Swale & Channel Capacity Calculator
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Swale & Channel Capacity Calculator
Work out the flow capacity and velocity of a swale, channel or ditch with Manning's equation — or find the flow depth needed for a target flow. Based on The SuDS Manual (CIRIA C753) Eq 24.12, with grass roughness guidance from Section 17.4.1.
Flow capacity & velocity
Enter the cross-section, longitudinal slope, roughness and flow depth. Returns area, hydraulic radius, velocity and flow (full Manning's).
Depth for a target flow
Solve the flow depth a channel needs to carry a given flow, then check the velocity and add freeboard.
Roughness & notes
Manning's equation, typical roughness values and the SuDS velocity limits.
| Surface | n |
|---|---|
| Grass swale, flow ≤ grass height (water quality) | 0.25 |
| Grass swale, deeper conveyance flow | ~0.05 |
| Short / mown grass | ~0.03 |
| Earth ditch, clean | ~0.022 |
| Earth ditch, weedy / stony | ~0.035 |
| Concrete-lined channel | ~0.013 |
| Riprap / stone | ~0.035 |
Manning's equation (C753 Eq 24.12). Q = (A / n) × R2/3 × S1/2, where A = flow area, R = hydraulic radius (A / wetted perimeter), S = longitudinal slope, n = roughness. Velocity V = Q / A.
Grass roughness. For grass swales, n varies strongly with flow depth (C753 Figure 17.7): use n ≈ 0.25 where the water depth is below or equal to the grass height (the water-quality treatment flow), and a lower average n for deeper conveyance flows. Increase n for larger or mixed planting.
Velocity limits. Keep velocities in extreme events below about 1.0 m/s (or up to 2.0 m/s only where slope stability, erosion and safety allow) to prevent scour. For water-quality treatment, lower velocities and longer residence times are better — use check dams on steeper swales. A Froude number below 1 indicates sub-critical (tranquil) flow.
This swale capacity calculator works out the flow capacity and velocity of a swale, channel or ditch using Manning's equation, or finds the flow depth needed for a target flow. It follows The SuDS Manual (CIRIA C753, Eq 24.12), with grass roughness guidance from Section 17.4.1. Swales are a preferred conveyance component under the national SuDS standards.
Capacity and velocity at a given depth
Enter the cross-section (trapezoidal swale, V-swale, or rectangular channel), the longitudinal slope, the roughness and the flow depth, and the calculator returns the cross-sectional area, hydraulic radius, velocity and flow using full Manning's. That tells you whether the channel can carry the design flow and whether the velocity is within a safe, non-eroding range.
Depth needed for a target flow
Working the other way, enter the flow the channel must carry and the calculator returns the depth required, so you can check freeboard against the swale's actual depth.
Choosing Manning's n
The roughness coefficient matters more than anything else in the calculation. A grass swale conveying shallow flow over the grass is far rougher than a mown or concrete-lined channel, and the calculator offers typical values for grass swales, earth ditches, riprap and concrete so you are not guessing.
Further reading: green SuDS: nature-based drainage · source control: SuDS at plot level · gov.uk SuDS technical standards.
Frequently asked questions
It estimates the flow and velocity in an open channel from its shape, slope and surface roughness. It is the standard method for sizing swales, ditches and channels.
Low enough to avoid eroding the grass but high enough to stay self-cleansing. The calculator returns the velocity so you can check it against the design range for your lining.
Yes — pick the rectangular or triangular cross-section and an appropriate roughness. Manning's equation applies to any prismatic open channel.
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