Measuring the in situ erosion shear stress of intertidal sediments with the Cohesive Strength Meter (CSM)

The shear resistance of muddy sediments directly governs the susceptibility of the sediment to erosion by tidal and wave induced currents. Measurements of the natural erosion shear stress are important in modelling estuarine systems and in determining the possible impact of human disturbance. A second-generation erosion instrument the Cohesive Strength Meter (CSM) designed to measure the critical erosion shear stress (τoCr) of intertidal sediments in situ is described. The CSM allows measurements of small-scale spatial and temporal variation of sediment stability to be made on a scale and rapidity previously unachievable. New features of specialized hardware are described and the instrument is empirically calibrated in terms of an equivalent horizontal bed shear stress using quartz sand. Some example results from recent field trials on the Königshafen (Sylt-Rømø Bight) are presented. The erosion characteristics of this intertidal flat varied on a cm scale due to the presence of patchy diatom biofilms. Areas with a diatom biofilm were more stable than those without, giving a biostabilization index of 6·2.