For a given discharge, the critical flow depth in an open

1. channel geometry only
2. channel geometry and bed slope
3. channel geometry, bed slope and roughness
4. channel geometry, bed slope, roughness and Reynold’s number

We obtain critical flow depth from specific energy equation.

E = y +	V²
	2g

For E_minimum,

∂E	= 0
∂y

Thus,	Q²T_e	= 1
	gA³

A = B. y_c.