Page 244 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 215

Reynolds numbers less than 500 signify dominantly viscous (laminar) flow.
When Re is greater than 2,000 viscous forces are insignificant. Reynolds found
that in pipeline flow, flow becomes fully turbulent at Re 12,000 regardless of
pipe diameter and fluid viscosity. On returning to the laminar-state the inertial
effects persisted until Re again drops to 2,000. In practice however, the actual
magnitude of Re varies widely with the boundary geometry in open stream
channels because of the arbitrary nature of the characteristic length L and
inherent differences in the pattern of flow.

Froude number
The effect of gravity on the state of flow is represented by the ratio of inertial
2
forces to gravity forces. This ratio velocity /flow depth x acceleration is defined
as:
2 ÿ2
2
V =dg ˆ L T =L:L Tÿ2 ˆ 0 4.15
2
The dimensionless quantity V =gL is called the Froude number F, where V is the
mean velocity of flow and L is a characteristic length. The Froude number is
computed by depth rather than by hydraulic radius. In open channel flow where
the boundaries are irregular, the mean depth or hydraulic depth represents L, i.e.,
the area of flow normal to the mean velocity divided by the width of the free
surface. Based upon the Froude number F and neglecting other forces, criteria
for flow classification are as follows:
· When F is less than unity, flow is sub-critical (tranquil); wave velocity
exceeds flow velocity so that a wave caused by an obstruction in the flow can
travel upstream.
· When F is greater than unity, flow is super-critical (shooting) and waves
cannot be propagated upstream.
· When F equals unity, the flow is said to be critical; it can be identified by the
celerity of small currents that occur in shallow water in response to
instantaneous changes in the local water depth.

4.3 Drainage systems

The boundary separating weathering from erosion marks the beginning of a
network of channels that provide conduits along which sediments from various
parts of the drainage area can come together in increasingly higher-order streams.
The drainage pattern in headwaters evolves from sheet flow and development of
fingertip channels in which surface run-off is restricted to periodic flow events by
the small size of individual catchments. Seepage from the interfluve is negligible
and stream flow is dependent upon surface run-off during periods of intense
rainfall or thaw. Sub-surface components of the drainage system may only