Home/Calculators/Open Channel Flow

🌊 Open Channel Flow Calculator

Manning's equation for trapezoidal, rectangular, and triangular channels — US customary units (ft, cfs, ft/s). Solve for flow rate, normal depth, critical depth, velocity, slope, or Froude number.

Inputs

Enter 0 for a triangular channel.
Enter 0 for a rectangular channel (vertical walls).
e.g. 0.005 = 0.5%

Results

Flow Rate, Q
—
Flow Rate, Q—
Mean Velocity, V—
Flow Area, A—
Wetted Perimeter, P—
Hydraulic Radius, R—
Top Width, T—
Froude Number, Fr—
Critical Depth, yₐ—
Specific Energy, E—
Q = (1.486 / n) · A · R2/3 · S₀1/2 (US units)
A = (b + z·y)·y P = b + 2y√(1+z²) R = A/P
Fr = V / √(g·A/T), g = 32.174 ft/s² E = y + V²/2g

About this calculator

This calculator solves Manning's equation in US customary units using the 1.486 conversion factor: Q = (1.486/n) · A · R^(2/3) · S₀^(1/2). It handles trapezoidal channel sections, which also cover rectangular channels (side slope z = 0) and triangular channels (bottom width b = 0).

Normal depth and critical depth are solved iteratively by bisection. Critical depth satisfies Q²T / (gA³) = 1 with g = 32.174 ft/s². The Froude number classifies the flow regime: Fr < 1 is subcritical (tranquil), Fr > 1 is supercritical (rapid). A velocity warning appears above 15 ft/s, where erosion and scour protection typically need attention.

Typical uses: roadside ditch and swale sizing, channel capacity checks, storm conveyance design, and verifying Civil 3D grading designs by hand. Results are for preliminary design — verify against your jurisdiction's drainage criteria.

Equations and methodology referenced from Firgelli Engineering Calculators — Open Channel Flow.