read the result
Reverse the usual Manning calculation.
A flow-at-depth check starts with y and calculates Q. A normal-depth check starts with target Q and varies y until the Manning discharge matches it. The solver uses a bounded bisection search, so the result remains tied to the same visible equation.
Governing relationships
For a rectangular channel, A = by, P = b + 2y, and R = A/P. The target discharge is matched to Q = (1/n) A R2/3 S1/2.
The result also reports mean velocity V = Q/A, hydraulic radius, and a rectangular-channel Froude number to indicate whether the solved flow is subcritical or supercritical.
Normal depth is not always the water depth.
- Backwater, controls, transitions, culverts, junctions, entrance effects, and downstream tailwater can make the actual depth different from normal depth.
- Freeboard, erosion, sediment transport, debris, side-slope stability, and maintenance access need separate checks.
- Use the trapezoidal channel tool when the section has side slopes, or the open-channel tool when depth is already known.