Open-Channel Regime — Depth Compared to Critical Depth If the flow depth y in an open channel is less than the critical depth y_c, how is the flow classified?

Introduction:
Open-channel flow regimes are often described using the Froude number Fr = V / sqrt(g * y). The relationship between actual depth and critical depth determines whether the flow is tranquil (subcritical), critical, or torrential (supercritical), with major implications for transitions, control structures, and energy dissipation.

Given Data / Assumptions:

• Prismatic channel with steady, uniform approximation locally.
• Depth y compared to critical depth y_c for the given discharge and geometry.
• Incompressible liquid; gravity-dominated flow.

Concept / Approach:
Critical flow corresponds to Fr = 1. For y < y_c, velocity must be higher to carry the same discharge, giving Fr > 1 (supercritical). Traditional hydraulics terms this regime ‘‘torrential’’ flow. For y > y_c, Fr < 1 and the regime is ‘‘tranquil’’ (subcritical).

Step-by-Step Solution:
Recall Fr = V / sqrt(g * y).If y decreases below y_c at fixed discharge, V increases and Fr > 1.Thus the correct classification is ‘‘torrential flow’’ (supercritical).

Verification / Alternative check:
Hydraulic jumps occur when torrential flow transitions abruptly to tranquil flow, a standard example demonstrating Fr > 1 upstream and Fr < 1 downstream.

Why Other Options Are Wrong:
‘‘Tranquil’’ corresponds to y > y_c; ‘‘critical’’ is y = y_c; ‘‘turbulent’’ refers to laminar vs turbulent (Reynolds-based), not Froude regime.

Common Pitfalls:
Confusing turbulence (a Re effect) with regime (a Fr effect); assuming depth alone determines discharge without considering velocity.

Final Answer:
torrential flow