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− | {{alert|A maintenance plan clarifying maintenance responsibility is REQUIRED. Effective long-term operation of bioretention practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility|alert- | + | {{alert|A maintenance plan clarifying maintenance responsibility is REQUIRED. Effective long-term operation of bioretention practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility|alert-danger}} |
Mulch Content and Depth - Fresh shredded bark mulch (Mn/DOT Type 6) should be used when possible to maximize nitrogen retention. If aged mulch is used, use the shredded type instead of the “chip” variety to minimize floating action. The mulch layer should not exceed 3” in depth. Too much mulch can restrict oxygen flow to roots. In addition, mulch should not be mounded around the base of plants since this encourages damage from pests and diseases. | Mulch Content and Depth - Fresh shredded bark mulch (Mn/DOT Type 6) should be used when possible to maximize nitrogen retention. If aged mulch is used, use the shredded type instead of the “chip” variety to minimize floating action. The mulch layer should not exceed 3” in depth. Too much mulch can restrict oxygen flow to roots. In addition, mulch should not be mounded around the base of plants since this encourages damage from pests and diseases. | ||
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=='''Construction Specifications'''== | =='''Construction Specifications'''== |
CADD based details for bioretention are contained in the Computer-aided design and drafting (CAD/CADD) drawings section. The following details, with specifications, have been created for bioretention systems:
A well blended, homogenous mixture of 55-65% construction sand: 10-20% top soil; and 25-35% organic leaf compost is necessary to provide a soil medium with a high infiltration/filtration capacity.
A well-blended, homogenous mixture of 50-70% construction sand and 30-50% organic leaf compost is necessary to provide a soil medium with a higher infiltration/filtration capacity.
Sand: Provide clean construction sand, free of deleterious materials. AASHTO M-6 or ASTM C-33 with grain size of 0.02”-0.04”
Organic Leaf Compost: Mn/DOT Grade 2
Topsoil in the mix will help with some nutrient removal, especially nutrients, but extra care must be taken during construction to inspect the soils before installation and to avoid compaction.
Field experiments show that pollutant removal is accomplished within the top 30” of soil depth with minimal additional removal beyond that depth (Prince George’s County, 2002). Therefore, the recommended depth of the prepared soil is 30 inches. However, if large trees are preferred in the design, a soil depth of 48”-52” should be utilized. The soil depth generally depends upon the root depth of the prescribed vegetation and content of underlying soils.
Gravel Filter Specifications - Under-drain gravel blanket shall be double washed stone, 1-1/2” in size. Pea Gravel shall be washed, river-run, round diameter, ¼ - ½ in size.
Recommended Maintenance Activities For Bioretention Areas (Source:
adapted from EPA, 1999)
Mulch Content and Depth - Fresh shredded bark mulch (Mn/DOT Type 6) should be used when possible to maximize nitrogen retention. If aged mulch is used, use the shredded type instead of the “chip” variety to minimize floating action. The mulch layer should not exceed 3” in depth. Too much mulch can restrict oxygen flow to roots. In addition, mulch should not be mounded around the base of plants since this encourages damage from pests and diseases.
Given that the construction of bioretention practices incorporates techniques or steps which may be considered non-traditional; it is recommended that the construction specifications include the following format and information:
This table shows cost Components for Bioretention Practices
(Link to this table)
Implementation Stage | Primary Cost Components | Basic Cost Estimate | Other Considerations |
---|---|---|---|
Site Preparation | Tree & plant protection | Protection Cost ($/area) x Affected Area | Removal of existing structures, topsoil removal and stockpiling |
Clearing & grubbing | Clearing Cost ($/area) x Affected Area | Removal of existing structures, topsoil removal and stockpiling | |
Topsoil salvage | Clearing Cost ($/area) x Affected Area | Removal of existing structures, topsoil removal and stockpiling | |
Site Formation | Excavation / grading | 4-ft Depth Excavation Cost ($/acre) x Area (acre) | Soil & rock fill material, tunneling |
Hauling material offsite | Excavation Cost x (% of Material to be hauled away) | Soil & rock fill material, tunneling | |
Structural Components | Under-drains | Under-drain cost ($/lineal foot) x length of device | Pipes, catch-basins, manholes, valves |
Inlet structure | ($/structure) or ($/curb cut) | Pipes, catch-basins, manholes, valves | |
Outlet structure | ($/structure) | Pipes, catch-basins, manholes, valves | |
Liner | Liner cost ($/square yard) x area of device) | Pipes, catch-basins, manholes, valves | |
Site Restoration | Filter strip | Sod cost ($/square foot) x filter strip area | Tree protection, soil amendments, seed bed preparation, trails |
Soil preparation | Topsoil or amendment cost ($/acre) x Area (acre) | Tree protection, soil amendments, seed bed preparation, trails | |
Seeding | Seeding Cost ($/acre) x Seeded Area (acre) | Tree protection, soil amendments, seed bed preparation, trails | |
Planting / transplanting | Planting Cost ($/acre) x Planted Area (acre) | Tree protection, soil amendments, seed bed preparation, trails | |
Annual Operation, Maintenance, and Inspection | Debris removal | Removal Cost ($/acre) x Area (acre) x Frequency | Vegetation maintenance, cleaning of structures |
Sediment removal | Removal Cost ($/acre) x Area (acre) x Frequency | Vegetation maintenance, cleaning of structures | |
Weed control | Labor cost ($/hour) x Hours per visit x Frequency | Vegetation maintenance, cleaning of structures | |
Inspection | Inspection Cost ($) x Inspection Frequency | Vegetation maintenance, cleaning of structures | |
Mowing | Mowing Cost ($) x Mowing Frequency | Vegetation maintenance, cleaning of structures |
Summary of Bioretention Variants for Permeability of Native Soils and Potential Land use Pollutant Loading
(Link to this table)
Bioretention Type1 | Variant | Underlying Soil Performance Criteria |
---|---|---|
Bioinfiltration (Infiltration/Recharge Facility) |
No underdrain | Higher recharge potential (facility drain time without underdrain is 48 hours or less) |
Biofiltration with underdrain at the bottom (Filtration/Partial Recharge Facility) |
Underdrain | Lower recharge potential (facility drain time without underdrain is > 48 hours) |
Biofiltration with internal water storage | Underdrain | Lower recharge potential (facility drain time without underdrain is >48 hours) |
Biofiltration with elevated underdrain (Infiltration/Filtration/Recharge Facility) |
Elevated underdrain | Higher nutrient loadings and/or quantity control |
Biofiltration with liner (Filtration Only Facility) |
Underdrain with liner | Hot Spot Treatment |
1The terminology has been changed from the original manual. The original Manual terminology is shown in parenthesis. For more information, see Bioretention terminology