Civil Engineering · Hydraulics
Manning's Equation
Open channel flow velocity and discharge for rectangular and trapezoidal channels using Manning's uniform-flow equation.
Standard method — stated scope limits apply
Channel
Enter channel geometry and roughness.
About This Calculation
Manning’s equation (1889) is the standard formula for uniform flow in an open channel — a storm drain, irrigation channel, or natural stream flowing under gravity rather than under pressure. It relates flow velocity to the channel’s roughness, its hydraulic radius (a shape-dependent ratio of flow area to wetted perimeter), and the slope driving the flow. It’s been the industry standard for well over a century precisely because it’s simple, robust, and matches real channel behavior closely across a very wide range of conditions.
Frequently Asked Questions
What is hydraulic radius, and why does it matter?
Hydraulic radius (R = flow area ÷ wetted perimeter) captures how efficiently a channel shape moves water — a wide, shallow channel has more wetted perimeter (more friction) per unit of flow area than a deep, narrow one, so it carries less flow for the same slope and roughness.
How do I choose Manning’s n for my channel?
It depends on the surface material and condition — smooth concrete (n≈0.012–0.017) is far less resistive than an untended earth channel choked with weeds (n≈0.030 or higher). Published tables list typical values by material; matching the actual surface condition (not just the material) matters more than people expect.
Does this work for a partially full pipe, not just an open channel?
The same equation applies to a partially full circular pipe flowing under gravity (not pressurized) — this calculator implements rectangular and trapezoidal channel shapes specifically; a circular partially-full pipe needs a different area/wetted-perimeter geometry not covered here.
What does the calculated velocity assume about the flow?
Uniform, steady flow — the same depth and velocity all along the channel. Real channels have transitions, bends, and changing cross-sections where flow is genuinely non-uniform; Manning’s equation is a good working approximation for a long, reasonably straight, constant-section reach.