Page 253 - Handbook of Gold Exploration and Evaluation

224 Handbook of gold exploration and evaluation

rapidly to any significant change in conditions of flow. Flow conditions vary
with every change in level of discharge and in the nature of the sediments in
transport. Flow regime thus adapts both to changes in channel form and velocity,
and friction on the bed surface and channel walls; and variations in sediment
grain size, sediment loading, river stage, stream type and disturbances caused by
depositional units such as alluvial bars and lag gravels. Given an appropriate
channel depth, most fluvial land-forms are developed in the low flow regime
following active sediment erosion and transport at high river stage

Time rate of energy expenditure
The concept of the time rate of energy expenditure is fundamental to all studies
of sediment transport and settling. Stream power is directly associated with flow
conditions and for every change in flow conditions, there is a generally
predictable reaction:

· Discharge variations result in scour during flooding and aggradation at low
water.
· Narrowing of the channel decreases the transport capacity of the stream and
promotes scouring.
· Changing the depth of a channel, without modification of either discharge or
width, requires a change in shape.
· Channel shortening increases the slope and transport capacity of a channel,
thus leading to local scour.
· Increasing channel roughness requires either the depth or slope to increase, or
both.
· Local changes in the nature of the bed material lead to local changes in
transport capacity.
· Selective sorting along one stream section eventually stabilises the channel in
that locality; however the flow will then be loaded below its capacity when it
enters into a zone of finer material, thereby causing scour.

The `law of least time rate of energy expenditure' states that during its evolution
towards an equilibrium condition, a natural stream chooses its course of flow in
such a manner that the time rate of energy expenditure per unit mass of water
along this course is a minimum. One consequence of the law is a requirement for
the channel slope to decrease in the downslope direction so that the time rate of
energy expenditure per unit weight of water is zero where the stream reaches its
ultimate base level. This requirement explains why the longitudinal profile is
usually concave (Stall and Yang, 1972). In applying the principle of stream
power to predicting channel forms, it may be concluded that the tendency to
minimise the expenditure of stream energy under the constraints of discharge
and sediment load imposed by conditions in the drainage basin determines the
river form.