Wave attenuation by vegetation

Report (2003)

Authors

Affiliation
None
Copyright: © 2003 Mol. A.
Waves Vegetation Wave attenuation Wave decay
More Info
expand_more

Published Date

13-03-2003

Reuse Rights

Other than for strictly personal use, it is not permitted to download, forward or distribute the text or part of it, without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license such as Creative Commons.

Source:

Z3040 WL report, also thesis University of Twente

Abstract

Measurements have been carried out at the Paulinaschor, a salt marsh in the Westerschelde, to obtain information of the effect of vegetation on wave attenuation. The data have been analyzed. It appears that wave height is strongly reduced by the vegetation, especially for low water depths. Further analysis of the data has been done, to achieve wave energy dissipation. An attempt has been made to formulate a theoretical approach, which is suitable for calculating wave energy dissipation due to vegetation on the basis of certain vegetation characteristics such as stem diameter, plant height and plant density. This theory has been tested by a comparison between the theoretical dissipations and the – so called – observed dissipations. This resulted in quite satisfying correlations; correlation coefficients of about 0.6 – 0.8 were calculated. By means of this analysis is a friction coefficient determined, describing the friction exerted by the vegetation. This coefficient depends on the various vegetation characteristics as mentioned before, but also at a second friction factor, that is more plant specific. Subsequently, the wave model SWAN has been suited for modelling waves over vegetation areas. The Collins friction factor is used for calibration. Values for this factor turned out to be 2 orders of magnitude bigger than the default value, for bare bottoms. A further study on this Collins coefficient showed that this coefficient is, except for a constant factor, the same as the friction coefficient that was calculated on the basis of the various characteristics. Using these calculated friction coefficients, converted to Collins coefficients, the SWAN model has been validated. The model results showed a good agreement with reality. Only the wave attenuation at the edge of the salt marsh did not correspond very well with the observed attenuation. A possible explanation could be that vegetation is modelled in SWAN through an enlarged bottom friction, in stead of 3D obstacles. Also due to the fact that the development of the orbital velocity in the vegetation is not known exactly, deviances between model outcome and observed attenuation may occur.