Infiltration Practices
Infiltrating stormwater runoff is a great way to reduce the amount of water and pollutants reaching our lakes and streams. There are three primary practices that may be used for infiltration, but which one should be used on your site? The table below summarizes design guidelines to help choose the appropriate practice. Once a practice is selected, more specific guidance and requirements can be found on their individual pages.
| Rain Garden | Bioretention Device | Infiltration Basin | |
|---|---|---|---|
| Watershed Size | ≤3,000 ft2 impervious, ≤5,000 ft2 total | <2 acres | <50 acres |
| TSS Pretreatment | Not required | Not required | 60% residential, 80% commercial |
| Max Ponding Depth | 8 inches | 12 inches (6-9 inches optimal) | 24 inches |
| Drawdown Time | <24 hours | ≤24 hours surface, 72 hours total | ≤24 hours |
| Surface Layer | Mulch | Mulch or class II matting | Not required |
| Engineered Soil Layer | Not required | 24 inches | Not required |
| Storage Layer | Not required | Required if native infiltration rate <3.6 in/hr, ≤48 Inches | Not required |
| Underdrain | Not required | Required if native infiltration rate <3.6 in/hr | Not required |
| Vegetation | Native plant plugs or seed recommended, turf grass allowed | Native plugs placed 12 inches apart | Native seed recommended, turf grass allowed |
| Applicable Standards | Infiltration, Green Infrastructure, Thermal | Infiltration, Sediment, Oil & Grease, Green Infrastructure, Thermal | Infiltration, Green Infrastructure, Thermal |