Percolation rate
Percolation rate is how fast your soil swallows water, in inches per hour, measured from a presoaked hole. It is the number that decides whether a swale soaks water or merely carries it, and whether a dry well is a well or a buried pond. It is also the number most often skipped — because it costs a day of waiting and people hate waiting.
The bands, and what each one permits
| Perk rate | Soil, typically | What it permits |
|---|---|---|
| < 0.25 in/hr | Heavy clay, compacted fill | Almost nothing infiltrates. Convey, don't soak — pipe it somewhere. |
| 0.25–0.5 | Silty clay, clay loam | Shallow swales, marginal. Design as a carrier with a slow-bleed bonus. |
| 0.5–2 | Loam, sandy loam | The working range: swales and small dry wells behave. |
| > 2 | Sandy, gravelly | Water leaves almost as it lands. Infiltration is easy; holding it is the problem. |
The 0.5 threshold is the desk's gate, echoed on the swale page: below it, plan conveyance; above it, infiltration earns its keep. Your local manual may set its own gate — read it.
The texture shortcut — and its trap
Soil texture predicts the band: sand drains fast, clay slow. The shortcut saves nothing if you guess the texture wrong, and compacted clay can hide under 2 inches of bought topsoil. Feel the soil at the depth the water will actually sit, not at the surface. When the band and the texture disagree, believe the test hole.
One hole is a screen, not a yard
A single perk hole is a spot check. Four holes across the area — the desk's minimum before sizing anything bigger than a small swale — begin to be a yard. The perk test job lays out the schedule; the point here is the limitation, stated plainly, so nobody mistakes a screen for a survey.
Where to go next
- Run a percolation test — the bucket-and-hole method, one afternoon.
- Dry well — what the rate permits underground.
- What 1 in/hr actually means — the unit trap, unwrapped.
Sources
- Minnesota Stormwater Manual, infiltration testing — stormwater.pca.state.mn.us
- USDA NRCS, soil texture and permeability — nrcs.usda.gov
- US EPA NPDES — epa.gov/npdes