The role of vegetation in determining patterns of the accretion of salt marsh sediment

Abstract

Abstract Vegetation contributes to the stability of the salt marsh surface both through the soil reinforcement provided by the root mass and by the ability of the aerial component of plant growth to increase the rates of sediment deposition. Salt marsh vegetation is variable in height, thickness and also in stiffness. These characteristics can all modify the role of the vegetation in trapping sediment. As the height of the vegetation increases so also does Manning’s ‘ n ’, coefficient of roughness, but the taller vegetation is more easily flattened by moving water. It is not necessarily, therefore, more effective in slowing water flow and increasing sediment deposition. Shorter vegetation (mean height = 50 mm) has been shown to be quite effective, particularly if it is stiff enough to resist being flattened by the water at the rates of flow experienced. The interplay of rates of sediment deposition is discussed with reference to data relating vegetation structure and pattern to local variations in accretion rates in salt marshes at Stiffkey, Norfolk, and Tollesbury, Essex, over a period of three years. At Tollesbury the mean annual rate of accretion was estimated to be 4.27 mm a −1 but there was no correlation between the vegetation height (mean height = 148 mm) and changes in surface level. At Stiffkey however the annual rate of accretion was estimated to be 3.08 mm a −1 , but there was a significant positive correlation between vegetation height (mean height = 75 mm) and accretion. There is evidence that the relationship between effective vegetation height and the process of sediment accumulation is complex. Factors that can affect the relationship include the nature of the sediment, the height and composition of the vegetation and the microtopography and elevation of the marsh itself.