the vegetation requires a certain bed level and vegetation improves sedimentation. The vegetation extent and bed level are highly correlated to the flotsam level, reducing the flotsam level when the other parameters increase. Due to the high correlation between the flotsam level and the

vegetated foreshore characteristics and the knowledge that the flotsam levels are determined by the local maximum wave run-up, it is possible to say that the flotsam levels can certainly serve as an indicator for the run-up reduction by vegetated foreshores.

64 The final research question was: Can flotsam measurements be approximated using a 2D wave model? The 2D model which was created in this research was used to determine the water level and wave conditions near the dike for all flotsam measurements. Using this information, the local maximum wave run-up could be determined (Chapter 5). The pattern found in the flotsam measurements could be reproduced well. The values of individual measurements were replicated with the maximum wave run-up calculations, but only when taking both measurement (±0.4 m) and model uncertainties into account (±20%). This means that the flotsam levels can be approximated using a 2D model and are useful as an indicator of local wave conditions but cannot be used to calibrate a 2D model due to the seeming inherent uncertainties related to the flotsam levels.

It is possible to make several recommendations based on this thesis for future research. Firstly it is very interesting to investigate the influence of the method to schematize vegetation. In the 1D model it became clear that when extrapolating from a calibrated situation to the design conditions, the ‘vegetation as additional bed friction method’ leads to less dissipation than the vertical cylinders method. However, the vertical cylinders method is not yet available in Delft3D-WAVE and could therefore not be tested. This can be overcome by replacing the version of SWAN manually and entering the SWAN commands via a separate script. The alternative is to wait for Delft3D to incorporate these features into the standard version which will likely be available at some point in the future.

Secondly, it would be interesting to compare the calibrated models for different storms. This must be done by placing pressure sensors on the salt-marsh to measure the wave

conditions, since wave measurements on the salt marshes are normally not performed. It is important to measure two storms within a time period where the vegetation characteristics changes are limited. By validating the model against a second storm, the dissipation by vegetation can be estimated with more certainty, leading to a higher confidence when scaling the model to the design conditions.

Thirdly, the schematization of the vegetation is based on a single biomass measurement of the vegetation and estimation from literature and photos. Having more and more accurate measurements of the vegetation could possibly add a lot of detail to the schematization of the vegetation. The values that should be determined are the average stem diameter, vegetation height and the biomass.

Finally, all wave calculations were performed in stationary mode. Comparing what the difference in 𝐻_𝑚0 is in the two approaches would be interesting.

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