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

Water Depth

In shallow water areas, water depth is the key control on the extent of wave attenuation. Although the relative level of wave attenuation afforded by saltmarsh is highly dependent on the water depth, the extent of wave attenuation over saltmarsh is greater than can be accounted for by water depth alone. The relationship between water depth and wave attenuation is stronger and less variable over vegetated saltmarsh than over unvegetated sand or mud flats (see figure below).

Relationship between wave energy attenuation and water depth for the sand flat and saltmarsh areas at Stiffkey, north Norfolk (from Möller et al., 1999)

In water depths of less than 1.1m, Möller et al. (2001) observed reductions in wave energy by an average of 87% over saltmarsh (37% over an equivalent width of sand flat); while in deeper water the average wave energy reduction was less at 72% and 27% over saltmarsh and sand flats respectively.

The results from Möller et al. (2001) confirm the potential for saltmarsh to contribute to coastal defence. In considering this value, it is important to understand variations in the attenuating efficiency of saltmarsh under different water depths. Over a certain water depth (estimated at 3.7m by Möller et al., 2001), the relationship between wave attenuation and water depth becomes the same as over unvegetated mud or sand flat. In many areas, it is likely that during very large storm surge events, the water level over saltmarsh areas would be increased above this threshold. This means that in the case of very large surge events, the level of protection against wave energy and overtopping at the landward margin of the saltmarsh may be the same as if it were fronted by unvegetated sand or mud flat. In contrast, Möller et al. (2001) showed that saltmarsh is very effective at mitigating small to medium energy wave events that still may cause damage to structures. The extent to which saltmarsh is able to aid in protection from very large events will, therefore, be site specific and dependent on the elevation of the saltmarsh and the magnitude of surge generated in the area.