Factors controlling critical shears for deposition and erosion of individual grains

Self, R.F.L., Nowell, A.R.M. and Jumars, P.A., 1989. Factors controlling critical shears for deposition and erosion of individual grains. Mar. Geol., 86: 181-199. We have quantified critical deposition and erosion shear stresses for individual, fine (24-141 µm), natural quartz particles in a flow chamber with radial parallel flow. In seawater (30%, 10-15`C), critical erosion shear stresses, as expected, are slightly larger than estimates from previous studies which strove for cohesionless conditions, while critical deposition shear stresses are three orders of magnitude larger than predicted by Hjulstrom (1935) and equal to the stress on a particle settling at Stokes' ' terminal fall velocity. Manipulative experiments indicate that critical deposition shear stress of medium to coarse silt is little affected by physical- chemical adhesion, bed roughness or microbial adhesion, while critical erosion shear stress is at least doubled by microbial adhesion or bed roughness Critical stresses for individual grains of comparable size are normally distributed, supporting the concept of well-behaved thresholds for both erosion and deposition. Because the thresholds do not differ greatly in magnitude for particles of a given size, organism-produced and-maintained features (e.g., pits and tubes) can be important in shifting local conditions from erosive to depositional and vice versa