Abstract
Vegetated shorelines have been increasingly recognized for their contribution to natural coastal protection due to their ability to dissipate wave energy. Within the UK, salt marshes are beginning to be included in flood defence schemes. Predicting wave dissipation over vegetation requires accurate representation of salt marsh canopies and the feedback relationship between vegetation and wave conditions. We present a modification to the SWAN vegetation model, which includes a variable drag coefficient and a spatially varying vegetation height. Its application is demonstrated by modelling wave propagation over UK salt marshes. The third generation wave model, SWAN includes a vegetation module for calculation of wave attenuation over vegetation. Wave dissipation is determined based on the vegetation properties and a drag coefficient. This drag coefficient, C_D, is used to calibrate the model, and a fixed value is used per model run. Empirically the drag coefficient has been found to vary with ambient wave conditions. Typically the drag coefficients are defined empirically as a function of either the stem Reynolds number, Rev, or the Keulegan-Carpenter number, KC. The parameter values have been shown to vary with vegetation type. In this paper, we modify the SWAN vegetation module to include a temporally varying CD. This allows the drag coefficient to vary with ambient wave parameters, which gives an improved prediction under time varying wave conditions (e.g. passage of a storm) and includes the change in wave conditions as they travel through the vegetation. We also incorporate spatially varying vegetation height into the model to further improve the representation of the complexity of vegetated shorelines. Using the new formulation we find improved prediction of wave dissipation over both idealized laboratory and field salt marsh vegetation.
Highlights
Recent years have seen the increasing recognition of wave dissipation by vegetation in aquatic and coastal environments
In this paper we present modifications to the SWAN-Veg module to better incorporate the complex hydrodynamics and varied vegetation structure across coastal vegetated wetlands
The spatially varying vegetation height was added to the SWAN-Veg module
Summary
Recent years have seen the increasing recognition of wave dissipation by vegetation in aquatic and coastal environments. This realization stems from growing field evidence of wave attenuation, both in salt marshes (Möller et al, 1999, 2001) and other types of vegetation (Kobayashi et al, 1993; Méndez et al, 1999; Paul and Amos, 2011; Bouma et al, 2014). The Morison equation describes the force of a wave on a cylinder to calculate the energy dissipation due to the cylinder This equation has been adapted for use in vegetated wetlands by Dalrymple et al (1984) and Mendez and Losada (2004), where the energy dissipation is calculated based on the properties of the plants including the plant height, plant diameter, number of plants and a drag coefficient. The drag coefficient, CD, is a simple method for parameterizing the approximations and physical processes not explicitly calculated
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