Field note / open-channel hydraulics
Start with the target flow.
The inputs are target discharge Q, bottom width b, bed slope S, and Manning roughness n. The unknown is flow depth y.
Recalculate the geometry at each trial depth.
At every trial y, the calculator evaluates A = by, wetted perimeter P = b + 2y, hydraulic radius R = A/P, and the resulting Manning discharge. A bisection search narrows the depth until the calculated flow matches the target.
Check the result beyond the number.
Velocity and Froude regime help describe the solved state, but they do not replace freeboard, erosion, sediment, debris, or control-structure checks. The actual water surface may be governed by downstream conditions rather than normal depth.
When to use another tool
- Use the rectangular open-channel calculator when depth is already measured or selected.
- Use the trapezoidal calculator when side slopes materially affect area and wetted perimeter.
- Use a backwater or gradually varied-flow model when the channel has controls, transitions, junctions, or tailwater effects.