Page 184 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Grit Chambers 139

0.70 in bulletins of the CSU Agricultural Experiment Station and the
U.S. Department of Agriculture. The report of Skogerboe et al.
0.60 (1967) was the first systematic calibration of the Parshall flume
coefficients since the original work of Parshall in the late 1920s
and the early 1930s and was used as the primary reference for
0.50
this section.
Water level tie: Figure 7.7 is a perspective drawing showing a
0.40
Q (m 3 /s) grit chamber and a Parshall flume together as a ‘‘system.’’ As
indicated, the water level at the grit chamber exit and the
0.30
flume inlet are at the same elevation. To achieve such a
water level ‘‘tie,’’ the relationship between the depth of
0.20 water in the grit chamber, d, and the depth in the flume, H a ,
is, d ¼ H a þ DZ, in which DZ is the difference between the
0.10 two depths. The elevation of the floor of the flume is ‘‘set’’ by
this relationship. The continuity between the water levels
defines the ‘‘hydraulic profile.’’
0.00
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1 1.2 7.2.2.2.1 Free Flow and Submerged Flow
0 1
h (m)
If the flow exceeds, what is called the ‘‘free-flow’’ condition, a
‘‘submerged-flow’’ condition exists. The ‘‘free-flow’’ condi-
FIGURE 7.5 Flow vs. depth, proportional.
tion means that a single measurement, H a (which is upstream
of the ‘‘throat’’), is sufficient to measure flow. If the flow is
‘‘submerged,’’ an additional measurement, H b (which is
downstream of the ‘‘throat’’), is necessary. The water level
downstream from the flume box should be low enough such
that a backup does not occur, that is, resulting in ‘‘submerged-
flow’’ condition. The ‘‘throat’’ is the narrow section of the
flume and is defined in terms of its width dimension.
Free flow: For the ‘‘free-flow’’ condition, which means that
‘‘super-critical’’ velocity occurs in the throat section, the
expression for flow is in terms of H a ,

Q ¼ CH n (7:6)
a
where
3
Q is the flow through a given Parshall flume (m =s)
C is the coefficient, specific to throat width, taken from
Table CD7.3
H a is the water depth measured at a section two-third of the
length of the entrance section upstream from the start of
FIGURE 7.6 Photograph of Parshall flume at Marcy Gulch the flume ‘‘throat’’ (m)
WWTP, Colorado, c. 2003. (Courtesy of Centennial Water and n is the exponent for flume equation, specific to throat
Sanitation District, Highlands Ranch, CO.) width, taken from Table CD7.3

Grit chamber
L
Transition
Q = v ·wd
H
w Parshall flume
Throat
w t
d
H a
ΔZ
n
Q = CH a
FIGURE 7.7 Parshall flume perspective showing grit chamber.