Science of Water : Concepts and Applications

94 The Science of Water: Concepts and Applications
Flumes can be categorized as belonging to one the three general families, depending on the
state of fl ow induced—subcritical, critical, or supercritical. Typically, fl umes that induce a critical
or supercritical state of fl ow are most commonly used. This is because when critical or supercritical
fl ow occurs in a channel, one head measurement can indicate the discharge rate if it is made far
enough upstream so that the fl ow depth is not affected by the drawdown of the water surface as it
achieves or passes through a critical state of fl ow. For critical or supercritical states of fl ow, a defi nitive
head–discharge relationship can be established and measured, based on a single head reading.
Thus, most commonly encountered fl umes are designed to pass the fl ow from subcritical through
critical or near the point of measurement.
The most common fl ume used for a permanent wastewater fl ow-metering installation is called
the Parshall fl ume, shown in Figure 3.36.
Formulas for fl ow through Parshall fl umes differ, depending on throat width. The formula below
can be used for widths of 1–8 ft, and applies to a medium range of fl ows.
where
Q = fl ow
H a = depth in stilling well upstream
W = width of throat
1.52 0.026
Q4WHaW √ Note: Parshall fl umes are low maintenance items.
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(3.29)
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