Field note / open-channel hydraulics
Side slopes change the hydraulic geometry.
The flow area is A = y(b + zy), where b is the bottom width, y is flow depth, and z is the horizontal-to-vertical side slope. The wetted perimeter includes the bottom and both sloping sides, so the hydraulic radius changes with the side slope as well as the depth.
Roughness and slope still matter.
Manning's equation uses area, hydraulic radius, bed slope, and a roughness coefficient to estimate mean velocity. Surface texture, joints, vegetation, sediment, maintenance, and channel irregularity can all affect the representative roughness used for a preliminary check.
What the quick check cannot prove
- Freeboard, overtopping, backwater, entrance losses, transitions, bends, and structures.
- Unsteady flow, sediment transport, erosion, debris, or changing boundary conditions.
- Whether a design meets a local drainage standard or flood-risk requirement.