Water Hydraulics - Wastewater Operators - page 103

Solution:

1200gpm
60 s/min7.48 gal/ft

= 2.67 cfs

3

×

q = a × v

2.67 = 2.5 × d × 4

d = 0.27 ft

Q = 2.5 × h 2.5

2.67 = 2.5 × h 2.5

h = 1.03 (with calculator, 1.06 INV y x 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 flow 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 car-
ries excessive solid materials or silt, which can deposit in the approach
channel behind the weir and destroy the conditions required for accu-
rate discharge measurements.

Note: Accurate flow rate measurements with a weir cannot be expected
unless the proper conditions and dimensions are maintained.

2.13.9.2 Flumes

A lume is a specially shaped constricted section in an open chan-
nel (similar to the Venturi tube in a pressure conduit). The special shape
of the flume (see Figure 2.36) restricts the channel area or changes the
channel slope, resulting in an increased velocity and a change in the
level of the liquid flowing through the flume. The flume restricts the
flow and then expands it in a definite fashion. The flow rate through the
flume may be determined by measuring the head on the flume at a single
point, usually at some distance downstream from the inlet.

Flumes can be categorized as belonging to one of three general
families, depending upon the state of flow induced: subcritical, criti-
cal, or supercritical. Typically, flumes that induce a critical or super-
critical state of flow are most commonly used because, when critical
or supercritical flow occurs in a channel, one head measurement can
indicate the discharge rate if it is made far enough upstream so the
flow depth is not affected by the drawdown of the water surface as it
achieves or passes through a critical state of flow. For critical or super-
critical states of flow, a definitive head-discharge relationship can be
established and measured, based on a single head reading. Thus, most
commonly encountered flumes are designed to pass the flow from sub-
critical through critical or near the point of measurement.