Open Channel Flow - Sediment Transport in Irrigation Canals: A New Approach - page 12

A rapidly varied flow is also known as a local phenomenon; examples
are the hydraulic jump and the hydraulic drop. Spatially constant flow
occurs when the average velocity is the same in all points; when the
velocity changes along or across the flow, the flow is spatially variable.
A clear example of a spatially varied flow is the flow through a gradual
contraction or in a canal with a constant slope receiving inflow over the full
length. Spatially varied flow shows some inflow and/or outflow along the
reach under consideration; the continuity equation should be adapted to
this situation. Examples are side channel spillways, main drainage canals
and quaternary canals in irrigation systems. Figure 2.4 gives some typical
locations in canals and rivers where gradually and rapidly varied flow
might occur.

R.V.F. G.V.F. R.V.F. G.V.F. R.V.F. G.V.F. R.V.F.

Flow over
a weir

Sluice Hydraulic

jump

Hydraulic drop

Contraction

Varried flow

Figure 2.4. Examples of
gradually and rapidly varied
flow (steady conditions).

R.V.F. = Gradually varied flow
G.V.F. = Rapidly varied flow

Another flow classification is based on the dimensionless numbers
that describe the relative influence of either the force due to viscosity or
inertia in relation to the gravitational force. In hydraulics, the force due
to viscosity is described as the resistance of a fluid to flow motion. The
resistance acts against the motion of fluid when it passes fixed boundaries
(e.g. the canal bottom or walls), but it also acts internally between slower
and faster moving adjacent layers. Viscosity is the internal fluid friction
that enables the acceleration of one layer relative to the other; it resists
the motion of a layer but also makes it possible to accelerate a layer.
Viscosity is the principal means by which energy is dissipated in fluid
motion, typically as heat.
The difference in velocity between adjacent layers is known as a vel-
ocity gradient. To move one layer at a greater velocity than the adjacent
layer, a force is necessary; resulting in a shear stress τ . Newton mentioned
that for straight, parallel and uniform flow, the shear stress τ between