A drainage swale is a shallow, vegetated or lined channel that collects and conveys stormwater runoff across a site. Swales are used along property lines, adjacent to roads, around parking lots, and through open space areas. They are less expensive to construct than storm drain pipes, provide some stormwater quality treatment, and are often required by local stormwater regulations as part of a low-impact development (LID) approach. But a swale that is undersized or improperly graded will overflow, erode, and create more problems than it solves.
Swale Cross-Section Geometry
The three common cross-section shapes for drainage swales are:
Trapezoidal
Flat bottom with angled side slopes. The most common shape for engineered swales. The flat bottom provides a defined flow channel and allows a wider range of flow depths. Typical bottom widths range from 2 to 8 feet, with side slopes of 3:1 (horizontal:vertical) or flatter. Steeper side slopes (2:1) are allowed in some jurisdictions but make mowing and maintenance difficult.
Parabolic
A smooth curved cross-section that mimics natural channels. Parabolic swales are aesthetically superior and easier to mow, but harder to construct precisely. They are common in residential subdivisions and parks.
V-Shape (V-Ditch)
A pointed bottom with no flat bottom width. V-ditches concentrate flow and are used where space is tight or where higher velocities are acceptable. They require erosion protection (concrete or riprap lining) because the concentrated flow produces higher velocities than a wider swale.
Design Using Manning's Equation
Swale capacity is calculated using Manning's Equation for open channel flow:
Q = (1.486 / n) x A x R^(2/3) x S^(1/2)
Where Q is flow in cfs, n is Manning's roughness coefficient, A is the cross-sectional flow area, R is the hydraulic radius (A / wetted perimeter), and S is the longitudinal slope.
Manning's n for Swales
| Lining Type | Manning's n |
|---|---|
| Short grass (mowed, 2-4 inches) | 0.030 - 0.035 |
| Tall grass (unmowed, 6+ inches) | 0.040 - 0.060 |
| Riprap (4-6 inch) | 0.035 - 0.045 |
| Concrete | 0.013 - 0.015 |
| Turf reinforcement mat (TRM) | 0.025 - 0.035 |
Worked Example
Design a grassed trapezoidal swale to carry 8 cfs from a 3-acre site. The swale has a longitudinal slope of 2.0% and side slopes of 3:1.
Using n = 0.035 (short grass), try a 4-foot bottom width with 1.0-foot flow depth:
- Top width = 4 + 2(3)(1.0) = 10 feet
- Flow area A = (4 + 10) / 2 x 1.0 = 7.0 SF
- Wetted perimeter P = 4 + 2 x sqrt(1.0^2 + 3.0^2) = 4 + 6.32 = 10.32 feet
- Hydraulic radius R = 7.0 / 10.32 = 0.678 feet
- Q = (1.486 / 0.035) x 7.0 x 0.678^(2/3) x 0.02^(1/2) = 42.46 x 7.0 x 0.770 x 0.141 = 32.3 cfs
The capacity of 32.3 cfs far exceeds the required 8 cfs. The actual depth of flow for 8 cfs is only about 0.45 feet (5.4 inches), with a velocity of approximately 2.1 ft/s. The design is adequate with generous freeboard.
Velocity and Erosion Limits
Maximum permissible velocity depends on the swale lining:
| Lining | Maximum Velocity (ft/s) |
|---|---|
| Bare soil (sandy) | 1.5 - 2.0 |
| Bare soil (clay) | 3.0 - 4.0 |
| Grass (established sod) | 4.0 - 6.0 |
| Turf reinforcement mat | 8.0 - 12.0 |
| Riprap (6-inch) | 8.0 - 10.0 |
| Concrete | 15.0 - 20.0 |
If the calculated velocity exceeds the permissible velocity for the chosen lining, either reduce the swale slope (by lengthening the flow path), widen the swale (to reduce depth and velocity at the same flow), or upgrade the lining material. Check dams (small weirs placed across the swale at intervals) can also reduce velocity on steep slopes by creating a series of level pools.
Freeboard
The swale must have freeboard above the design water surface to contain the flow during larger storm events and to account for debris, sediment accumulation, and wave action. Minimum freeboard is typically 6 inches (0.5 feet) above the design water surface for the 10-year storm. The top of the swale (lip of the channel) must be at least this much higher than the calculated flow depth.
Outlet Design
Every swale needs a defined outlet. Common outlets include:
- Connection to storm drain inlet. A catch basin or area drain at the downstream end of the swale collects the flow and directs it into the pipe system.
- Discharge to a larger channel or creek. The swale outlets at a natural drainage course, with appropriate erosion protection (riprap, energy dissipater) at the transition.
- Spread to a vegetated area. For stormwater quality swales (bioswales), the flow may be spread across a vegetated buffer at the outlet. This is only appropriate for small flows.
An undersized or missing outlet is one of the most common swale design failures. If the swale fills up and has nowhere to go, water overtops the banks and creates uncontrolled erosion across the site.
Maintenance Considerations
Swales require regular maintenance to function properly. Grass must be mowed to maintain the design roughness. Sediment that accumulates in the bottom must be removed periodically. Erosion at the inlet and outlet must be repaired. If vegetation dies (from drought, shade, or chemical spill), the bare soil is vulnerable to erosion at the next storm. Include a maintenance access route in the site plan so that mowers and equipment can reach the swale without crossing landscaped areas or structures.
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