Page 255 - Handbook of Gold Exploration and Evaluation

226 Handbook of gold exploration and evaluation

4.17 Shield's analysis of entrainment function.

Cohesiveness may also occur when small grains fill the voids between larger
outstanding grains, thus producing a cohesive action, which bonds the grains
together both mechanically and electrostatically. The ensuing reduction in sur-
face roughness results in reduced turbulence at the bed and smooths the surfaces
over which flow takes place. The lifting action of the flow is reduced and
increased velocities are needed for the entrainment of sediment, which is then
torn from the bed in clusters that vary in size according to the local cohesiveness
of the bed materials and the velocity of the flow.

Non-cohesive sediment
Air fills the spaces between purely `cohesionless' particles thus promoting the
ready passage of water through the soil and relative motion between individual
grains. For such deposits the theoretical critical stress can be arrived at
analytically as a function of particle size and weight and the dimensions of
cross-sectional areas exposed to the flow.
Very extensive sediment transport experimentation has since been devoted to
the development of completely general models, which relate the bulk flow rate
of sediment to prevailing hydrodynamic conditions for solids that are generally
assumed to be of quartz density. It is now generally agreed that the theoretical
prediction of critical conditions proceeds from a torque balance on a grain in the
bed (e.g. Everts, 1993; Yalin, 1977; Slingerland, 1977). The sediment is