Manning's Flow & Size the Water a Channel Carries
Computes canals
A canal must carry the water without silting or scouring — enter the width, depth, side slope, bed slope and roughness n to get the discharge in m³/s, velocity, flow area and hydraulic radius by Manning's equation.
Enter your channel
Next: keep velocity in the non-scouring band (≈0.3–1.5 m/s for earth canals); if it runs faster, flatten the bed slope or line the channel, and if too slow it will silt up — raise the slope or narrow the section.
Manning: V = (1/n)·R^(2/3)·S^(1/2); Q = V × A; R = A / wetted perimeter.
Manning's flow — key facts
- Velocity
- V = (1/n)·R^(2/3)·S^(1/2)
- Discharge
- Q = A × V
- Flow area
- (width + slope·depth) × depth
- Hydraulic radius
- area ÷ wetted perimeter
- n — concrete
- ≈ 0.011–0.015
- n — earthen
- ≈ 0.020–0.030
- Non-scour velocity
- ≈ 0.3–0.9 m/s earthen
- Privacy
- Runs in your browser; nothing uploaded
A channel must carry the flow without silting or scouring
Designing a canal or field drain is a balance: it must move enough water, but at a velocity that neither lets silt and weeds settle nor erodes the bed and banks. Manning's equation ties those together — velocity rises with a steeper slope, a larger hydraulic radius and a smoother lining, and discharge is simply that velocity times the flow area. Get the section, slope and roughness right and the channel runs clean for years.
This tool returns the discharge, velocity, flow area and hydraulic radius for a trapezoidal or rectangular channel, so you can size a canal to a target flow and check the velocity against the non-scour window. Pair it with the Channel Flow, Weir Flow and Canal Seepage Loss tools for a full conveyance design.
Size to a target flow
Find the section that carries your design discharge.
Check the velocity
Stay inside the non-silting, non-scouring window.
Compare linings
See how a lower n lifts capacity for the same size.
Any section
Trapezoidal or rectangular — set side slope to zero.
Frequently Asked Questions
What is Manning's equation?+
Manning's equation gives the velocity in an open channel as V = (1/n) × R^(2/3) × S^(1/2), where n is the roughness coefficient, R is the hydraulic radius (flow area divided by wetted perimeter) and S is the channel bed slope. Discharge is then Q = A × V, the flow area times the velocity. The tool computes the area, wetted perimeter, hydraulic radius, velocity and discharge for a trapezoidal channel from your dimensions.
How does the calculator handle a trapezoidal channel?+
For a trapezoidal channel it sets the flow area to (bottom width + side slope × depth) × depth, and the wetted perimeter to bottom width + 2 × depth × √(1 + side slope²). The hydraulic radius is area ÷ wetted perimeter. A side slope of 0 makes it a rectangular channel, so the same tool covers both shapes — just set the side slope to zero for a vertical-walled channel or lined flume.
What is the Manning roughness n?+
n captures how rough the channel surface is and how much it slows the flow; smoother surfaces have a lower n and carry more water. Typical values are about 0.011–0.013 for smooth concrete, 0.013–0.017 for ordinary lined canals, 0.020–0.030 for earthen channels, and 0.030–0.050 or more for weedy or rocky ones. Choosing n for the channel's real condition — including vegetation and maintenance — is the largest source of uncertainty in the result.
What is the side slope value I enter?+
The side slope is the horizontal run per unit of vertical rise of the bank — a side slope of 1.5 means the bank goes out 1.5 m horizontally for every 1 m of depth (often written 1.5:1). Steeper, more stable soils take a smaller value; loose or sandy soils need a flatter (larger) side slope to stay put. Enter 0 for vertical walls, as in a lined rectangular flume.
Why does velocity matter as well as discharge?+
Discharge tells you how much water the channel carries, but velocity tells you whether the channel will survive. Too slow and silt and weeds settle out, shrinking capacity; too fast and the bed and banks erode. A common design window for earthen irrigation channels is roughly 0.3–0.9 m/s — fast enough to stay clean but slow enough not to scour. The tool reports velocity so you can check it against that window.
How do I increase a channel's discharge?+
Discharge rises with a larger cross-section (more width or depth), a steeper bed slope, or a smoother lining (lower n). Increasing depth helps most because it raises both the flow area and the hydraulic radius. Lining an earthen channel with concrete can roughly double its capacity for the same size by cutting n. The tool lets you change each input and watch the discharge respond.
Is Manning's equation valid for all channels?+
Manning's equation is an empirical formula for steady, uniform, turbulent flow in open channels, which covers most irrigation canals, field drains and natural waterways at design flow. It assumes the slope and section are roughly constant over the reach and that flow is fully turbulent. For rapidly varied flow, very steep chutes or pressurised pipes it does not apply, but for everyday farm channel design it is the standard method this tool uses.
Does the result depend on units?+
Yes — this tool works in SI units, so enter widths and depths in metres and the bed slope as a dimensionless fraction (for example a 1-in-1000 slope is 0.001). The discharge comes out in cubic metres per second and the velocity in metres per second. The Manning coefficient itself is the SI form; if you have a value from an imperial-unit source it is the same n, because n is taken as dimensionless across both systems in common practice.