From c of Figure 3.5, at d/D = 75/150 = 0.5, K = 1.02; therefore,
2(9.81)(26000) nmo 3. nmi 3, = 0.032 m /s ~ 0.031 m /s
Therefore, design values: approach diameter = 150 mm, throat diameter = 75 mm,
pressure differential = 26 kN/m Ans
Figure 3.6 shows the plan and elevation of a Parshall flume. As indicated, the flow enters the flume through a converging zone, then passes through the throat, and out into the diverging zone. For the flume to be a measuring flume, the depth somewhere at the throat must be critical. The converging and the subsequent diverging as well the downward sloping of the throat make this happen. The invert at the entrance to the flume is sloped upward at 1 vertical to 4 horizontal or 25%. Parshall flumes measure the rate of flow proportional to the cross-sectional area of flow. Thus, they are area meters. They also present obstruction to the flow by making the constriction at the throat; thus, they are intrusive meters.
As defined by Chow (1959), the letter designations for the dimensions are described as follows:
W = size of flume (in terms of throat width) A = length of side wall of converging section 2/3A = distance back from end of crest to gage point B = axial length of converging section C = width of downstream end of flume D = width of upstream end of flume E = depth of flume F = length of throat G = length of diverging section
K = difference in elevation between lower end of flume and crest of floor level = 3 in. M = length of approach floor
N = depth of depression in throat below crest at level floor P = width between ends of curve wing walls R = radius of curved wing walls
X = horizontal distance to Hb gage point from low point in throat Y = vertical distance to Hb gage point from low point in throat.
The standard dimensions of the Parshall flume are shown in Table 3.1. If the steps used in deriving the equation for rectangular weirs are applied to the
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