Swale
A swale is a shallow, planted channel that holds runoff back and lets it soak. A ditch is built to move water away fast; a swale is built to slow it. If your soil swallows less than half an inch of water per hour, plan the swale as a carrier, not a soaker — the perk rate decides which one you own.
Where the boundary runs
Four things get called swales. Three of them are not. The line that matters is intent: a swale is sized and planted to detain; the others are built to convey or store.
Boundary table
| Name | Shape | Job | Water leaves by |
|---|---|---|---|
| Swale | Shallow, wide, planted | Detain and soak | Infiltration + slow overflow |
| Ditch | Narrow, steep-sided, bare | Move water away fast | Outlet, culvert, road |
| French drain | Trench of rock with pipe | Collect and carry below ground | Pipe to outlet or dry well |
| Rain garden | Basin, not channel | Hold one storm, soak it | Infiltration only, 24–48 h |
A swale moves water slowly along a line; a rain garden holds it in a bowl. Both are on this desk's beat; only one is a channel.
What a swale cannot do
- Hold water past 72 hours. A swale that stays wet is a mosquito farm and a failed design. The rule and its origin: the 72-hour note.
- Take fast water on bare soil. Grass-lined channels erode above roughly 1.5 feet per second. Steep runs need check dams or riprap.
- Replace a drain on bad soil. Below about 0.5 in/hr perk rate, the swale is a carrier to somewhere else — usually a French drain or dry well. Pretending otherwise is the most common swale failure.
Numbers to hold onto
| Number | Typical range | Note |
|---|---|---|
| Longitudinal grade | 1–4% | Below 1% water stalls; above 4% it scours. Check dams stretch the upper end. |
| Side slopes | 3:1 or flatter | Steeper sides slump and can't be mowed. |
| Drawdown | ≤72 h | From the end of the storm. |
| Setback from foundation | ≥10 ft | Common minimum; read your code — the one-hour job. |
| Max grass-channel velocity | ≈1.5 ft/s | Over this, armor the channel or slow the slope. |
Misreads
People call a mowed drainage ditch a swale because both are channels. The ditch has steep sides, no berm, and an outlet to the street; the swale is wide, flat-sided, and ends in soil. People also call a shallow French drain trench a swale before the pipe goes in. Name it by its job, not its trench.
Where to go next
- Size a swale from three numbers — the desk job that turns this page into a plan.
- Berm — the ridge this page keeps mentioning.
- Grass or rock in the channel — the note with the trade-offs.
Sources
- Minnesota Stormwater Manual, vegetated swale sections — stormwater.pca.state.mn.us
- US EPA NPDES, stormwater management basics — epa.gov/npdes
- USDA NRCS, conservation practice standards — nrcs.usda.gov