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Head over a Rectangular Sharp Weir

Easyflood controlweirspillway
An emergency spillway discharges over a rectangular sharp-crested weir with a crest length of 6 m and a weir coefficient of 1.84 (SI units). For a design discharge of 15 m3/s, the head over the crest is most nearly:
  1. A1.2 m
  2. B1.4 m
  3. C2.0 m
  4. D4.1 m
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Answer: A. 1.2 m

For a rectangular sharp-crested weir, Q = Cw L H^(3/2). Solving for the head: H = (Q/(Cw L))^(2/3) = (15/(1.84×6))^(2/3) = (1.359)^(2/3) ≈ 1.2 m. Treating the relationship as linear in H gives 1.4 m, and omitting the crest length entirely gives about 4.1 m.
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