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

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  1. Drop at a downstream section
  2. Rise at a downstream section
  3. Rise at a upstream section
  4. Not undergo any change

Answer (Detailed Solution Below)

Option 1 : Drop at a downstream section
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Concept:

When the width of the channel is reduced the term q (Discharge per unit width) increases. The no choke condition means critical depth of flow is not reached.

Case 1: Width of channel is B

The specific energy curve is drawn for the channel

F1 Killi 28.6.21 Pallavi D2

Case 2: Width of channel is B1 (B1 ≤ B)

The specific energy curve corresponding to the width B1 is drawn.

F1 Killi 28.6.21 Pallavi D3

When we consider the subcritical portion the graph,

We find that for the same specific energy the depth decreases when there is reduction in width of the channel. Hence there is a drop at a downstream section.
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