Channel Flow Calculator & Canal Discharge (Manning's)
Sizes canals
Find a canal's discharge and velocity with Manning's equation — from the section, bed slope and lining get the flow in L/s, m³/s and m³/h, with an erosion/silting check.
Enter your channel
Next: good non-scour velocity ✓ — aim for a non-scouring velocity of roughly 0.3–1.5 m/s in earth canals; if it runs faster, line the channel (concrete/brick) or flatten the bed slope.
Manning: V=(1/n)R^(2/3)S^(1/2); Q=V×A.
Channel flow — key facts
- Manning's V
- (1/n)·R^⅔·S^½
- Discharge
- Q = V × A
- Hydraulic radius
- area ÷ wetted perimeter
- Concrete n
- ≈ 0.013
- Clean earth n
- ≈ 0.022
- Non-scour velocity
- ≈ 0.3–1.5 m/s (earth)
- Canal slope
- ≈ 0.0005–0.002
- Privacy
- Runs in your browser; nothing uploaded
Size a canal that carries — and lasts
An irrigation channel has to carry enough water without either silting up or scouring away. Both come down to velocity and discharge, which Manning's equation ties to the channel's shape, bed slope and lining roughness. Too gentle a slope or too rough a bed and the flow crawls, drops its silt and grows weeds; too steep and an earthen canal erodes. The right design threads between the two.
This tool computes the velocity and discharge (in L/s, m³/s and m³/h), the flow area and hydraulic radius for a rectangular or trapezoidal section, and flags scour or silting risk. Use it to size a new field channel, check whether an existing canal can carry your pump's flow, or decide if lining it would help. Pair it with the Pipe Size, Pump Power and Sprinkler tools to design the whole conveyance system.
Check the capacity
See if a channel can carry your required flow before you dig.
Avoid scour & silt
Keep velocity in the safe band for an earthen or lined canal.
Compare linings
See how concrete vs earth changes capacity via roughness.
Design the section
Trade depth, width, slope and side slope to hit the flow.
Frequently Asked Questions
What is Manning's equation?+
Manning's equation estimates the velocity of water in an open channel: V = (1/n) × R^(2/3) × S^(1/2), where n is the roughness coefficient, R the hydraulic radius (area ÷ wetted perimeter) and S the bed slope. Discharge Q = V × A. It's the standard formula for designing canals, drains and channels, and this tool applies it.
How do I calculate canal discharge?+
Find the flow area and wetted perimeter from the channel's shape and water depth, compute the hydraulic radius, then apply Manning's equation for velocity and multiply by area for discharge. The tool does it for rectangular and trapezoidal channels and reports Q in L/s, m³/s and m³/h.
What is the hydraulic radius?+
It's the flow cross-sectional area divided by the wetted perimeter (the length of channel boundary in contact with water). A larger hydraulic radius means the water 'feels' less friction relative to its volume, so it flows faster. It's central to Manning's equation.
What is Manning's n?+
The roughness coefficient — higher for rougher boundaries. Typical values: concrete-lined ≈ 0.013, brick/masonry ≈ 0.015, clean earth ≈ 0.022, gravel ≈ 0.025, weedy earth ≈ 0.030, natural stream ≈ 0.035. Smoother linings (lower n) carry more water at the same slope and size. Pick your lining in the tool.
What is bed slope and how do I measure it?+
Bed slope (S) is the fall of the channel bed per unit length, e.g. 0.001 means 1 m drop over 1,000 m. Measure the level difference over a known length and divide. Irrigation channels are usually gently sloped (0.0005–0.002) to move water without scouring.
What velocity should a canal flow at?+
Aim for a non-scouring, non-silting velocity — roughly 0.3–1.5 m/s for earthen canals. Below ~0.3 m/s sediment settles and weeds grow; above ~1.5–2 m/s earth canals erode (lined channels tolerate more). The tool flags silting or scouring risk for your design.
Rectangular or trapezoidal channel?+
Trapezoidal channels (sloped sides) are the norm for earthen canals because vertical earth banks collapse; the side slope (z, horizontal per vertical) keeps them stable. Rectangular sections suit lined or masonry channels. The tool handles both — choose the shape and, for trapezoids, the side slope.
How do I increase a channel's capacity?+
Make it bigger (deeper/wider), steepen the bed slope, or smooth the lining (lower n). Lining an earthen canal with concrete cuts roughness and seepage, raising capacity and letting you use a higher velocity. Test each change in the tool to see the discharge it delivers.
Why does my channel overflow or silt up?+
Overflow means the flow exceeds the channel's capacity — enlarge it or reduce inflow. Silting means the velocity is too low — steepen the slope, narrow the section, or clean and reshape it. The velocity and discharge here tell you which problem you have and how to fix it.
Can I use this for a drain or natural stream?+
Yes — Manning's equation applies to any open channel. Use the appropriate n (higher for natural, weedy or stony beds) and approximate the section as rectangular or trapezoidal. For very irregular natural channels the estimate is rougher, but still a useful first approximation.