W

FIGURE 3.7 Discharge coefficient for the Parshall flume.

Example 3.7 (a) Design a Parshall flume to measure a rate of flow for a maximum of 30 cfs. (b) If the invert of the incoming sewer is set at elevation 100 ft, at what elevation should the invert of the outgoing sewer be set?

Solution: As mentioned previously, in design problems, the designer is at liberty to make any assumption provided she or he can justify it. This means, that two people designing the same unit may not have the same results; however, they must show that their respective designs will work for the purpose intended.

(a) From Table 3.1, for a throat width of 1 ft to 8 ft, the depth E is equal to 3 ft. Thus, allowing a freeboard of 0.5 ft, Ha = 3.0 - 0.5 = 2.5 ft. Also, for a throat width of 9 in. = 0.75 ft, the depth E = 2.5 ft, giving Ha = 2 ft for a freeboard of 0.5 ft. From Figure 3.7, the following values of K are obtained for various sizes of throat width, W:

W (ft)

W (m)

E (ft)

Ha (ft)

Ha (m)

Ha/W

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