Question

# The discharge over a broad crested weir is maximum when the depth of flow is

A
H3
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B
2H3
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C
H2
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D
2H5
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Solution

## The correct option is B 2H3 Broad crested weir supports nappe so that pressure variation is hydrostatic at section (2). Applying Bernoulli's equation between (1) and (2) 0+0+H=(h−z)+V222g+z ⇒V222g=H−h V2=√2g(H−h) Thus, the velocity across cross section 2-2 is constant. Qth=L×h×√2g(H−h) Qactual=CdL×h×√2g(H−h) In broad crested weir, flow adjusts itself to have maximum discharge for the available head H, the critical depth is achieved by itself. dQdh=0 ⇒h=23H, for maximum discharge h=23H=Critical depth By putting h=23H, in equation, Qactual=1.7CdLH32

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