Construction specifications for bioretention
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Construction specifications for bioretention

Construction Details

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:

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 the section on 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.

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.

Warning: 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

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.