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. Measurement s of the natural erosion shear stress are important in modelling estuarine systems and in determining the possible impact of human disturbance. A seco nd-generation erosion instrument the Cohesive Strength Meter (CSM) designed to measure the critical erosion shear stress tau(oCr) of intertidal sedime nts in situ is described. The CSM allows measurements of small-scale spatia l and temporal variation of sediment stability to be made on a scale and ra pidity previously unachievable. New features of specialized hardware are de scribed and the instrument is empirically calibrated in terms of an equival ent horizontal bed shear stress using quartz sand. Some example results fro m recent field trials on the Konigshafen (Sylt-Romo Bight) are presented. T he erosion characteristics of this intertidal flat varied on a cm scale due to the presence of patchy diatom biofilms. Areas with a diatom biofilm wer e more stable than those without, giving a biostabilization index of 6.2. ( C) 1999 Academic Press.