There is a long history of the use of engineered works for coastal flood and erosion protection in the UK and around the world. It has long been assumed that saltmarsh has the beneficial effect of attenuating wave energy and, thereby, contributes to this protection. Interest in quantifying the ability of saltmarsh to attenuate wave energy has been further stimulated by concern over future accelerated sea level rise and the associated increases in the cost of strengthening and raising flood protection structures (should this be viable). Widespread loss of saltmarsh from UK estuaries may be further reducing shoreline protection.

Research in the UK has shown that saltmarsh areas attenuate significantly more wave energy than unvegetated intertidal areas (see figure below).

Reduction of significant wave height between sandflat (outer), the saltmarsh edge (middle) and saltmarsh (inner) stations (from Möller et al. (1999)

Studies into the attenuation of waves over saltmarsh have consistently demonstrated greater wave height and energy reduction over saltmarsh compared to unvegetated sand or mudflats, but the extent and nature of the attenuation is highly variable, for example, at Stiffkey, on the north Norfolk coast, an average wave height reduction of 61% has been measured over saltmarsh, which is approximately four times the 15% reduction observed over an equivalent width of adjacent sand flat (Möller et al., 1999). Total wave energy reduction over the saltmarsh of 82% was nearly three times that over the unvegetated sand flat (average 29%). On the Dengie Peninsula, at Tillingham, wave height attenuation of 21% was measured over the mudflats and 87% over saltmarsh, while at Bridgewick, offshore wave heights actually increased across mudflats by 24% and decreased by 44% over a much narrower (10m) strip of saltmarsh (Möller and Spencer, 2002). Associated decreases in wave energy were observed over the saltmarsh at Tillingham (35%) and Bridgewick (79%). In The Wash, overall wave energy attenuation over saltmarsh of between 72% and 97% has been measured, with associated decreases in wave height of between 64% and 91% (Cooper, 2001). Results from these three studies can be viewed here.

There are a number of factors (see figure below) that influence the extent of wave attenuation by saltmarsh, including water depth, bed roughness and marsh edge characteristics and vegetation characteristics.

Factors influencing wave energy dissipation over saltmarshes (from Möller et. al., 2002)