Section 4: Soil and Water Conservation Engineering
Fluid Mechanics
Subtopic: Flow in open channels
Question Statement:
In an open channel conveying a given constant discharge $Q$, the flow regime is said to be critical when:
Options:
(A) The specific energy $E$ attains its minimum value
(B) The Froude number $Fr$ exceeds 2.5
(C) The flow depth is twice the critical depth
(D) The bed shear stress drops to absolute zero
Answer Key:
A
Step-by-Step Solution & Explanation
Critical flow in open channels corresponds to:
1. Minimum specific energy for a given discharge ($\frac{dE}{dy} = 0$).
2. Maximum discharge for a given specific energy.
3. Froude number $Fr = \frac{V}{\sqrt{g D}} = 1.0$.
Option A is the foundational thermodynamic and hydraulic definition.