Civil Engineering MCQAE Level Civil Engineering
For a broad-crested weir, the maximum discharge per unit width occurs when the depth of flow over the crest y_c satisfies:
Irrigation EngineeringDischarge Measurement WeirMEDIUM
Choose the correct answer
A
y_c = H (full upstream head)
B
y_c = 2H/3 (two-thirds of upstream head = critical depth on crest)
C
y_c = H/2
D
y_c = H/4
Correct Answer
B. y_c = 2H/3 (two-thirds of upstream head = critical depth on crest)
Why is this the answer?
For a broad-crested weir (critical flow on the crest): y_c = 2H/3, where H = upstream head above crest. Maximum q = (2g/3)^(1/2) x (2H/3)^(3/2) = 1.705 H^(3/2) per unit width (ideal). This is the critical flow condition on the horizontal crest.
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