Energy Dissipators MCQ with answer and solution

Concept

When the tail water depth is only slightly less than the required jump depth, a baffle wall can raise the water level and form the jump.

Formula

y₂/y₁ = 0.5 × [√(1 + 8 F₁²) - 1]

Step-by-step solution

1. A hydraulic jump forms when the tail water depth equals the conjugate depth y₂. 2. If the tail water is less than y₂, we have a tail water deficiency and the jump moves downstream. 3. If the deficiency is small, a baffle wall on the apron creates extra depth and resistance, so the jump forms on the apron. 4. If the deficiency is large, we need a depressed stilling basin, a sloping apron or a ski jump bucket. 5. So for a small deficiency, construct a baffle wall.

Explanation

A small shortage of tail water can be fixed by a simple baffle wall. The wall raises the water level upstream of it and forces the jump to form on the apron. Bigger shortages need bigger works, such as a stilling basin or a ski jump.

Why the other options are wrong

B: A sloping apron is used when the deficiency changes with discharge. C: A depressed stilling basin is used when the deficiency is large. D: A ski jump bucket is used when the tail water is very low, so a jump cannot form at all.

Shortcut

Small deficiency means a baffle wall. Large deficiency means a stilling basin or ski jump.

Common mistake

Choosing a stilling basin for every deficiency case without checking how large the deficiency is.