Draw a trapezoidal or circular channel with the water surface at a given depth (or solve normal depth from a design flow), and read the hydraulic geometry, Manning's velocity and flow, and the Froude regime. The visual companion to the Manning's flow calculator. Runs entirely offline; inputs persist in this browser via localStorage.
n is where the uncertainty lives — a lined channel is predictable, a natural or vegetated one swings ±30% with the season.
| Flow depth | — |
| Flow area A | — |
| Wetted perimeter P | — |
| Hydraulic radius R = A/P | — |
| Top width T | — |
| Velocity V | — |
| Flow Q = V·A | — |
| Froude number / regime | — |
| d/D ratio | — |
V = (1.486/n) · R^(2/3) · √S and Q = V · A (US customary; the 1.486 is the unit factor, 1.0 in SI). Trapezoidal: A = (b + Zy)y, P = b + 2y√(1+Z²), T = b + 2Zy. Circular with central angle θ = 2·acos(1 − 2y/D): A = (D²/8)(θ − sinθ), P = Dθ/2, T = D·sin(θ/2).
Manning's can't be inverted for depth in closed form, so solving a design flow bisects Q(y) to the target. For a circular pipe, discharge peaks near d/D ≈ 0.94, not full — past that, added wetted perimeter costs more than the added area gains, so a "full" pipe carries slightly less than a 94%-full one. The drawing makes this visible: the shaded segment at max discharge doesn't reach the crown. A design flow above the peak is reported as exceeding gravity capacity.
n dominates the uncertainty. A concrete-lined channel is predictable to a few percent; a grass or riprap channel varies with season, maintenance, and depth of flow.