State the definitions of subcritical and supercritical flow using Froude number Fr = V/√(g y).

• A. Subcritical: Fr < 1; Supercritical: Fr > 1
• B. Subcritical: Fr > 1; Supercritical: Fr < 1
• C. Subcritical: Fr = 1; Supercritical: Fr ≠ 1
• D. Subcritical: Fr ≤ 1; Supercritical: Fr ≥ 1

Answer: (A)

The main idea here is how the Froude number compares the water’s inertia to gravity-driven wave speed. Fr = V / sqrt(g y) tells us whether surface waves can move upstream against the flow. When Fr is less than one, gravity dominates and waves travel faster than the water; the flow is subcritical, meaning disturbances can propagate upstream and the flow changes gradually, with deeper water and lower velocity. When Fr is greater than one, the water’s inertia dominates and it outruns the surface waves; the flow is supercritical, features higher velocity and shallower depth, and disturbances cannot move upstream easily, often leading to rapid transitions like hydraulic jumps. The boundary case Fr = 1 is the critical condition where flow speed matches wave speed. This is why the definitions pair subcritical with Fr < 1 and supercritical with Fr > 1.