Measuring the shear strength of cohesive sediment in the field Robert C. Grabowski 1,2 1 School of Geography, Queen Mary, University of London, UK (r.c.grabowski@gmail.com ) 2 Cranfield Water Science Institute, Cranfield University, UK ABSTRACT: Shear stress is a fundamental driver of geomorphic change and is at the centre of a range of geomorphological processes including the deformation of glacier tills, mass movements on hillslopes, riverbank erosion and the stability of intertidal sediment. Consequently, measurements of a sediment’s resistance to shear stress, i.e. shear strength, is essential in our field. Numerous techniques have been developed to quantify both surface and internal shear strength for soils and sediments, but most are limited to the laboratory. Measurements of shear strength, particularly for cohesive sediment, are best conducted in the field in the first instance, because the extraction, transport and storage of cores prior to analysis in the laboratory cause physical, chemical, and biological changes to the sediment characteristics that alter its shear strength. The handheld shear vane and the cohesive strength meter (CSM) are portable devices that can be used to measure internal and surface shear strength, respectively, in the field. The shear vane quantifies the undrained geotechnical shear strength of the sediment, i.e. resistance to deformation / fracture, and the CSM measures resistance to surface erosion by water. These tools can be combined with geomorphological mapping, stratigraphy, and sedimentological and biological analyses to support a range of investigations into geomorphological forms and processes. KEYWORDS: shear vane, erosion threshold, erodibility, jet test, CSM Introduction Sediment is continually subjected to physical stress in the environment, whether from the crushing weight of a glacier, the scouring flows of water, or the relentless pull of gravity. Forces push, pull and manipulate sediment. When these forces are imposed parallel to the sediment surface, the stress generated is termed a shear stress (force per m 2 ; Pascal, Pa) (Figure 1). Shear stress is of paramount importance to geomorphologists because it is a driver of geomorphic change. The term ‘shear stress’ is used in various ways in earth science, and a clarification is needed at this early point in the chapter (for more information, see Fookes et al. 2007, Ch. 3). Shear stress can describe forces applied parallel to the sediment surface (e.g. surface drag by flowing water) that result in a surface response (e.g. entrainment of surficial sediment) (Figure 1b). It can also be used to describe forces applied at the sediment surface (e.g. a moving glacier), but which are transmitted into the bed to exert internal shear stress that results in sediment deformation or fracture (Figure 1c). Finally, shear stress can be used to describe a force that acts upon the whole sediment bed (e.g. gravity) and which induces a shear stress that can result in sediment deformation or mobilisation (e.g. hillslope mass movement) (Figure 1d). In this chapter, I take an inclusive view of shear stress. Geomorphic processes in nature often involve a combination of stresses. A flowing glacier not only exerts a shear stress on the sediment bed, but the glacier’s weight also imposes an ISSN 2047-0371 British Society for Geomorphology Geomorphological Techniques, Part 1, Sec. 3.1 (2014)