Weirs are apertures in the top of a dam, across a channel through which flows the liquid to be measured (Figure 2.31a). The aperture may be rectangular (Figure 2.31b), trapezoidal (Figure 2.31c), or V-notch (Figure 2.31d). The special case of a trapezoidal weir with side slopes of 1:4 (Figure 2.31c) is known as a Cippoletti weir ; this form leads to a simplified flow calculation. V-notch weirs generally have a notch angle from 30 to 90 ° , depending on required flow capacity. The head is measured as the difference in level of the pool at an adequate distance upstream from the weir as compared to the horizontal crest of a rectangular or trapezoidal weir, or the bottom point of the V of a V-notch weir. Heads less than 0.1 ft (30 mm) for minimum measured flow or more than 1.0 ft (300-mm) for maximum flow are generally to be avoided, although a 1.25-ft (380-mm) head can be tolerated under favorable conditions. These limits are easily met by practical design, given that a 30 ° V-notch will measure a minimum flow of 1 GPM (3.8 l/m), whereas the maximum value for a rectangular or trapezoidal weir is limited only by practical crest length. V-notch weirs are used for smaller flows. A 30 ° V-notch weir has a practically constant coefficient from 3.0 to 300 GPM (11.4 to 1140 l/min) with flow proportional to the fivehalves power of the head. Coefficient increases roughly 2% for flow down to 1 GPM (3.8 l/min) and changes relatively little for flow up to 500 GPM (1893 l/min). For notch angle up to 90 ° , flow varies as the tangent of half the notch angle. Notch angle exceeding 90 ° is not recommended. Rectangular or Cippoletti weirs are used for larger flows. A rectangular weir with a crest 2 ft (0.6 m) long develops a head of about 0.2 ft (60 mm) for 250 GPM (946 l/min) and 1.0 ft (305 mm) for 2700 GPM (10,221 l/min). For this weir, flow is directly proportional to crest length and to the threehalves power of the head. The weir plate may be located in a dam in a natural channel or in a weir box (Figure 2.31e). The stilling basin ahead of the weir should be large enough so that the upstream velocity does not exceed 0.33 ft/sec (0.01 m/sec). Width and depth immediately ahead of the weir should be sufficient so that the wall effect of the bottom and sides of the channel has negligible effect on the pattern of flow through the notch. It is important that the flow break clear from the sharp edge of the notch with an air pocket maintained immediately beyond and below the weir plate. The channel downstream from the weir must be sufficiently wide and deep so that, at maximum flow, there is ample clearance between flow through the notch to downstream liquid level so that this air pocket is maintained (Figure 2.31a). The upstream edge of the weir should be sharp and straight. It is usual practice to bevel the downstream edge of the weir at 45 ° to about a 1/32-in. (0.8-mm) edge. For rectangular and Cippoletti weirs, the crest must be carefully leveled. Accuracy of the relation between flow and head (level) to ± 2% is attainable, based on the dimensions of the primary device. Reference 1 gives full data on installation and operation of weirs. The following equations establish the relationships between flow and measured head, provided that the installation and operation of the weir are as recommended in this section and also in the cited references. For a V-notch weir THE PARSHALL FLUME Developed by R.L. Parshall at the Colorado Experiment Station of the Colorado Agricultural College, in cooperation with the Division of Irrigation of the U.S. Department of Agriculture, 2 this device is a special type of venturi flume (Figure 2.31f). The loss of head is about one-quarter of that for a weir of equal capacity. Compared to weirs, approach velocity effects are practically eliminated so that a large upstream stilling basin is not required. The relatively high velocities in the system tend to flush away deposits of silt and other solids that might accumulate and alter measurement. There are no sharp edges, no pockets, and few critical dimensions; also, the device can belocally fabricated from available materials.

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