Bioretention - Virginia Tech · Bioretention. Photo Tutorial to Accompany Bioretention Design...
Transcript of Bioretention - Virginia Tech · Bioretention. Photo Tutorial to Accompany Bioretention Design...
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Bioretention
Photo Tutorial to Accompany Bioretention Design Specification
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Applications: Micro-ScaleImpervious Area Treated = 250 to 2,500 square feet
Residential Lots Commercial Rooftop
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Applications: Small-Scale
Parking Lot Edges Parking Lot Medians
Courtyards Right-of-way/Urban Bioretention
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Applications: Bioretention Basins
Bottom of ED Pond
Institutional
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Design, Construction, Maintenance Guidance
• Pretreatment • Geometry• Vegetation/Surface Cover• Construction• Maintenance
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Pretreatment (types)
Grass Filter Strip
Grass Channel
Stone Flow SpreaderStone/Rip Rap Apron
Forebay
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Results of Poor or No Pretreatment
Debris from Curb Cut Blocks Outlet Flow from Curb Cut Creates By-Pass Channel
Sediment clogs filter surface
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Geometry: Long Flow Path, Good Treatment
Grass inlet channel lengthens flow path
Long flow path from inlet to outlet
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Geometry: Short Flow Path, Less Treatment
Last curb cut
Outlet
These practices have a lack of storage and treatment due to:
•Curb inlets too close to outlets
•Outlet structure flush with filter surface
•Direct or almost direct conveyance from inlet to outlet
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Turf-treeTree-shrub-mulch
Perennial Garden Herbaceous-tree
Vegetation & Surface Cover (options)
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Other Types of Surface Cover
Stepping stones for pedestrian flowStone/cobble
Concrete cells -- herbaceous
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Vegetation: Poor Examples
Too much rock
Sparse, not enough herbaceous Plant substitutions lead to mortality, not enough herbaceous
Sparse, ground too soggy
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Typical Construction Sequence
Install secondary E&S, excavate from side
Install geotextile on SIDES ONLY (optional), add gravel to create stone storage layer up to design elevation
Install underdrain pipes and pack #57 stone around the pipes
Scar, rip, or till bottom to promote some infiltration
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Construction Sequence (cont.)
Install plant materials and water until established
Add soil media in 12” lifts from sides. Bring to design elevation and add more after settling
Install surface cover: mulch, stone, matting, etc.
Add 3” choker stone (#8 or #89) on top of underdrain stone instead of filter fabric
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Typical Construction Problems
Installed too early during construction
Stone too high at inlet – blocks flow Small grade changes divert flow from inlet
Improper soil mix
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Maintenance (representative)
Remove sediment and trash from pretreatment and filter bed
Water, add/replace plants, replace mulch, check invasives Rehabilitate clogged or slow draining media if necessary
Remove sediment from inlets, curb cuts, pavement edges
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Maintenance Issues (typical)
Sediment buildup at grass edge diverts flow
Too much mulch added – diverts flow from inlets Overgrown or taken over by invasives
Unhealthy or dead plants