The program
Special care must be taken to secure proper aeration beneath
the overflowing sheet at the crest. Aeration is usually accomplished by placing vents
on both sides of the weir box under the nappe.
Other conditions for accuracy of measurement for this type of weir
are generally the same as for those of the contracted rectangular weir,
except for the absence of side contraction. However,
the crest height 3.
HFig. 1 Definition sketch for a standard suppressed rectangular weir. The formula for the standard suppressed rectangular weir is the Francis equation. In U.S. Customary units, this equation is:
in which
The accuracy of the discharge coefficient is
Equation 1 has a constant discharge coefficient (3.33), which facilitates
computations. However, the coefficient does not
remain constant for a ratio of head-to-crest = 10 ft
and heads in the range 0.4 ft ≤ L ≤ 1.6 ft.
Thus, the equation applies
particularly to such conditions.
USBR experiments on 6-in, 1-ft, and 2-ft weirs
has shown that the Francis equation also applies fairly well to shorter crest lengths
H,
provided L ≤ 1/3.
H/L
No extensive tests have been made to determine the applicability of Eq. 1
to weirs with < 0.2 ft.
These small heads do not give accurate flow measurements
because the nappe of water going over the crest may not spring free of
the crest.
HEquation 1 is subject to the following restrictions: Head ≥ 0.2 ft.*H*Length ≥ 4 ft.*L*Height-to-head ratio ≥ 3.*P/H*Length-to-head ratio: ≥ 3.*L/H*
The head 4
from the weir. The sidewalls must extend a distance of at least H0.3 downstream from
the crest.
The overflow jet must be adequately ventilated to the atmosphere.
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