Page 238 - Handbook of Gold Exploration and Evaluation
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Sedimentation and detrital gold 209
P W
hA=A
h 4.8
The weight of the water column is
hA, where
is the weight density of the
water. At sea level with air as the fluid, P becomes P A the atmospheric pressure
of an element of matter at sea level. The total pressure at the plane is then:
P P W P A
h P A 4.9
Pressure is one component of potential energy and as already noted a gradient of
gravitational potential energy is needed between two points for flow to take
place. The direction of the movement is in the direction of the lowest pressure.
The rate of motion is proportional to the spatial rate of change (gradient) of
potential energy between the two points.
Mechanical potential energy
As applied to natural flow conditions, an element of water moving from rest in
the headwaters of a stream contains mostly potential energy, i.e., the product of
its density , acceleration due to gravity g and its elevation above sea level z. At
any point downstream, losses of potential energy with decreasing z are
compensated for by gains in kinetic energy. Some of this energy is expended in
eddying, turbulence and changes in momentum, particularly at the foot of rapids
and waterfalls. A further interchange of energy takes place when the stream
changes direction. Flowing around a bend, the velocity increases on the outside
and is retarded along the inside of the bend. Elements of water in a horizontal
line across the bend thus have equal quantities of potential energy and total
energy but different quantities of kinetic energy and pressure energy. At sea
level, any remaining energy is dissipated in turbulence and intermolecular
friction.
4.2.4 Forces acting on fluids
Forces acting on fluids and on solids and fluids in relative motion react differ-
ently according to differences in the physical characteristics of the solids and the
relative magnitude and direction of hydraulic forces, which act both to induce
and resist movement thereby determining the nature of the flow. The net rate of
entrainment, transport and deposition of bed-load materials depend upon the
degree of balance between the individual phases of such exchange. The forces
involved in the reactions are termed either `body' forces or `surface' forces.
Body forces
Body forces act from a distance upon the whole bulk of a fluid element or solid
immersed in a fluid. Typical body forces comprise:
