Critical flow condition in open channels The flow in an open channel is termed “critical” when the Froude number Fr equals what value?

Introduction / Context:
The Froude number Fr = V / √(g D_h) compares flow inertia to gravity effects (D_h is hydraulic depth). It classifies free-surface flows as subcritical, critical, or supercritical—vital for control structures, spillways, and rapidly varied flow predictions.

Given Data / Assumptions:

• Steady, one-dimensional open-channel flow.
• Hydraulic depth D_h = A / T (area over top width).
• Froude number Fr = V / √(g D_h).

Concept / Approach:

By definition, critical flow occurs when specific energy is minimized for a given discharge, and mathematically when Fr = 1. Subcritical flows have Fr < 1 (gravity-dominated), while supercritical flows have Fr > 1 (inertia-dominated).

Step-by-Step Solution:

Verification / Alternative check:

At critical conditions, specific energy E has a stationary point with respect to depth, consistent with GVF theory.

Why Other Options Are Wrong:

(a) and (c) correspond to subcritical and supercritical regimes; (d) and (e) contradict the definition.

Common Pitfalls:

Confusing hydraulic depth with hydraulic radius; using local depth in irregular sections without proper definition of D_h.

Final Answer:

equal to 1.0