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. 

Soil Media:

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


Additional Considerations:

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. 


Design Highlights:

  • Follow the guidelines in the Iowa Stormwater Management Manual (ISWMM) Chapter 2E-4
  • Delineate drainage areas into two acres of impervious surface per biocell
  • Identify the subdrain outlet location
  • Calculate the WQv in cubic ft
  • Determine the ponding depth (6″ – 9″)
  • Calculate the required surface area
  • Determine the elevation if the top of the overflow pipe
  • Calculate the quantities of materials (rock, chip, sand, compost, topsoil, mulch, tile, etc.)
  • Develop a planting plan



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


  • Develop a maintenance plan
  • Manage vegetation to maintain a pleasing appearance
  • Perform weed control as needed
  • If native vegetation is used, consider burning annually if local code allows
  • Inspect annually for scour erosion at point of entry
  • Inspect annually for sediment accumulation. Biocells trap sediment that will need to be removed to restore efficacy
  • Remove accumulated trash and debris
  • Mulch can float and smother small plants or plug outlets. Reposition mulch to maintain a 2″ uniform layer
  • Replace mulch and dead plants until the plant community is well established.

Water Quality Volume

WQv = P * Rv * DA * 43,560 SF/ac * 1ft/12in

Required Surface Area


Af = WQv * df / [K * (hf + df) * tf]

Accordion Content

Polk Soil and Water Conservation District

1513 North Ankeny Blvd., Suite 3

Ankeny, IA, 50023

(515) 964 1883 ext. 3

Monday – Friday

7:30 am – 4:00 pm

Polk Soil and Water Conservation District is an Equal Opportunity Employer.

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