Open-Channel Flow Calculator
How much flow a ditch, swale or channel carries at a given depth — and how fast. Manning equation for rectangular, trapezoidal and triangular sections, from the slope and surface roughness.
How Much a Channel Can Carry
Water in a ditch, swale or culvert running partly full is open-channel flow — driven by gravity down a slope, not by pressure. The Manning equation is the workhorse for estimating how much it carries at a given depth, and how fast.
k is 1.49 in US units (1.0 in metric), n the roughness, A the flow area, R the hydraulic radius (area divided by wetted perimeter) and S the channel slope. The two-thirds power on R is why a deeper, more efficient section carries disproportionately more than a wide shallow one.
Roughness and Hydraulic Radius
The roughness n captures how much the surface drags on the flow — about 0.013 for smooth concrete, 0.022 to 0.030 for earth and grass, 0.035 for a natural stream. Halving n nearly doubles the flow. The hydraulic radius R rewards compact, deep sections: a channel that is all width and no depth wastes wetted perimeter on friction.
Watch the Velocity
The same equation gives the mean velocity, which decides whether a channel scours or silts. Earth and grass channels generally want to stay below about 5 ft/s to avoid erosion; concrete tolerates much more. Too slow, on the other hand, lets sediment drop out and clog the channel.
Frequently Asked Questions
What is normal depth?
The steady depth at which gravity and friction balance, so the flow neither accelerates nor backs up. This calculator gives the flow at the depth you enter, assuming that uniform condition.
How do I size a channel for runoff?
Find the peak design flow first (the Rational Method does this), then adjust the channel depth, width or slope here until the capacity meets or exceeds it, with freeboard to spare.
What is side slope z?
The horizontal run per unit of vertical rise on the channel banks – a z of 2 means the bank goes out 2 feet for every 1 foot of depth. Steeper soils need flatter (larger z) banks.
Related calculators
- Stormwater Runoff Calculator — the peak flow your channel must carry.
- Hydraulic Diameter Calculator — the related geometry for non-round sections.
- Pipe Flow Calculator — for full, pressurized pipe flow.
- Engineering Unit Converter — flow and velocity units.