In a rectangular horizontal channel, width of channel decreases at a section. If flow in the channel is subcritical, then at that section?

UJVNL AE 2012

⭕ (a) Depth of flow will increase

⭕ (b) Depth of flow will remain same

⭕ (c) Depth of flow will decrease

⭕ (d) None of the above

Explanation: In a rectangular horizontal channel, when the width decreases and the flow is subcritical (Fr < 1), the flow behaves like a gravity wave and responds upstream. Due to the narrowing, the flow accelerates and the depth decreases. This is consistent with the principles of gradually varied flow and continuity.

Effect of Channel Width on Subcritical Flow

In open channel hydraulics, flow behavior changes significantly with geometry. For subcritical flow, where gravitational forces dominate, a decrease in channel width causes a reduction in flow depth to maintain continuity and energy.

Key Concepts:

• Subcritical flow is characterized by Fr < 1 (slow and deep flow).
• In subcritical conditions, changes in downstream geometry affect upstream conditions.
• Reduction in width increases velocity → reduces depth (continuity equation).

Conclusion:

The correct answer is (c) Depth of flow will decrease. This principle is critical in hydraulic design and analysis of gradually varied flow profiles.