Page 257 - Handbook of Gold Exploration and Evaluation

228 Handbook of gold exploration and evaluation

explored in the previous section. Predictions of sediment transport rates, and
prediction of bed form and thus of flow resistance are two additional aspects of
sediment transport of particular interest to placer engineers and geologists.

Bed-load

The bed-load is transported by both traction and saltation under conditions of
shear flow (refer to Chapter 8). Particles in traction move by sliding and rolling
in the direction of flow without losing contact with the bed. `Saltation', from the
Latin `saltare' (to jump), describes the motion of any particle that is too heavy to
remain in suspension in prevailing stream conditions but by virtue of its size and
shape, may be re-entrained as soon as it reaches the bed. Movement takes place
in a zone of viscous shearing within which particle concentration and hence the
apparent density of the fluid determines the collision conditions between
individual particles. Large particles protruding from the bed are subject to
greater lift and drag forces than particles with smaller cross-sections.
Differences in the gross character of flow over rough and smooth surfaces
provide plausible explanations for differences in the nature of flow zones. They
do not, however, explain in quantitative terms how the free settling of grains is
modified by fluid turbulence in a polydispersed concentration of grains. Slinger-
land and Smith (1986) note that predictions cannot be made of actual transport
rates or of the ultimate fates of size density fractions in a mixture undergoing
unsteady, non-uniform flow and therefore under aggrading or degrading bed
conditions at the current state of knowledge. Instead, they visualise the gross
character of the flow by considering a vertical cross-section in the downstream
direction with a planar channel bottom (Fig. 4.19). A straight channel segment is
assumed for this exercise in which neither depth nor average velocity changes
occur downstream thus providing a state of steady uniform flow. The frictional
resistance of the boundary balances the force exerted by the gravitational flow
on the channel bottom and sides because the flow rate is constant. The temporal
mean tractive or boundary shear stress o is given by:
o ˆ f gRS 4.19

where o ˆ is the fluid density, R the hydraulic radius and S the slope of the
streambed and water surface, both of which are equal in uniform flow. For
natural stream channels where width greatly exceed the depth, if follows that
R J (the flow depth) and thus that:

o ˆ f gJS 4.20
Grass (1983) suggests if a coefficient of variation equal to 0.4 is adapted for
the instantaneous shear stress at the bed, the grains could experience shear
stresses at least twice as great as the temporal mean given in eqns 4.16 and 4.17.