Features of Bioretention Stormwater Storage Systems
From paved or impermeable surfaces, stormwater runoff enters the bioretention ponds via swales and drainpipes that guide water to the system. Frequently located near the base of a retaining wall, rain gardens and bioretention areas often have graded land sloping toward the low-lying containment area.
It is always recommended that the drainage pipes in the retaining wall and clean gravel backfill remain a minimum of 12 inches (300 mm) above the high water mark for any water application, including bioretention ponds. This area is wrapped in a filter fabric to prevent fines from migrating into the clean drainage areas.
The retaining wall or grassy slope down to the containment area serve to help direct, filter and disperse stormwater evenly into the bioretention pond. The surface layer of the bioretention pond or basin is typically native vegetation that absorbs the water nutrients and mulch as an initial filter for runoff debris. The layers below can include organic soil, sand and an optional underdrain beneath with native subsoil at the bottom for infiltration.
Underdrain systems or overflow inlets and storm drains help remove the filtered, cleaner water. These release points allow treated runoff to enter other waterways and be re-used. When dry, bioretention ponds and rain gardens are not typically used as a multifunctional space due to their marshy or vegetated appearance.