Page 251 - Handbook of Gold Exploration and Evaluation

222 Handbook of gold exploration and evaluation

Channel velocities vary throughout the water body due to the interrelationship of
such factors as the presence of a free stream surface, sediment loadings, channel
geometry, and friction along the channel bed and walls. The maximum velocity
appears to occur below the free surface at a distance of about 0.05 to 0.50 of its
depth, the closer the banks the deeper the maximum (Chow, 1959).

Velocity distribution
The actual velocity distribution depends in each case upon the configuration of
the channel bed and the shape of the channel upstream of the velocity measure-
ment site. Examples of the effects of changing sectional shape upon the
distribution of velocity are given in Fig. 4.16. This figure shows a series of
velocity curves constructed from measurements of velocity taken at various
stages of flow and discharges in Behana Creek, at Aloomba, North Queensland,
Australia.
An infinite variety of flow rates and sediment loadings impose constantly
changing physical constraints on streams flowing through locally different
geological structures. Progressive increases in discharge and decreases in the
size of particles in transport are normal responses to downstream changes in
width, depth and roughness. Shape differences depend largely upon the extent to
which flow is retarded by frictional resistance along its boundaries. Roughness
has a marked affect on energy losses due to friction. If the sediments are fine
grained and the banks are more resistant to erosion than the bed, the cross-
section will be narrow and deep. If the sediments are coarse, armouring of the
bed will protect it against erosion and the stream section will tend towards
lateral expansion and shoaling.

Grading concept
Theoretically, if flow conditions remain unchanged through time to a final state
of equilibrium the longitudinal profile will achieve a smooth concave upward
curve. In practice, grading is a complicated process and many factors intervene
to prevent this happening and compromises are sought. In 1948, Machin defined
a graded stream as one in which, over a period of years; channel form and slope
are delicately adjusted to provide with available discharge, just the velocity
required to transport the incoming load. He classified the Shoshone River,
Wyoming with an average gradient of 10 m/km as graded, the Illinois River with
a gradient of less than 1 m/km, ungraded. The distinction was made on the basis
of differences in the nature of the materials in transport. The Shoshone River
must handle material up to 25 mm to 250 mm diameter; bed-load in the Illinois
River is mainly silt and clay.
Modern views of grading hold that any stream may be called graded if, by
adjusting its geometry to achieve an average state of operation, it achieves a