Bioretention cells are also commonly referred to as “Rain Gardens”. Porous Landscape Detention (PLD) is also a type of bioretention. Bioretention is typically utilized in more intensely developed areas – commercial, multi-family developments, etc. Often, bioretention cells are designed for confined spaces such as:
- Parking lot islands
- Street medians
- Landscaped areas between drives, parking areas, roads and sidewalks, etc.
- Planter boxes collecting roof drains, street drains, etc.
Bioretention works to enhance infiltration, and to provide some water quality capture volume (holding volume for settling of suspended solids). Bioretention cells can also act as a “filter” for storm runoff. The growing media in a bioretention cell can trap sediment and pollutants. Additionally, biological uptake can occur in bioretention cells, which enhances the removal of pollutants within storm runoff.
The soil mixture/growing media is critical for a bioretention cell. See recommendations on soil mixtures in the Urban Storm Drainage Criteria Manual (USDCM) Volume 3, by the Urban Drainage and Flood Control District, December 2010.
It is critical that bioretention cells be protected during the construction process from sources of sediment. In typical construction, large areas of land can be left exposed to erosion. If a bioretention cell is downstream of areas of exposed/unvegetated land, there is potential for large volumes of sediment to be discharged into the cell during a storm event. This could result irreparable damage to the growing media of the cell.
Care needs to be taken when bioretention cells are placed near the foundations of structures, roadways, etc. Often, bioretention cells are designed as a focal point for surrounding drainage. Saturation of the underlying soil in the vicinity of a bioretention cell can occur, adversely impacting foundations, the stability of roadways, etc. Again, recommendations in the Urban Storm Drainage Criteria Manual (USDCM) Volume 3, by the Urban Drainage and Flood Control District, December 2010, should be consulted.
Photos: A. Cvar