In short: a swale must be deep and wide enough to carry your site's peak runoff without overtopping. Estimate that peak flow with the Rational Method (area × rainfall intensity × runoff coefficient), then solve Manning's equation for the flow depth a trapezoidal channel needs to carry it — add freeboard on top for a safety margin. The calculator above does both steps live.
These figures are a planning aid based on standard hydraulic formulas. For municipal approval, easements, or sites with unusual soils or steep grades, verify sizing with a licensed civil engineer familiar with your local drainage code.
How to size a drainage swale
Sizing a swale is a two-step problem. First you need to know how much water it has to move — the peak flow from a design storm. Then you need channel dimensions that can carry that flow without the water climbing over the banks.
Because a trapezoidal channel's area and wetted perimeter both change with depth, there is no simple formula to solve for depth directly — the calculator narrows in on it numerically, the same way an engineer would with a spreadsheet or channel-design chart.
Worked example. Take a 0.5-acre lawn (C = 0.25) in a 2.5 in/hr design storm: Q = 0.25 × 2.5 × 0.5 = 0.31 cfs. For a channel with a 2 ft bottom width, 3:1 side slopes, a 2% longitudinal slope and mowed-turf roughness (n = 0.035), that flow needs a depth of about 1.30 in (0.108 ft) — a top water width of about 2.65 ft and a velocity near 1.24 ft/s. Add 3 in of freeboard and the constructed channel should be about 4.30 in deep with a 4.15 ft top width.
Freeboard is not optional. The calculated flow depth is the water level in a perfectly behaved channel with no debris, sediment build-up, or storm larger than assumed. Freeboard — extra depth above that line — absorbs those real-world margins. Even a shallow swale should carry a few inches of freeboard.
Why slope and roughness change the required depth
For a fixed flow, a steeper longitudinal slope lets gravity move the water faster, so a shallower, narrower channel can carry the same volume — that is the √S term in Manning's equation. Flatten the slope and the calculator asks for more depth (and often more width) to move the same flow at a lower velocity.
Roughness works the opposite way. Manning's n represents how much the channel lining resists flow. Mowed turf (n ≈ 0.035) lets water move more freely than dense, unmown vegetation (n ≈ 0.045); a rougher lining needs a deeper or wider channel to pass the same design flow, because more of the water's energy is lost to friction with the vegetation.
Together, slope and roughness set the channel's velocity as much as its depth — and velocity matters for erosion. A very steep, smooth channel can move the right volume of water fast enough to strip vegetation and scour soil, even if the calculated depth looks small. You can read the full method on our methodology page.
Runoff coefficients (Rational Method)
The land-cover presets this calculator uses. Higher coefficients mean more of the rain becomes runoff rather than soaking in.
| Land cover | Runoff coefficient (C) |
|---|---|
| Lawn / turf, flat (<2% slope) | 0.15 |
| Lawn / turf, average slope (2–7%) (default) | 0.25 |
| Lawn / turf, steep slope (>7%) | 0.35 |
| Gravel or bare soil | 0.50 |
| Pavement, roof or other impervious area | 0.90 |
Manning's roughness for vegetated channels
The roughness presets this calculator uses for the channel lining.
| Channel lining | Manning's n |
|---|---|
| Bare earth | 0.025 |
| Sparse vegetation or gravel | 0.030 |
| Mowed lawn / turf grass (default) | 0.035 |
| Dense grass / heavy vegetation | 0.045 |
Worked examples
Residential backyard swale
1.30 in deep
0.5 acres of average lawn (C 0.25) in a 2.5 in/hr storm needs about 0.31 cfs of capacity. A 2 ft bottom, 3:1 sides and a 2% slope carry it at roughly 1.24 ft/s.
Roadside swale, paved street
6.31 in deep
2 acres of pavement (C 0.90) in a 4 in/hr storm produces 7.2 cfs. On a 4 ft bottom, 4:1 sides and a 1% slope, velocity reaches about 2.25 ft/s — worth a closer look at erosion control.
Steep-lot swale
1.07 in deep
0.3 acres of steep lawn (C 0.35) in a 2 in/hr storm needs only 0.21 cfs. A steep 5% slope with dense vegetation (n 0.045) keeps the channel shallow at about 1.33 ft/s.
What else affects the result
Soil and infiltration. This calculator sizes the swale to convey the design flow, the way a hydraulic engineer checks channel capacity. It does not account for infiltration into the soil along the way, which in practice can reduce the flow reaching the downstream end of a long, permeable swale.
Vegetation maturity. A newly seeded swale behaves more like bare earth (lower n, less erosion resistance) until turf establishes. Many designers use a lower roughness and check erosive velocity for the establishment period, then re-check with mature vegetation.
Cross-section shape. Real swales are not always a clean trapezoid — shallow, rounded ("parabolic") cross-sections are common and behave similarly for capacity purposes. Setting the bottom width to 0 here approximates a triangular or rounded shape reasonably well for planning.
Check dams and grade control. On slopes steeper than about 5%, many designs add periodic check dams to break up the slope and cut velocity, rather than relying on a single uniform grade over the whole swale length.
Frequently asked questions
How deep should a drainage swale be?
It depends on how much runoff it must carry. A small residential swale draining half an acre of lawn might only need a flow depth of about an inch, while a swale collecting runoff from a paved street or several lots can need six inches or more. Always add freeboard — extra depth above the calculated water level — for a margin of safety.
What slope should a swale have?
Most vegetated swales work well with a longitudinal slope between about 1% and 5%. Below roughly 0.5% water tends to pond and vegetation can struggle; above about 5% velocities rise quickly and the channel may need check dams or a more erosion-resistant lining.
What Manning's roughness value should I use for a grass swale?
A well-maintained, mowed turf swale is commonly modeled with n around 0.035. Denser, less-frequently-mowed vegetation increases resistance to roughly 0.045, which requires more depth to carry the same flow. Bare earth or sparse cover is closer to 0.025–0.030.
How wide should a swale be?
Bottom width is typically 2 to 4 feet for a residential swale and wider for larger contributing areas. Side slopes of 3:1 to 4:1 (horizontal to vertical) are common because they are gentle enough to mow safely; steeper side slopes save space but need erosion control or a stabilized lining.
Do I need to add freeboard to my swale depth?
Yes. Freeboard is extra depth built in above the calculated design water level to absorb debris, sediment, minor blockages and errors in the rainfall or runoff estimate. A minimum of about 3 to 6 inches is common practice for small vegetated swales.
What's the difference between a swale and a French drain?
A swale is a shallow, open, vegetated channel that conveys surface runoff visibly across the ground. A French drain is a buried perforated pipe surrounded by gravel that collects and moves subsurface or concentrated water underground. Many yards use both — a swale for sheet flow and a French drain near a foundation or wet spot.
Sources & references
- Manning, R. (1891). On the flow of water in open channels and pipes. Transactions of the Institution of Civil Engineers of Ireland, 20, 161–207.
- Federal Highway Administration. Hydraulic Engineering Circular No. 15 (HEC-15): Design of Roadside Channels with Flexible Linings.
- Kuichling, E. (1889). The relation between the rainfall and the discharge of sewers in populous districts. Transactions of the ASCE, 20, 1–56.
- NOAA Atlas 14, Precipitation-Frequency Atlas of the United States — for local design-storm intensities.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)