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

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

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

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

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.