Predictions of saltmarsh suitability as a coastal defence measure based on numerically-modelled extrapolations from a present day Digital Terrain Model

The ability of saltmarshes to dissipate incoming waves, coupled with their sensitivity to relative sea level rise and increased storminess in the future, means that predictions of saltmarsh suitability as coastal defence mechanisms are much needed. This study focused on the saltmarsh at Freiston Shore, the Wash, eastern England. It analysed past topographic and vegetative changes, determined current topographic and sea level rise states, and quantified rates of wave attenuation. A century-long numerical model was used to extrapolate these trends to predict future states of the marsh based on 9 scenarios of sea level rise and accretion rate. The ratio of significant wave height at the rear of the marsh to waves at the seaward end of the marsh was estimated based on field-obtained wave transformation functions dependent on depth and vegetation type. On average, significant wave height reduced by 88.5% over a 205m section of part-mudflat and part-saltmarsh surface. The greatest rates of wave attenuation were located in a zone containing the most seaward extent of seasonally permanent vegetation. The model highlighted differences in inter-scenario wave attenuation. It concluded that the threat of saltmarsh erosion or flooding caused by wave attack under non-stormy conditions is low in either an accretion or sea level rise dominant situation over the next century. However, saltmarsh wave attenuation capability was found to reduce significantly during storm surge periods, and it is likely that storm related increases in wave height and water level would be the chief cause of saltmarsh erosion in the future