§ 6.2.1. Uniform Flow

For a given channel condition of roughness, discharge and slope, there is only one (1) possible depth for maintaining a uniform flow. This depth is referred to as normal depth.

The Manning's Equation is used to determine the normal depth for a given discharge.

Q = (1.49/n) A R ^2/3 S ^1/2 (Eq. 6-1)

Where,

Q = Total discharge, cfs

n = Roughness coefficient

A = Cross-sectional area of channel, ft

R = Hydraulic radius of channel, feet (R=A/P)

S = Slope of the frictional gradient, ft/ft

P = Wetted perimeter, feet

Uniform flow is more often a theoretical abstraction than an actuality. True uniform flow is difficult to find in nature or to obtain in the laboratory. The engineer must be aware of the fact that uniform flow computations provide only an approximation of what will occur but that such computations are usually adequate and useful and, therefore, necessary for planning.

The computation of normal depth for trapezoidal sections can be performed by using Figure 6-1 in Appendix E of this manual.