Environmental Engineering Reference
In-Depth Information
2.12.1.2 Uniform and Varied Flow
Flow can be a function of time and location. If the flow quantity is
invariant, it is said to be steady. Uniform flow is flow in which the depth,
width, and velocity remain constant along a channel; that is, if the flow
cross-section does not depend on the location along the channel, then
the flow is said to be uniform. For varied or nonuniform flow, a change in
one factor produces a change in the others. Most circumstances of open
channel flow in water/wastewater systems involve varied flow. The con-
cept of uniform flow is valuable, however, in that it defines a limit that
the varied flow may be considered to be approaching in many cases.
Note: Uniform channel construction does not ensure uniform flow.
2.12.1.3 Critical Flow
Critical flow (i.e., flow at the critical depth and velocity) defines a
state of flow between two flow regimes. Critical flow coincides with mini-
mum specific energy for a given discharge and maximum discharge for a
given specific energy. Critical flow occurs in flow measurement devices
at or near free discharges and establishes controls in open channel flow.
Critical flow occurs frequently in water/wastewater systems and is very
important in their operation and design.
Note: Critical flow minimizes specific energy and maximizes discharge.
2.12.1.4 Parameters Used in Open Channel Flow
The three primary parameters used in open channel flow are
hydraulic radius, hydraulic depth, and slope ( S ).
2.12.1.4.1 hydraulic radius
The hydraulic radius is the ratio of area in flow to the wetted
perimeter:
a
P
r
h =
(2.23)
where:
r h = hydraulic radius.
a = the cross-sectional area of the water.
P = wetted perimeter.
Why is hydraulic radius important? Good question. Probably the
best way to answer this question is by illustration. Consider, for exam-
ple, that in open channels it is of primary importance to maintain the
proper velocity. This is the case, of course, because if velocity is not
maintained then flow stops (theoretically). In order to maintain velocity
at a constant level, the channel slope must be adequate to overcome fric-
tion losses. As with other flows, calculation of head loss at a given flow
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