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 a buffer of sod and planting beds. The rain garden also accepts overflow from the outdoor cistern whenever its capacity is exceeded.
The ponding area of the rain garden is approximately 7″ deep and is designed to never remain ponded for more than 14 hours. 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.
The rain garden soil media is comprised of 83% sand, 8.5% onsite topsoil, and 8.5% shredded leaf mulch. Beneath the soil layer a 4″ perforated underdrain is contained within an 8″ layer of crushed gravel. After entering the underdrain, water flows into the gravel bed detention area.
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.