The channel width is to be contracted. The minimum width

The channel width is to be contracted. The minimum width to which the channel can be contracted without affecting the upstream flow condition is

When width is contracted, the depth decreases to critical depth, i.e. flow is critical means minimum specific energy.

E_min =	3	y_c
	2

1.74 =	3	×		q²		^{1 / 3}
	2			g

∴ B = 4.1 m.