Salt marsh substrate composition and responses to applied stress

Salt marshes provide a variety of ecosystem services, including habitat provision, pollutant storage and attenuation of waves and currents. As such, understanding the resistance of the marsh sedimentary platform to erosion is important, particularly as marsh edge erosion is common on many shores. This resistance is likely to be strongly influenced by marsh biological, geochemical and sedimentological/geotechnical properties. Currently there is little systematic research into the within- and between-marsh variability in these properties and how they affect both marsh edge and marsh surface erosion processes. This thesis compares Tillingham marsh, eastern England, where the sediment is clay/silt-dominated and the marsh canopy is species-rich, to Warton marsh, Morecambe Bay, NW England, where the sediment is sand/silt-dominated and the vegetation species-poor. Soil shear strength and compressibility are determined by applying geotechnical methods which have not previously been applied to salt marsh environments to determine marsh resistance. These results are compared to commonly-used in situ methods for determining substrate strength, and then variations in substrate strength and compressibility are linked to measured variations in marsh composition. This research finds that particle size is a key control on marsh resistance to particle detachment, with root- and organic content providing an important secondary influence on marsh resistance, particularly to bulk failure processes. By comparing the angle of internal friction measured by shear box and ring shear tests, this research is able to isolate the influence of roots on substrate shear strength. This research enhances understanding of the links between marsh substrate composition and marsh substrate behaviour under applied stress. By linking this knowledge of how marsh composition affects marsh stability to marsh erosion processes and rates, this research sheds light on key determinants of marsh resistance to edge erosion, and thus improves our ability to predict future erosion, which is ultimately essential for the informed implementation of both nature-based coastal flood defences, and coastal restoration schemes.NERC, British Geological Surve