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Sedimentation and detrital gold 225

4.4 Entrainment, transport and sorting

Weathering on slopes marks the beginning of mobilisation of sediments by the
various agents of erosion (glaciers, rainwater and wind). Spoil gouged out of the
valley floor and walls by glacial erosion (plucking and abrasion) may be
transported for considerable distances before being dumped at the foot of the
glacier. Rainwater disturbs surface particles by impact when it strikes the ground
and continued precipitation leads to sheet flow and rivulets, which wash the
lighter particles away. Water seeping downwards along seepage planes fills the
voids between particles and provides lubrication for the mass to move as a
whole. Wind sweeping over the ground entrains small waste particles, which are
carried away from the surface by deflation processes involving traction, saltation
and for dust-sized particles, suspension.

4.4.1 Mechanism of entrainment

Mathematical explanations of entrainment conditions at the threshold of
movement are typically restricted to analytical expressions of the resultant
force acting on a spherical particle free to move on a horizontal planar bed. A
number of general models related to the transportation of sediment in gravity
treatment plant are discussed in Chapter 8, particularly those modified from
empirical relationships proposed by Bagnold (1966) for conditions of steady,
uniform, simple shear flow of neutrally buoyant spherical particles. Application
of this approach to bed-load movement as a whole however, necessitates
describing various aspects of the phenomenon by certain functions of unknown
form. In 1936, the approach by Shields was to make certain gross assumptions
and then to confirm and supplement the analysis experimentally. His analysis of
the entrainment function (Fig. 4.17) foreshadowed other works including Rigby
and Hamblin (1972). Variation in character between sandy (cohesionless) and
clayey (cohesive) material is due to properties such as structure and chemical
composition.

Cohesive sediment

The most common `cohesive' materials are beds composed of very fine par-
ticles. Particles smaller than 0.06 mm diameter (e.g., clays and fine silts) have
very large surface area/mass ratios and are bonded together mechanically or by
electrostatic attraction or by both. Chemical reactions occur mostly at the
surface of contact between water and mineral particles, particularly in the
presence of colloidal organic materials. In clayey soils the process of cation
exchange of mineral elements between colloid molecules and the soil solution
rearranges the molecules so that positively charged and negatively charged ions
will ultimately be attracted to and held onto their surfaces. The very large
surface area of clay particles provides a great water-holding capacity.