Drainage Swale: Design, Slope and Swale vs Ditch
A drainage swale is a shallow, wide channel shaped in the ground to steer runoff where you want it to go. Most are lined with grass, which slows the water, filters it and lets some soak in. Swales are common along driveways, between houses and at the edges of parking areas.
Driveway rainfall runoff calculator →Swale vs ditch
| Swale | Ditch | |
|---|---|---|
| --- | --- | --- |
| Shape | Shallow, wide, gentle sides | Deeper, narrower, steeper sides |
| Lining | Usually grass | Grass, rock or bare |
| Flow | Slow | Faster |
| Mowing | Easy | Hard |
| Typical use | Yards, driveways, lots | Roads, farms, larger flows |
Basic design targets
- Longitudinal slope: roughly 1 to 2 percent for grass swales. Under about 0.5 percent water may pond; steeper slopes risk erosion.
- Side slopes: 3 to 1 or flatter; 4 to 1 if you want to mow it.
- Depth: often 6 to 18 inches for residential work.
- Bottom width: wide and rounded or flat, not V-shaped, so water spreads out.
- Outlet: a storm drain, ditch, dry well or rain garden that can take the flow.
Sizing the flow
The amount of water depends on the area draining to the swale and how hard it rains. Paved areas shed most of the rain that falls on them. The driveway rainfall runoff calculator estimates runoff from a driveway.
Example (made-up numbers): a 600-square-foot driveway in a 1-inch storm sheds about 600 × (1/12) = 50 cubic feet of water, roughly 374 gallons. A swale 40 feet long with a 3-foot-wide flow area and 4 inches of water depth holds about 40 cubic feet at once, so it needs to keep moving water to the outlet.
Building a swale
- Mark the route and check the fall with a level line or laser.
- Excavate to shape, keeping the bottom smooth and the sides gentle.
- Compact loose fill lightly so it does not settle unevenly.
- Seed and cover with erosion control blanket, or lay sod.
- Protect the outlet with rock if water exits with force.
- Water the grass until it is established.
Rock and check dams
Where slopes are steeper or flows are larger, line the swale with rock over fabric (see landscape fabric under gravel) or add small rock check dams to slow the water.
Swales and driveways
- Run a swale along the low side of a driveway to keep water off the pavement edge.
- Use a driveway water diverter to send water off the surface into the swale.
- Where the ground stays wet, pair the swale with a French drain along the driveway.
- Consider a permeable driveway where space for a swale is limited.
A bioswale is a swale planted with deeper-rooted plants and sometimes built over engineered soil to treat runoff more fully. Check local rules before redirecting water toward neighbors or a public road.
Questions people ask
What is a drainage swale?
A shallow, broad channel, usually grassed, shaped to carry surface runoff slowly to a safe outlet while letting some of it soak in.
How deep should a drainage swale be?
Residential swales are often only 6 to 18 inches deep, with wide, gentle sides. Depth depends on how much water it must carry.
What slope should a swale have?
Along its length, roughly 1 to 2 percent is a common target for grass swales: enough to drain but slow enough to avoid erosion. Steep swales may need rock or check dams.
What side slopes are used on a swale?
Side slopes of 3 to 1 or flatter are common, and 4 to 1 makes mowing easier.
What is the difference between a swale and a ditch?
A swale is shallow and wide with gentle sides and usually grass. A ditch is deeper and narrower, with steeper sides, and carries more water faster.
Do I need a permit for a drainage swale?
Possibly. Changing drainage toward a neighbor or a public right-of-way is regulated in many places. Check with your local building or public works department.
Related guides
- French drain along a driveway
- Asphalt driveway water diverter
- Permeable driveway
- Landscape fabric under gravel
- Catch basin repair
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