A sluice gate (underflow gate) acts as a submerged orifice when:
Choose the correct answer
Gate is fully open (a = H)
Downstream water level rises above critical depth at vena contracta — tailwater drowns the gate
Upstream head is low
Gate width b is very large
Correct Answer
B. Downstream water level rises above critical depth at vena contracta — tailwater drowns the gate
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Sluice gate (vertical gate partially opened): Free flow — downstream level below critical depth at vena contracta; Q = Cd × b × a × √(2gH). Submerged (drowned) when: downstream depth y2 > critical depth at gate → downstream level raises, flow reduces. Requires submergence factor correction. Free flow preferred for measurement accuracy.
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