Conditions for uniform flow in open channel

Uniform flow is flow in which depth, area, velocity and discharge are the same at every section along the channel. The following conditions must be satisfied.

1. Prismatic channel: the cross-section (shape and size) is constant along the length.
2. Constant bed slope: the bed slope $S_0$ does not change.
3. Constant roughness: the boundary roughness (Manning's $n$) is the same all along.
4. Steady flow: the discharge does not change with time, and is constant along the channel (no lateral inflow or outflow).
5. Sufficient length: the channel is long and straight enough that the effect of entrance and exit controls dies out.
6. No control or obstruction in the reach (no weir, gate, drop, or bend) and no backwater.
7. Equilibrium of forces: the gravity force component along the slope equals the boundary shear resistance, $\gamma A L S_0=\tau_0 P L$. This gives

$$S_0=S_w=S_f\quad\text{(bed, water surface and energy line parallel)}$$

Strictly, true uniform flow is rare in nature. It is assumed for design of canals and for long reaches of rivers.