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Rain Garden

General

Rain gardens are shallow depressions that are designed to collect stormwater and promote infiltration, minimizing the amount of runoff from a site. These infiltration areas are planted with native vegetation, which act as a natural sieve, absorbs excess nutrients, and filters out pollutants.

Rain gardens should be located to intercept runoff along its natural path. When directing runoff naturally, grassed swales may be used as a conveyance structure. Rain gardens may be used on most any area of the site, excluding steep slopes, wetlands, floodplains, or in threatened or endangered species habitat. Rain gardens, while effective, generally are not designed for large storm events and, as a result, are best used in conjunction with other management practices.

Advantages

  • Reduces the amount of runoff from a site
  • Improves aesthetics and provides habitat for mosquito predators and other wildlife
  • Appropriate for either new or retrofit applications
  • Low maintenance

Disadvantages

  • Water quality impact from high traffic areas is unknown
  • Longevity of the practice is dependent upon sediment accumulation and maintenance

Design

Basin

Rain gardens should be designed to handle the 2-year, 24-hour storm and are most efficient with a storage volume that is equal to 10% of the impervious area of the site, with a maximum infiltration ponding depth of 12 inches. Side slopes of 6:1 or flatter are recommended to ensure the safety of the practice and to promote the establishment of vegetation.

Vegetation

Rain gardens are planted or seeded with deeply rooted native vegetation systems because of their ability to absorb water, hardiness, natural beauty, and their ability to mitigate compaction. Plants must be selected to meet the needs of the site, wants of the individual users and, tolerate both wet and dry conditions. Please refer to the Native Plants page for all other selection criteria and specifics.

To improve the year-round aesthetics of this practice, select species that bloom at various times throughout the spring and summer.

Soils

Rain gardens are very versatile structures and can be constructed on most any type of soil. Clay soils will generally pond runoff water for at least 72 hours, while well drained or sandy soils will infiltrate water more quickly. Fine textured soils will require shallower ponding depths and increased area.

Construction

  • Obtain all necessary permits and locate any underground utilities before construction begins
  • Rain gardens should be located at least 10 feet from buildings
  • To improve infiltration, compacted soils should be deep tilled to a depth of at least 12 inches.

Maintenance

  • Rain gardens should be mulched until vegetation has become established, and once vegetation is established, it should be mulched as needed to help keep weeds down
  • Plants should be watered at least weekly for the first 3 months, depending on the weather
  • Vegetation should be weeded occasionally during the first year and at least twice a year (or as needed) after that
  • All dead vegetation should be cut and removed once a year in the spring to allow for new vegetation growth

Method to Determine Practice Efficiency

Rain gardens are designed as an infiltration practice and do not significantly reduce the amount of suspended sediment in stormwater runoff. As a result, no efficiency is given for this practice. For purposes of Dane County Ordinances, rain gardens are not credited with sediment removal.