Wave Dissipation over Vegetation
Large-scale modelling of waves with spectral wave models such as SWAN is indispensable for design of coastal structures and the assessment of flood risk. The wave dissipation computational modelling works out well for average bathymetries with e.g. sandy material. One of the exceptions is the modelling of wave dissipation due to submerged or emerging aquatic vegetation. Wave dissipation due to vegetation can be represented as bottom friction (implicit modelling) or as an additional dissipation function (explicit modelling). The second one assumes that vegetation can be represented as cylinder or plates (canopies). Wave dissipation over any kind of species has been measured at numerous locations all over the world, providing insight in the physical process and validation material for the various models that have been developed so far. Within the BE-SAFE project, wave measurements over salt marshes in the Dutch Wadden Sea were carried out during several storms. The modelled and measured spectral energy distribution still showed significant and not yet understood differences.
Example of mismatch between modelled (dashed lines) and measured (continuous line) spectral wave distribution at the salt marsh edge (red) and at the dike (blue).
Project POVW/BESAFE at the Wadden Sea, Groningen. The Netherlands.
Recently Jacobsen et al. (2019) published a new, frequency-dependent dissipation model for waves propagating over a canopy. This is considered as a promising development showing significant differences specially for submerged canopies. The goal of the graduation project is to improve the presently available vegetation module in SWAN, e.g. by implementing the formulation of Jacobsen et al. (2019). This might include a further refinement of the formulation and reanalysis data. The model by Jacobsen et al. (2019) is formally limited to rigid vegetation, so the effect of flexibility of the stems on wave dissipation under varying wave climates will also be considered.
Reference: Jacobsen, McFall, Van der A (2019). A frequency distributed dissipation model for canopies. Coastal Engineering, 150,135-146
https://www.sciencedirect.com/science/article/abs/pii/S0378383918303892
Info:
The main objectives of the thesis project are:
1) Understand the processes related to spectral wave dissipation for submerged vegetation
2) Improve the wave vegetation module of SWAN to provide better results on the wave energy spectra for projects.
a. Implement (Jacobsen et al., 2019 ) in SWAN and validate it with experimental and field data.
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