Wave height • Water depth • Width & edge characteristics • Topography & bed friction • Vegetation density

Topography & bed friction

In addition to water depth and topography, wave attenuation over intertidal flats and saltmarsh is generated by a wide range of other influencing factors, including viscous friction, percolation and sea bed boundary layer friction. Sea bed friction is the most significant of these factors in terms of the increased effect of saltmarsh on wave transformation.

Numerical model results indicate that the main factor responsible for increased wave attenuation across saltmarsh is an increase in the bed surface friction factor due to the roughness of the vegetation (Möller et al., 1999). Results of field studies and numerical modelling at Stiffkey have been used to estimate values for the “friction factor” f mod to reflect surface roughness over saltmarsh and sand flat areas. In the table below, results from a study by Möller et al. (1999) gave values of less than 0.02 for sand flat increasing to over 0.07 for saltmarsh.

Friction factors for sand flat and saltmarsh environments, calculated from the wave attenuation model by Möller et al. (1999)

These results indicate that if saltmarsh areas were lost and replaced by sand or mud flat, average wave heights would increase at the backing land or sea defences. Möller et al. (1999) estimated that if the saltmarsh at Stiffkey was lost completely, it would cause a two-fold increase in wave heights at the shore, due to the reduction in the surface friction factor. This effect would vary depending on the site, in particular the elevation of the saltmarsh and the nature of the vegetation.

In addition to saltmarsh vegetation, other factors can increase sea bed friction and increase energy dissipation across an area of salt marsh. Landward of the marsh edge, wave energy dissipation is affected by marsh surface micro-topography, including presence/absence of salt pans and creek systems, and the nature and density of the vegetation cover. Quantifying the influence that these factors have on the extent of wave attenuation is very difficult, due to their highly variable and dynamic nature. The relative level of wave attenuation by unvegetated sand flats can also be highly variable, due (in part) to the presence of ripple bedforms.