Science of Water : Concepts and Applications

Water Hydraulics 93
Solution:
1200gpm
267 . cfs
3
60 smin7.48 gal/ft
Q = AV
2.67 = 2.5d4
d = 0.27 ft
Q = 2.5h 2.5
2.67 = 2.5h 2.5
h = 1.03 (calculator: 1.06 INV y 2.5 = 1.026 or 1.03)
0.27 ft (original depth) + 1.03 (head on weir) = 1.3 ft
It is important to point out that weirs, aside from being operated within their fl ow limits, must
also be operated within the available system head. In addition, the operation of the weir is sensitive
to the approach velocity of the water, often necessitating a stilling basin or pound upstream of the
weir. Weirs are not suitable for water that carries excessive solid materials or silt, which deposit in
the approach channel behind the weir and destroy the conditions required for accurate discharge
measurements.
√ Important Point: Accurate fl ow rate measurements with a weir cannot be expected unless the
proper conditions and dimensions are maintained.
FLUMES
A fl u m e is a specially shaped constricted section in an open channel (similar to the Venturi tube in
a pressure conduit). The special shape of the fl ume (see Figure 3.36) restricts the channel area or
changes the channel slope, resulting in an increased velocity and a change in the level of the liquid
fl owing through the fl ume. The fl ume restricts the fl ow, and then expands it in a defi nite fashion.
The fl ow rate through the fl ume may be determined by measuring the head on the fl ume at a single
point, usually at some distance downstream from the inlet.
FIGURE 3.36 Parshall fl ume.
Converging
inlet
Flow
Stilling well for
measuring hear
Top view
Throat
Diverging
outlet