Estuarine, Coastal and Shelf Science (1999) 49, 281–294 Article No. ecss.1999.0512, available online at http://www.idealibrary.com on Measuring the in situ Erosion Shear Stress of Intertidal Sediments with the Cohesive Strength Meter (CSM) T. J. Tolhurst a , K. S. Black a , S. A. Shayler b , S. Mather a , I. Black c , K. Baker d and D. M. Paterson a a Gatty Marine Laboratory, The University, St Andrews, Fife KY16 8LX, U.K. b British Oceanographic Data Centre, CCMS, Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, CH43 7RA, U.K. c 26 Park Avenue, Shoreham-by-Sea, West Sussex BN43 6PH, U.K. d Racal, The Sussex Innovation Centre, Sussex University, East Sussex BN1 9SB, U.K. Received 20 November 1998 and accepted in revised form 27 April 1999 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 ( oCr ) 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 Ko ¨ nigshafen (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.  1999 Academic Press Introduction The erosion of fine-grained cohesive sediments (mud) is a ubiquitous phenomenon in marine and estuarine environments. Sediment erosion occurs once a critical value of shear stress (denoted  oCr ) exerted by moving fluids is exceeded. The critical erosion shear stress is an important parameter in sediment transport mech- anics. Below this value little or no erosion occurs, whereas once exceeded significant erosion occurs (Teisson et al., 1993). It is also an important free variable in numerical models of cohesive sediment transport, as it parameterizes the bottom boundary condition (Dyke, 1996). In contrast to non-cohesive sediments (sands), it is presently not possible to predict the critical erosion stress of cohesive sediments from one or more easily measurable parameters, such as grain size, bulk density, or water or organic content (Dade et al., 1992). The factors contributing to the shear strength of cohesive sediments are numerous, interact in a complex manner and remain poorly understood (Amos, 1995; Paterson, 1997). Further- more, the visco-elastic properties of estuarine (and fully marine) mud, means that realistic values for critical erosion shear stress can only be obtained by in situ measurement programs. In situ methods of measuring critical erosion shear stress ( oCr ) Black and Paterson (1997) recently reviewed the instrumentation that has been developed to date to measure the critical erosion shear stress directly on undisturbed marine and estuarine mud. Of the 21 devices, 13 were designed for subaerial deployment on estuarine, intertidal flat environments and most of these are benthic flumes. The flumes are inverted channels of varied geometrical configurations placed on submerged or subaerially exposed mud and filled with (sea-) water that is then recirculated until sedi- ment erosion is induced. Both direct and remote measurements of sediment constituents in the water column are used to determine the critical erosion threshold (Wiltshire et al., 1998). Instruments that induce sediment erosion of the bed by other means have also been devised. These include the Particle Erosion Simulator device of Tsai and Lick (1986), 0272–7714/99/080281+14 $30.00/0  1999 Academic Press