There are two basic types of rain gardens – under-drained and self-contained. Both types of rain gardens are used to improve stormwater quality, reduce runoff volumes and generally facilitate infiltration of cleaned water. Which type of garden is selected to be built is a balance of volumes of water to be treated, existing soil conditions, available space, and budget for the project.
In some cases where infiltration is not desired, the underdrain system can move excess water into a conventional storm sewer pipe system. Cases where infiltration would not be desirable would be if the bottom of the garden has less than 4’ of clearance to the seasonal mean high water table or if the adjacent soils are contaminated and the cleaned water from the garden would become recontaminated by coming in contact with the adjacent native solids.
Rain gardens are designed to be drained within four hours after a 1” rain event. Under-drained rain gardens typically are designed to drain within 2 hours of the design storm event. This is achieved through the use of highly porous planting media and underdrains which carry the cleaned rainwater away from the garden. As a result, the plants selected for the bioretention cell need to be able to withstand both the extremes of flooding and drought. Plants on the upper edges of the garden are often xeric in their cultural requirement descriptions with plants lower in the garden being more adapted to floodplain conditions. Many riparian edge species are particularly well suited to the extreme environments of rain gardens.
Rain gardens with no underdrain typically hold moisture longer, particularly in the lower areas of the garden. Plants selected for this garden should be able to tolerate inundation for a more extended period of time. However, as in the case of the underdrained rain garden, the surface is drained within four hours, although the soil may be saturated. As in the bioretention cell, soils are amended with a very porous planting media, minimally to a depth of 8” and ideally to a depth of 2’- 3’. The lower the amount of soil amendment added when the garden is built, the more necessary it is to have plants adapted to prolonged periods of wetness. As with the underdrained rain garden, the plants on the upper edges of the garden will need to be more xeric in their cultural requirements than the plants in the lower areas.
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