Wave attenuation over salt marsh vegetation. A numerical implementation of vegetation in SWAN
Meijer, M.C. (2005). Wave attenuation over salt marsh vegetation. A numerical implementation of vegetation in SWAN. MSc Thesis. TU Delft, Faculty of Civil Engineering and Geosciences, Hydraulic Engineering: Delft. viii, 53 + appendices pp.
Salt marshes are transitional areas between soil and water. Until recently these areas were usually diked and drained in order to create new agricultural land, but nowadays fortunately more awareness exists about the importance of these inter tidal areas as nature reserves and as natural coastal protections. Although awareness on the coastal protection value of salt marshes has increased in recent years, it is still uncertain how and to what extent waves are reduced by these vegetated intertidal areas. In order to get more insight into the wave damping skills of salt marsh vegetation, it was decided to make SWAN suitable for modelling situations with vegetated foreshores.The numerical description of the effect of vegetation on waves that was used, is based on representing the vegetation by vertical, ridged cylinders and was derived by Dalrymple et al.(1984). The physical vegetation properties that are considered in this formulation are the relative vegetation height, vegetation diameter and density, and the drag coefficient. The calibration parameter in the Dalrymple et al.(1984) formulation, which is of significant importance for determining the wave dissipation due to vegetation, is the drag coefficient (Cd). By varying the drag coefficient different types of vegetation (both stiff and flexible) can be modelled. The value of the drag coefficient is dependent on the Reynolds number and on the shape and orientation of the object. A more severe study of the drag coefficient, first for a single cylinder in flow, subsequently for a multi cylinder array, gave insight in the relation with both wave and vegetation characteristics. A number of processes that are not accounted for in the Dalrymple formulation are also discounted in the drag coefficient. The model was made suitable to model homogeneous, horizontally varying (zoned vegetation) and vertically varying (f.i. Mangroves) vegetation fields.As a final validation the model was applied on field measurement results from the Paulina Marsh in the Westerschelde. In this case all parameters were known except for the drag coefficient, which was calibrated. The results were satisfactory, the value for the drag coefficient ranged between 0.5 and 3.0 and the relation with the vegetation height was as expected.The implementation of vegetation in SWAN was found successful for all types of vegetation. However, due to the dependency of the drag coefficient on wave and vegetation characteristics, the model can only be applied on specific situations where wave and vegetation measurements have been taken. In order to make the model generally applicable, more research needs to be done on the dependency of the drag coefficient on wave and vegetation characteristics. When more is known about these relations, a first prediction can be made on the value of the drag coefficient in a specific situation. This can be accomplished by doing more physical modelling tests and by taking wave measurements over vegetated foreshores.
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