Quantifying saltmarsh vegetation and its effect on wave height dissipation: Results from a UK East coast saltmarsh

The degree to which incident wind waves are attenuated over intertidal surfaces is critical to the development of coastal wetlands, which are, amongst other processes, affected by the delivery, erosion, and/or resuspension of sediment due to wave action. Knowledge on wave attenuation over saltmarsh surfaces is also essential for accurate assessments of their natural sea-defence value to be made and incorporated into sea defence and management schemes. The aim of this paper is to evaluate the use of a digital photographic method for the quantification of marsh vegetation density and then to investigate the relative roles played by hydrodynamic controls and vegetation density/type in causing the attenuation of incident waves over a macro-tidal saltmarsh. Results show that a significant statistical relationship exists between the density of vegetation measured in side-on photographs and the dry biomass of the photographed vegetation determined through direct harvesting. The potential of the digital photographic method for the spatial and temporal comparison of marsh surface vegetation biomass, density, and canopy structure is highlighted and the method was applied to assess spatial and seasonal differences in vegetation density and their effect on wave attenuation at three locations on a macro-tidal saltmarsh on Dengie Peninsula, Essex, UK. In this environmental setting, vegetation density/type did not have a significant direct effect on wave attenuation but modified the process of wave transformation under different hydrodynamic conditions. At the two locations, characterised by a relatively tall canopy (15e26 cm) with biomass values of 430e500 g m 2, dominated by Spartina spp. (>70% of total dry biomass), relative incident wave height (wave height/water depth) is identified as a statistically significant dominant positive control on wave attenuation up to a threshold value of 0.55, beyond which wave attenuation showed no significant further increase. At the third location, characterised by only slightly less biomass (398 g m 2) but a shorter (6 cm) canopy of the annual Salicornia spp., no significant relationship existed between wave attenuation and relative wave height. Seasonally (between September and December) significant temporal increase/decrease in vegetation density occurred in one of the Spartina canopies and in the Salicornia canopy, respectively, and led to an expected (but not statistically significant) increase/decrease in wave attenuation. The wider implications of these findings in the context of formeprocess interactions on saltmarshes and their effect on marsh evolution are also discussed.