Froude number classification in open-channel flow: when is the flow subcritical (tranquil)?

Introduction / Context:
The Froude number Fr is a key dimensionless parameter in open-channel hydraulics used to classify flow regimes. Correctly identifying subcritical, critical, and supercritical states guides design decisions for channels, spillways, and energy dissipation structures.

Given Data / Assumptions:

• Prismatic channel with steady, uniform flow conceptually.
• Hydraulic depth D_h = A/T (area over top width) used for wide channels; for rectangular sections, D_h ≈ flow depth y.
• Froude number defined as Fr = V / √(g * D_h).

Concept / Approach:

Flow classification is based on the relative importance of inertia to gravity waves. When Fr < 1, surface gravity waves can propagate upstream, and the flow is subcritical (tranquil). At Fr = 1, flow is at the critical state. When Fr > 1, the flow is supercritical (rapid), dominated by inertia; disturbances cannot move upstream.

Step-by-Step Solution:

Verification / Alternative check:

For a wide rectangular channel, substitute D_h ≈ y. Example: if V = 0.8 m/s and y = 1.0 m, g = 9.81 m/s^2, then Fr ≈ 0.8 / 3.13 ≈ 0.26 < 1, confirming subcritical flow.

Why Other Options Are Wrong:

(b) Fr = 1.0 defines the critical condition, not subcritical. (c) Fr > 1.0 is supercritical. (d) and (e) are inconsistent with the standard definition and would misclassify many flows.

Common Pitfalls:

Using geometric depth instead of hydraulic depth in irregular sections; ignoring that local accelerations can change Fr along a transition.

Final Answer:

less than 1.0