Open-channel/Weir flow – discharge over a rectangular sharp-crested weir with velocity of approach Given H1 = H + Ha, where H is the head over the crest and Ha is the velocity-of-approach head, select the correct expression for discharge Q over a rectangular sharp-crested weir (Cd is coefficient of discharge, L is effective crest length).
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AQ = Cd · L · √(2g) · (H1 − Ha)
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BQ = Cd · L · √(2g) · (H1^(3/2) − Ha^(3/2))
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CQ = Cd · L · √(2g) · (H1^2 − Ha^2)
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DQ = Cd · L · √(2g) · (H1^(5/2) − Ha^(5/2))
Answer
Correct Answer: Q = Cd · L · √(2g) · (H1^(3/2) − Ha^(3/2))
Explanation
Introduction / Context:Discharge over a sharp-crested rectangular weir depends on the effective head causing flow. When approach velocity is non-negligible, an additional head term appears. This question checks recognition of the correct functional dependence on head.
Given Data / Assumptions:
- Rectangular sharp-crested weir with coefficient Cd.
- H is static head over crest; Ha is velocity-of-approach head.
- H1 = H + Ha represents total effective head at approach section.
- Standard, free, fully aerated nappe; neglect end contractions beyond effective L.
Concept / Approach:For a sharp-crested weir, the theoretical discharge is proportional to the integral of velocity across depth, yielding the 3/2 power law in head. Accounting for velocity of approach, the effective head terms enter as H1^(3/2) − Ha^(3/2).
Step-by-Step Solution:
Base relation (without approach velocity): Q = (2/3) Cd L √(2g) H^(3/2).With approach velocity: replace H by H1 and subtract the contribution of Ha to avoid double counting.Thus, Q = (2/3) Cd L √(2g) (H1^(3/2) − Ha^(3/2)).The constant 2/3 is often absorbed into empirical Cd or presented explicitly; options use the compact √(2g) form with the 3/2 exponents.Verification / Alternative check:Dimensional analysis confirms Q has units of L^3/T when head is to the 3/2 power multiplied by L √(2g). Linear or quadratic head dependences (options a and c) do not match the classical derivation.
Why Other Options Are Wrong:
- (a) Linear in head: incorrect functional form.
- (c) Quadratic in head: inconsistent with weir theory.
- (d) 5/2 power: belongs to broad-crested or other profiles under different assumptions, not the sharp-crested rectangular case with this correction.
Common Pitfalls:Forgetting to subtract Ha^(3/2), which leads to overestimation of Q when approach velocity is significant.
Final Answer:Q = Cd · L · √(2g) · (H1^(3/2) − Ha^(3/2))