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dec home > wsmd home > erp > green infrastructure > green stormwater infrastructure > bioretention

Bioretention

 

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Rain Gardens

Rain gardens are typically surface depressions with native plantings and amended soils used to capture, slow, infiltrate, and treat runoff from impervious surfaces, including rooftops, streets, parking lots and driveways. There are several design modifications for this practice to get the desired result and are flexible in size and infiltration rates. Ultimately bio-retention practices are designed to pond to depths of 6-18 inches with a complete drawdown in 48 hours. They provide volume control, groundwater recharge, moderate peak flows and provide an aesthetic alternative to a traditional stormwater pond.

Steps to Install a Rain GardenBioretention area built at 133 State Street in Montpelier - design sponsored by EPA's 'Green Capitols' program.

• Assess the soil drainage - conduct an infiltration test
• Calculate slope - assess other aspects of the rain garden location
• Construct a non-erodable outlet or spillway to discharge overflow
• Install amended soil for drainage using a mixture of loose aggregate and compost
• Plant native species able to withstand drought and wet conditions
• Mulch plantings and maintain garden by weeding, pruning, etc.

Design Guidance for Placement

To test the drainage of the possible rain garden location, dig a 6-8 inch deep and wide hole and fill with water. If the water does not drain within 12 hours, the location is not appropriate for a rain garden.

Rain gardens should be placed where their potential can be maximized. For example, although placing a rain garden under a mature tree will intercept runoff, the tree is most likely taking up more water than the garden would take up; therefore, a rain garden is unneeded in this location.

Placement of rain gardens should:

    • Be 100 feet from wells
    • Be at least 10 feet from building foundations
    • Not be above septic systems
    • Not be above soils where the water table is within 24 inches of the surface
    • Avoid utility crossings

Design Guidance for Size

Soil type will affect the size of the garden. The correct size garden will maximize groundwater recharge and ensure proper drainage. A general size estimate for the rain garden should be:

• 60% of the upstream impervious area for clay soils
• 10-20% of the upstream impervious area for sandy soils
• Between 20-60% of the upstream impervious area for loamy soils
(info on soil types can be found in the

Shapes of the rain gardens may vary, but are most effective when:
• curvy (increased surface area)
• situated with the longest length of the garden perpendicular to the slope of the land

rain garden sizing example

Smaller distributed rain gardens are more effective than a single large scale one.

Design Guidance for Depth

Rain gardens are typically between 4-8 inches in depth depending on slope. When slope is:

• <4%, the depth should be 3-5 inches
• between 5-7%, the depth should be 6-7 inches
• between 8-12%, the depth should be 8 inches
• >12% slope, should be individually assessed by a site designer

rain garden design standards

rain garden calculations

rain garden planview

Design Guidance for Construction (continued)

After removing 8-10 inches of soil, creating berms, and forming the shape of the rain garden, the following layers should be applied to the level bottom of earth in order from bottom to top.

• 2-4 inches of stone or sand
• filter fabric
• mix 2-4 inches of existing soil and organic compost (50/50 mix) and spread evenly (acidic soils may need lime application; clay soils may need a higher percentage of compost mix)

• plant native species suitable for the conditions
• 2-3 inches of mulch
**It is important that all layers of the rain garden be level.

A grass swale, forebay or gravel entrance should be installed to slow the velocity of runoff to prevent channels from cutting into the garden.

rain garden profile

Engineered Rain Gardens with Underground Drainage System (should be designed by an individual with relevant expertise - this could include a landscape architect or professional engineer).

rain garden design options

 

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