Define section factor

Section factor is a geometric property of a channel section used in solving for depth. Two forms are used.

For critical flow

$$Z=A\sqrt{\frac{A}{T}}=\sqrt{\frac{A^3}{T}}$$

Critical flow occurs when $Z=\dfrac{Q}{\sqrt g}$. It depends only on the depth and shape of the section and is used to find the critical depth.

For uniform flow

$$AR^{2/3}=\frac{nQ}{\sqrt{S_0}}$$

The right side is known from the data, so the normal depth is the depth at which $AR^{2/3}$ equals this value; it is found by trial or from charts.

Both section factors are functions of depth only for a given channel shape; they are tabulated or plotted against $y/D$ or $y/b$.