Bioretention cells (or biocells) are one of the most widely used green infrastructure practices for managing stormwater. They are landscaped depressions that capture stormwater runoff, usually from large imprevious surfaces like parking lots or streets. Above ground, biocells look much like a flower garden, but below ground they have an engineered subgrade design to filter pollutants and aid to control stormwater runoff.
1. Hardwood Mulch: 2″ – 3″ of mulch
2. Curb cut or other inlet: allows water into the biocell
3. Modified Soil: 18″ – 30″ of sand, topsoil, and compost mixture
4. Stone Choke Layer: 2″ of thick, washed, 3/8″ chip
5. Stone Base Layer: 8″ – 12″ thick, washed, 1″ rock
6. Subdrain: perforated subdrain tile ensures the system never stays saturated
7. Existing Soils: typically altered and compacted soils with poor percolation rates
8. Overflow Standpipe: non-perforated pipe to ensure designed ponding depth of 6″ – 9″. Flows exceeding designed ponding depth exist through the standpipe
9. Plants: plants provide a pleasing appearance while protecting the soil surface while absorbing water and nutrients.
Research continues to refine the best blend for bioretention media meant to capture pollutants. Current recommendations include blending of sand, topsoil, and compost in these proportions:
75% – 90% Sand
0% – 25% Topsoil
0% – 10% Compost
Biocells are typically designed to manage the WQv volume event but can be designed to manage larger rainfall events if desired or needed.
Biocells must be installed downslope, at least 10 ft away from buildings with basements, but preferably 30ft – 40ft.
There must be 2 ft of separation between the bottom of the cell and normal high water table
Provide pretreatment to runoff to minimize sediment collection within the biocell
Native plants are recommended for their deep roots that make them drought resistant while helping to maintain high organic matter levels, and high porosity.
Modified soil percolates at 1″ per hour, so water shouldn’t pond for more than 6 – 9 hours before moving into the soil media
Do not use bioretention on brownfield sites, adjacent to hotspots, or other sensitive areas unless special precautions are used to overcome risk of groundwater contamination.
Step 1: Excavation and installation of subdrain
Step 2: Installation of rock and overflow pipe
Step 3: Placing the soil media
Step 4: Planting and mulching
Water Quality Volume
WQv = P * Rv * DA * 43,560 SF/ac * 1ft/12in
Required Surface Area
Af = WQv * df / [K * (hf + df) * tf]
Polk Soil and Water Conservation District
1513 North Ankeny Blvd., Suite 3
Ankeny, IA, 50023
Polk Soil and Water Conservation District is an Equal Opportunity Employer.
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