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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=(hz)+V222g+z

V222g=Hh

V2=2g(Hh)

Thus, the velocity across cross section 2-2 is constant.

Qth=L×h×2g(Hh)

Qactual=CdL×h×2g(Hh)

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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