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

Width & edge characteristics

Assuming a constant rate of wave attenuation, the overall width of saltmarsh fronting a shoreline would be related directly to the total extent of wave attenuation. However, the results shown above show that the level of wave attenuation is not proportional to the width of saltmarsh. This is because the rate at which wave energy is attenuated across saltmarsh is not consistent across the whole salt marsh width. Field measurements on the Dengie Peninsula indicate that more than 40% of the wave energy arriving at the saltmarsh edge is attenuated across the first 10m of the permanently vegetated saltmarsh with, on average, the following 28m of saltmarsh attenuating a further 60% of the remaining energy. The importance of the saltmarsh margin in attenuating wave energy is shown particularly clearly in results from Dengie, where a saltmarsh width of just 10m attenuated 79% of wave energy at Bridgewick. The morphology and vegetation of the saltmarsh margin are, therefore, particularly important in determining the nature and extent of total wave energy attenuation.

The morphology of the saltmarsh and intertidal flat boundary can vary considerably and will influence the way that wave energy is dissipated across the critically important saltmarsh margin. The nature of the saltmarsh margin depends on whether the saltmarsh is undergoing erosion or accretion, and is also influenced by the exposure of the area to wave energy. A gradual sloped margin typical of an accretionary saltmarsh will dissipate wave energy in a very different way to a bold saltmarsh cliff, normally associated with an erosional system.

Möller and Spencer (2002) investigated the difference in wave attenuation patterns across two contrasting saltmarsh margins on the Dengie Peninsula . The two sites were a low angle ramp at Tillingham and a steep cliffed saltmarsh frontage at Bridgewick. Wave attenuation rates at the saltmarsh margin were twice as high at the cliffed site compared with the low angle smooth transition at Tillingham, with an average energy dissipation of approximately 8%/m and 4%/m, respectively.

Steep saltmarsh boundaries impact on the nature as well as the extent of wave energy dissipation. Steep faced saltmarsh boundaries reflect wave energy and cause rapid shoaling, increasing wave energy immediately fronting the cliffed frontage (Möller and Spencer, 2002).

Although the increased attenuation rate at a steep margin protects the saltmarsh and shoreline further landward, it is likely to generate ongoing erosion of the cliffed face, and progressive reduction in saltmarsh width. In contrast, wave attenuation rates over a ramped saltmarsh edge are more consistent, with an initially high level of wave reduction, followed by a steady attenuation rate of approximately 0.5%/m after approximately 80 m of saltmarsh. This configuration is much less likely to experience ongoing erosion caused by wave action.