Shear strength and erosion resistance of salt marsh substrates: Which method to use?

Abstract

Salt marshes provide a variety of ecosystem services, including habitat provision, pollutant storage and attenuation of waves and currents. However, marsh edge, surface and interior erosion is common on many shores due to shear and impact forces applied by waves or currents, as well as gravitational failure. To forecast the future provision of these ecosystem services, the resistance of marsh substrates to hydrodynamic forcing must be determined. At present, a variety of methods exist to measure resistance to failure (e.g. shear vane, torvane and Cohesive Strength Meter; CSM), but what these different methods are measuring, and therefore the differences that result from using different methods, are often poorly specified – or even ignored.While shear vane and torvane devices have, until recently, been used interchangeably in salt marsh studies, the data collected is highly variable within individual datasets, between sites using the same technique, and between techniques at an individual site. In UK marshes, we find that the shear vane apparatus records a higher undrained shear strength than the torvane on the same substrate, possibly due to greater interaction between roots and the larger blades of the shear vane instrument (compared to the torvane).Our results also show that the vane-based and CSM devices measure two different processes, with vanes quantifying the bulk resistance of the substrate to shear and mass movement, while the CSM records the process of grain-by-grain erosion from the substrate surface. These two distinct measurements were not related, so our research challenges the common assumption that substrates with a higher resistance to bulk failure processes also have a high erosion threshold.