SETTLING VELOCITIES AND ENTRAINMENT THRESHOLDS OF BIOGENIC SANDS (SHELL FRAGMENTS) UNDER UNIDIRECTIONAL FLOW

Abstract

Settling velocities and entrainment thresholds of biogenic sedimentary particles, under unidirectional flow conditions, are derived on the basis of settling tower and laboratory flume experiments. Material consisting predominantly of equant blocks (shell fragments of Cerastoderma edule, density, ρ s =2800 kg m-3 ) or of mica-like flakes and elongate rods (Mytilus edulis fragments, ρ s =2720 kg m-3 ) are used in separate series of experiments. Differences in the measured settling and threshold properties are related primarily to particle shape. The selection of a characteristic length scale for non-spherical grains is investigated by comparing two approaches used to define the grain size (D ) of the sediment samples: grain settling and sieve analysis that are used to derive data for the threshold criteria, in terms of the Shields and Movability diagrams. The empirical curves effectively predict the threshold conditions for any grain shape, provided that grain size is defined in terms of grain settling velocity. However, a functional distinction is made between the characteristic `hydraulic' grain size, defined by grain settling for grain transport applications, and the actual (physical) grain size defined by sieve analysis.