Rain Garden Practice
Rain gardens, also known as bioretention basins, are shallow, gently-depressed areas which hydrologically resemble sites of intermittent ponding on the forest floor. They are designed to pond water on the surface during storm events and allow the water to infiltrate once the storm passes. The infiltration process removes pollutants from the water and makes it available for uptake by plants. WSSI's 1,536 square foot rain garden accepts surface flow from approximately 9,500 square feet of impervious parking and driving surfaces behind the WSSI building. The rain garden is separated from impervious areas by an 11,700 square foot buffer of sod and planting beds, which provides initial runoff filtering and frames the rain garden in more conventional landscaping. The rain garden also accepts overflow from the outdoor cistern whenever its capacity is exceeded. Overflow from the cistern is released just beneath the mulch layer so that it receives the full benefit of filtration through the soil media.
The ponding area of the rain garden is approximately 7" deep. At a design draw-down rate of 0.5 inches per hour the rain garden should never remain ponded for more than 14 hours, eliminating the risk of long-term standing water that can serve as a mosquito breeding habitat. Observation has shown that WSSI's rain garden draws down much faster than anticipated, which will allow for future clogging without resulting in failure. Once stormwater has reached the maximum ponding depth of 7” above the mulch layer, any additional water can be released to the adjacent porous asphalt parking lot through an emergency spillway.
The rain garden soil media was mixed and placed on-site using light construction equipment, and is comprised of 83% sand, 8.5% onsite topsoil, and 8.5% shredded leaf mulch. The media was designed to allow water to permeate through very quickly while still providing enough soil and organic matter for plants to thrive. Beneath the soil layer is an 8" layer of crushed gravel, which provides storage capacity for the captured water and a base for the 4" perforated underdrain. After entering the underdrain, water flows into the gravel bed detention area.
The entire rain garden basin is wrapped in a non-woven geotextile fabric. This fabric allows water to infiltrate freely but prevents fine particles found in the in-situ clay soils from migrating into the growing media and causing clogging and failure. The same type of fabric separates the soil layer from the crushed gravel layer. WSSI has never experienced failure due to the fabric clogging.
Conditions in the rain garden are typically dry due to the highly permeable soil media, but plants must be able to withstand having "wet feet." To vegetate the rain garden, WSSI chose a variety of native species that thrive under such conditions, including Joe Pye Weed and River Birch. Selection of proper plant species is not only important for aesthetics, but also increases the infiltration rate as root networks expand and create macropores in the growing media.
More Information on WSSI’s Rain Garden