The location of the head measurement section is at 3 4 times H max upstream

The location of the head measurement section is at 3

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The location of the head measurement section is at 3-4 times H max upstream from the weir block. 7.7.2.3 Flumes A flume is a flow measurement device, which is formed by a constriction in a channel. The constriction is a narrowing in the channel mostly combined with a hump.
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7.7 S T RU C T U R E S 123 Flumes are in particular recommended in streams where siltation of a weir is ex- pected to be a mayor problem. However, it should be realized that the construction is complex (and costly) and that attention must be given to the appropriate rating. It is essential to construct the flumes to the dimensions as specified in the stan- dard designs. For the same reason the heads should be measured at the positions specified. A typical design of a flume is provided in Figure 7.39, which has rectangular cross-sections. There are variations to this design with different cross-sectional shapes such as U-shaped trapezoidal shaped cross-sections. Figure 7.39: Typical design of a flume An example of a short throated flume is the Parshall flume. It has a rectangular cross-section and comprises three main parts; a converging inlet section with a level floor, a throat section with a downward sloping floor and a diverging outlet section with an upward sloping floor (see Fig. 7.40). The control section of the flume is not located in the throat as on the previous flumes mentioned but near the end of the level floor, or crest in the converging section. Flumes are supposed to operate under free flow conditions, i.e. the flow is unaf- fected by the downstream water level and only the upstream level ( h 1 ) needs to be measured. If the downstream water level h b exceeds a certain level, free flow con- ditions no longer exist and the flume is said to be submerged. The level that must not be exceeded depends on the size of the flume, and is for 1ft to 8ft (0.30m to 2.4m) flumes 0.7 h 1 and for 10ft to 50ft (3.0m to 15.2m) flumes 0.8 h 1 . If the flume becomes submerged, h b must be measured and the flow adjusted accordingly.
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124 S T R E A M F L OW M E A S U R E M E N T S In case of high risk of submergence a tapping for measuring the downstream wa- ter level is used. Different sizes of Parshall flumes, related to the design discharges to be measured, can be applied. Their typical dimensions and discharge formulas have been well investigated (see Tab. 7.2 and 7.3. A Parshall flume is typically identified by the width of the throat. One will talk of an 8ft Parshall flume. The discharge equation of a Parshall flume is: Q = Kh n 1 (7.78) where K and h are derived from the table to the applicable flume. Figure 7.40: Diagrammatic illustration of the Parshall flumes At some gauging stations there is a combination of flumes and sharp-crested weirs. The head is then not measured at the prescribed point in the flume, but upstream of the flume. Experiments at some flumes have indicated that the relation between the head in the flume and the upstream head is ± 0.83. When the upstream pool level is measured 0.83 is therefore used as a conversion factor.
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