Open Channel Flow MCQ with answer and solution

Concept

Flow type depends on the actual depth compared with the critical depth (or on the Froude number).

Formula

Fr = V/√(g×D); Fr < 1 sub-critical, Fr = 1 critical, Fr > 1 super-critical

Step-by-step solution

1. Critical depth y_c is the depth at which specific energy is minimum (Fr = 1). 2. If y > y_c, velocity is low, so Fr < 1 and the flow is sub-critical (tranquil). 3. If y = y_c, the flow is critical. 4. If y < y_c, velocity is high, so Fr > 1 and the flow is super-critical (rapid). 5. Only option D matches this rule.

Explanation

When depth is more than critical depth, the water moves slowly and Fr < 1, so the flow is sub-critical. When depth is less than critical depth, the flow is fast and super-critical. So statement D is correct.

Why the other options are wrong

A: y > y_c gives sub-critical flow, not super-critical. B: y = y_c gives critical flow, not super-critical. C: y < y_c gives super-critical flow, not sub-critical.

Shortcut

Deep water flows slowly (sub-critical). Shallow water flows fast (super-critical).

Common mistake

Mixing up the two terms and thinking a greater depth means super-critical flow.