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Drainage Area

• limited to less than 2 to 3 acres
• preferably less than 1 acre

Ponding Depth

• maximum 6" recommended for soils with an infiltration rate of at least 2"/hr
• maximum 3 to 4" recommended for soils with low infiltration rates or high hydraulic loadings (combine with a smaller drainage area)
• ponding depth may be increased if using sandy soils and underdrains to increase filtration
• any pooled water should be drawn down within 4-6 hours after a storm event

Plants

• must be able to tolerate
1. expected pollutant loadings
2. highly variable soil moisture conditions
3. ponding water fluctuations
• cannot be an invasive species
• the use of native species is recommended
• minimum recommended caliper size for trees is 1"
• a minimum of 3 species of trees and 3 species of shrubs is recommended to insure diversity
• avoid species that require regular maintenance

Soil

• homogeneous mix of
• 50% construction sand
• 20-30% topsoil with less than 5% maximum clay content
• 20-30% organic leaf compost
• pH between 5.5 and 6.5
• recommended minimum depth of 2 to 2.5 feet without large tree plantings
• if shallow rooted plants are used, soil depth may be reduced to 1.5 feet
• recommended depth of 4 to 4.5 feet with large trees
• soil infiltration rate should exceed 1.5"/hr

Mulch

• maximum 2 to 3 inches deep
• should be fresh, not aged
• apply uniformly, do not pile around the base of trees
• do NOT use grass clippings

Groundwater

• water table depth below the facility invert at least 2' (or an underdrain may be used)

Slope

• if greater than 15%, technique not recommended or use a weep-garden design (see The Bioretention Manual)

Pollutant Concerns

design considerations based on pollutants will differ depending on the location of the rain garden

• if sited to treat primarily landscaped areas, the major pollutants of concern are fertilizers such as nitrogen and phosphorus
• for these, the removal efficiency increases with depth suggesting that deeper cells reaching approximately 2 to 3 feet should be used (see Watershed Benefits...)
• if sited to treat primarily parking lots, rooftops, and sidewalks, the main pollutants of concern will be metals, solids, and oil & grease
• a shallow bioretention area with a full mulch layer can be used to focus on metal removal (see Watershed Benefits...)

Underdrain

• ***required for all rain gardens in residential areas
• build with an accessible cleanout well
• do not locate within the groundwater zone of saturation
• must have a hydraulic capacity greater than the planting soil infiltration rate
• may outfall to a suitable location such as common space area, stream valley, drainage swale, roadside open-section, or existing enclosed drainage system

Surface Overflow

• design for overflow is necessary, especially if the drainage area is highly impervious
• in open areas, the rain garden may be graded so that the overflow discharges into or near the baseline of a grass swale (the bottom of the rain garden invert should be a maximum of 3-4" below the swale invert)

Inflow

• special design considerations are necessary to direct the impervious drainage area's runoff to the rain garden
• inlet deflector blocks may be used in parking areas
• grassed swales and filter strips may be used in open common areas
• drainage dispersion techniques may also need to be used for highly impervious drainage areas
• when inflow exceeds 3 cfs the designer should evaluate the potential for erosion

Location

• place the rain garden near the source of the runoff generation
• avoid locating the garden near building areas, well heads, and septic systems
• locate away from traveled areas such as public pathways to avoid compaction

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¹ The Bioretention Manual, Prince George's County Department of Environmental Resources Programs and Planning Division, Maryland, 2001.