Environmental Engineering Reference
In-Depth Information
under consideration. In unsteady flow the variables (magnitude and direc-
tion of the velocity, pressure, flow path, etc.) vary with time at the spatial
points in the flow. Examples are surge waves in irrigation canals after a
sudden opening or closing of a gate. In many open-channel problems it is
sufficient to study the flow behaviour under steady conditions. However,
when the change in flow condition with respect to time is of major con-
cern, the flow should be treated as unsteady. In surges, for instance, the
water level will change instantaneously as the waves pass by, and the time
element becomes vitally important in the design of control structures.
In uniform flow the cross section (shape, side slope and area) through
which water flows remains constant in the flow direction, the flow depth
is the same in every section. In addition, the velocity does not change
in magnitude and direction with distance. A uniform flow may be steady
or unsteady, depending on whether or not the depth changes with time
or during the time interval under consideration (see Figure 2.2). Steady
uniform flow is the fundamental flow in open channel hydraulics. The
establishment of unsteady, uniform flow would require that the water
surface fluctuates from time to time while remaining parallel to the channel
bottom. Obviously, this is a practically impossible condition.
Change from
time to time
Constant depth
Figure 2.2. Steady and
unsteady, uniform flow.
Uniform flow
Unsteady uniform flow
In varied flow, the cross section (shape, side slope and area) changes in
the flow direction, the flow depth changes along the channel. Varied flow
may be either steady or unsteady. Since unsteady uniform flow is rare, the
term 'unsteady flow' is used exclusively for unsteady varied flow. Varied
flow may be further classified as either rapidly or gradually varied. The
flow varies rapidly if the depth changes abruptly over a relatively short
distance; otherwise, it varies gradually. Figure 2.3 shows an example of
unsteady, gradually and rapidly varied flow (G.V.F. and R.V.F.).
G.V.F.
R.V.F.
Figure 2.3. Unsteady, gradually
and rapidly varied flow.
Unsteady flow
 
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