Multiple choice

The flow in a rectangular channel is subcritical. If width of the channel is reduced at a certain section, the water surface under no-choke condition will

  1. drop at a downstream section

  2. rise at a downstream section

  3. rise at a upstream section

  4. not undergo any change
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A Correct answer
Explanation

In a subcritical flow, the flow is controlled by downstream conditions. Reducing the width (a constriction) increases the velocity and decreases the depth at the constriction. To maintain the subcritical state, the water surface drops at the constriction and downstream.

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