Construction specifications for bioretention
Construction Details CADD based details for bioretention are contained in Appendix D. The following details, with specifications, have been created for bioretention systems:
- Bioretention Facilities General Plan
- Bioretention Facilities Performance Types Cross-Sections
- Infiltration / Recharge Facility
- Filtration / Partial Recharge Facility
- Infiltration / Filtration / Recharge Facility
- Filtration Only Facility
Materials Specifications
Soil Medium / Filter Media Content
Mix A: Water Quality Blend
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.
Sand: Provide clean construction sand, free of deleterious materials. AASHTO M-6 or ASTM C-33 with grain size of 0.02”- 0.04”
Top Soil: Sandy loam, loamy sand, or loam texture per USDA textural triangle with less than 5% clay content
Organic Leaf Compost: (MnDOT Grade 2) 2 (see also a fact sheet in Chapter 12-3, entitled Using Compost as a Soil Amendment)
Mix B: Enhanced Filtration Blend
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.
Soil Medium / Filter Media Depth 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.
Table 12.BIO.5 Recommended Maintenance Activities For Bioretention Areas (Source:
adapted from EPA, 1999)
- As needed
- Prune and weed to maintain appearance.
- Mulch replacement when erosion is evident.
- Remove trash and debris.
- Mow filter strip.
- Semi-annually
- Inspect inflow points for clogging (off-line systems). Remove any sediment.
- Inspect filter strip/grass channel for erosion or gullying. Re-seed or sod as necessary.
- Trees and shrubs should be inspected to evaluate their health and remove any dead or severely diseased vegetation.
- Annually
- Inspect and remove any sediment and debris build-up in pre-treatment areas.
- Inspect inflow points and bioretention surface for build up of road sand associated with spring melt period. Remove as necessary and replant areas that have been impacted by sand/salt build up.
- 2 to 3 years
- Replace mulch over the entire area.
- Replace pea gravel diaphragm or filter fabric if warranted.
- The planting soils should be tested for pH to establish acidic levels. If the pH is below 5.2, limestone should be applied. If the pH is above 7.0 to 8.0, then iron sulfate plus sulfur can be added to reduce the pH.
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.
Construction Specifications
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:
A. Temporary Erosion Control
- Install prior to site disturbance
- Protect catch basin/inlet
- It is HIGHLY RECOMMENDED that future bioretention locations not be used as temporary sedimentation basins. If used as temporary sedimentation basins, the bioretention practice should be over excavated a minimum of 18” below sedimentation basin grade
(Link to this table)
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 |
B. Excavation, Backfill and Grading
- Timing of grading of infiltration practices relative to total site development
- Use of low-impact, earth moving equipment (wide track or marsh track equipment, or light equipment with turf-type tires)
- Do not over-excavate
- Restoration in the event of sediment accumulation during construction of practice
- Alleviate any compacted soil (compaction can be alleviated at the base of the practice by using a primary tilling operation such as a chisel plow, ripper or sub-soiler to a minimum 12” depth
- Gravel backfill specifications
- Gravel filter specifications
- Filter fabric specifications
C. Native Plants, Planting and Transplanting (MN Plant List in Appendix E)
- Site preparation of planting areas
- Timing of native seeding and native planting
- Weed control
- Watering of plant material
D. Construction Sequence Scheduling
- Temporary construction access
- Location of silt fence installation to protect BMPs and downstream receiving waters
- Removal and storage of excavated material
- Installation of underground utilities
- Rough grading
- Seeding and mulching disturbed areas
- Road construction
- Final grading
- Site stabilization
- Installation of semi-permanent and permanent erosion control measures
- Silt fence removal
E. Construction Observation
- Adherence to construction documents
- Verification of physical site conditions
- Erosion control measures installed appropriately
Table 12.BIO.7 Summary of Bioretention Variants for Permeability of Native Soils and Potential Land use Pollutant Loading
- Bioretention Type
- Variant
- Underlying Soil Performance Criteria
- Infiltration/Recharge Facility
- No under-drain
- Higher recharge potential (facility drain time without under-drain is < 48 hours)
- Filtration/Partial Recharge Facility
- Under-drain
- Lower recharge potential (facility drain time without under-drain is > 48 hours)
- Infiltration/Filtration/Recharge Facility
- Elevated under-drain
- Higher nutrient loadings and/or quantity control
- Filtration Only Facility
- Under-drain with liner
- Hot Spot Treatment
B. Excavation, Backfill and Grading
- Timing of grading of infiltration practices relative to total site development
- Use of low-impact, earth moving equipment (wide track or marsh track equipment, or light equipment with turf-type tires)
- Do not over-excavate
- Restoration in the event of sediment accumulation during construction of practice
- Alleviate any compacted soil (compaction can be alleviated at the base of the practice by using a primary tilling operation such as a chisel plow, ripper or sub-soiler to a minimum 12” depth
- Gravel backfill specifications
- Gravel filter specifications
- Filter fabric specifications
C. Native Plants, Planting and Transplanting (MN Plant List in Appendix E)
- Site preparation of planting areas
- Timing of native seeding and native planting
- Weed control
- Watering of plant material
D. Construction Sequence Scheduling
- Temporary construction access
- Location of silt fence installation to protect BMPs and downstream receiving waters
- Removal and storage of excavated material
- Installation of underground utilities
- Rough grading
- Seeding and mulching disturbed areas
- Road construction
- Final grading
- Site stabilization
- Installation of semi-permanent and permanent erosion control measures
- Silt fence removal
E. Construction Observation
- Adherence to construction documents
- Verification of physical site conditions
- Erosion control measures installed appropriately
(Link to this table)
Table 12.BIO.7 Summary of Bioretention Variants for Permeability of Native Soils and Potential Land use Pollutant Loading
- Bioretention Type
- Variant
- Underlying Soil Performance Criteria
- Infiltration/Recharge Facility
- No under-drain
- Higher recharge potential (facility drain time without under-drain is < 48 hours)
- Filtration/Partial Recharge Facility
- Under-drain
- Lower recharge potential (facility drain time without under-drain is > 48 hours)
- Infiltration/Filtration/Recharge Facility
- Elevated under-drain
- Higher nutrient loadings and/or quantity control
- Filtration Only Facility
- Under-drain with liner
- Hot Spot Treatment