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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

	Rain Garden	Bioretention Device	Infiltration Basin
Watershed Size	≤3,000 Ft² Impervious, ≤5,000 Ft² Total	<2 Acres	5-50 Acres
Pretreatment	Not Required	Not Required	60% Residential, 80% Commercial
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 in apart	Native seed recommended. Turf grass allowed
Applicable Standards	Infiltration\Thermal\Green Infrastructure	Infiltration\Sediment\Oil & Grease\Thermal\Green Infrastructure	Infiltration\Thermal\Green Infrastructure