Presented by Rebecca Balke, P'E', Comprehensive Environmental Inc'

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Bioretention and Underdrain Filters
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Overview

• Benefits
• Sizing
• Vegetated Underdrain Soil Filter Field
• Bioretention Cells
• Options
• Geotextile Fabric
• Layout
• Important Considerations

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Benefits

• Excellent pollutant removal
• High absorbtion
• Channel protection through slow release
• Cooling of stormwater through soil media

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Sizing

• Must retain 1.0 inch times the impervious area
  plus 0.4 inch times the landscaped developed area
• Must drain within 24 hours

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Vegetated Underdrain Soil Filter Field
Figure 7-1 of Volume III
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Vegetated Underdrain Soil Filter Field
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Grass
Max 18
Pretreatment
4-6 slotted, schedule 40 PVC or SDR35
Energy Dispersion
Figure 7-1 of Volume III
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Bioretention Cell Treatment with Style
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Bioretention Cell
Native plant species
2-3 well aged mulch
Max 6 Ponding
4-6 slotted, schedule 40 PVC or SDR35
Figure 7-3 of Volume III
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Soil Filter Bed Option 1
18 soil filter 50-55 Sand 20-30 Topsoil with
minimal clay content and between 15-25 fines
passing the 200 sieve 20-30 shredded bark or
wood fiber mulch with lt 5 fines passing the 200
sieve
Resultant mixture should have between 5-15
passing the 200 sieve
Compact Soil by Saturation Only
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Soil Filter Bed Option 2
18 soil filter Coarse loamy sand
Resultant Mixture
20-30 shredded bark or wood fiber mulch with lt
5 fines passing the 200 sieve
Compact Soil by Saturation Only
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Pipe Bedding/Transition Zone Option 1
6 coarse gravel MEDOT 703.22 Type B
Min 4
12-14 crushed stone MEDOT 703.22 Type C
Underdrain pipes must be placed no further than 8
feet apart.
Min 4
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Pipe Bedding/Transition Zone Option 2
Underdrain pipes must be placed no further than 4
feet apart
Min 4
12-14 coarse gravel MEDOT 703.22 Type B
Min 4
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Geotextile Fabric
Geotextile fabric between gravel/crushed stone
bedding/ transition layer and adjacent soil only.
No fabric between soil filter and bedding layer.
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Layout

• Use existing site features to develop layout

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Pretreatment

• Grassed swales
• Grass or meadow filter strips
• Sediment traps

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Important Considerations

• Very important to minimize fines and meet the
  specifications
• Involve someone with plant experience when
  selecting plantings (a plant list is provided in
  Volume I, Appendix B)
• Some contractors will begin to specialize in
  building these systems to spec consider
  involving them
• Do not install soil media until catchment is
  stable

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