Operation and Maintenance


The most frequently cited maintenance concern for bioretention is surface and under-drain clogging caused by organic matter, fine silts, hydrocarbons, and algal matter. Common operational problems include:

  • Standing water
  • Clogged filter surface
  • Inlet, outlet or under-drains clogged
Recommendations described in this chapter are aimed at preventing these common problems.

Design Phase Maintenance Considerations

Implicit in the design guidance in the previous sections is the fact that many design elements of bioretention systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include: limiting drainage area, providing easy site access (REQUIRED), providing pre-treatment (REQUIRED), and utilizing native plantings.

Construction Phase Maintenance

Proper construction methods and sequencing play a significant role in reducing problems with operation and maintenance (O&M). In particular, with construction of bioretention practices the most important action for preventing operation and maintenance difficulties is to ensure that the contributing drainage area has been fully stabilized prior to bringing the practice on line (this is a REQUIRED practice).

Inspections during construction are needed to ensure that the bioretention practice is built in accordance with the approved design and standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided in Appendix D.

Post-construction Operation and Maintenance

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 life span of the facility, but will also improve aesthetics and property value. Some important post-construction considerations are provided below along with RECOMMENDED maintenance standards. A more detailed checklist of maintenance activities and associated schedules is provided in Appendix D.

  • A site specific O&M plan that includes the following considerations should be prepared by the designer prior to putting the stormwater filtration practice into operation:
    • Operating instructions for outlet component
    • Vegetation maintenance schedule
    • Inspection checklists
    • Routine maintenance checklists
  • A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority.
  • Adequate access must be provided for all bioretention facilities for inspection, maintenance, and landscaping upkeep, including appropriate equipment and vehicles.
  • The surface of the ponding area may become clogged with fine sediment over time. Core aeration or cultivating of non-vegetated areas may be required to ensure adequate filtration.
  • Bioretention areas should not be used as dedicated snow storage areas:
    • Areas designed for infiltration should be protected from excessive snow storage where sand and salt is applied
    • Specific soil storage areas should be assigned that will provide some filtration before the stormwater reaches the infiltration areas.
    • When used for snow storage, or if used to treat parking lot runoff, the bioretention area should be planted with salt tolerant, and non-woody plant species
  • Bioretention areas should always be inspected for sand build-up on the surface following the spring melt event.
  • General maintenance activities and schedule are provided in Table 12.BIO.4.

Table 12.BIO.8 Design Infiltration Rates

  1. Hydrologic Soil Group A* This rate is consistent with the infiltration rate provided for the lower end of the Hydrologic Soil Group A soils in the Wisconsin Department of Natural Resources Conservation Practice Standard: Site Evaluation for Stormwater Infiltration
    1. Infiltration Rate [inches/hour] - 1.6”
      1. Soil Textures - Gravel, sandy gravel and silty gravels
      2. Corresponding Unified Soil Classification
        1. GW - Well-graded gravels, sandy gravels
        2. GP – Gap-graded or uniform gravels, sandy gravels
        3. GM - Silty gravels, silty sandy gravels
    2. Infiltration Rate [inches/hour] - 0.8”
      1. Soil Textures - Sand, loamy sand or sandy loam
      2. Corresponding Unified Soil Classification
        1. SP - Gap-graded or uniform sands, gravelly sands
  2. Hydrologic Soil Group B
    1. Infiltration Rate [inches/hour] - 0.6”
      1. Soil Textures - Silt Loam
      2. Corresponding Unified Soil Classification
        1. SM - Silty sands, silty gravelly
    2. Infiltration Rate [inches/hour] - 0.3”
      1. Soil Textures - Loam
      2. Corresponding Unified Soil Classification
        1. MH – Micaceous silts, diatomaceous silts, volcanic ash
  3. Hydrologic Soil Group C
    1. Infiltration Rate [inches/hour] - 0.2”
      1. Soil Textures - Sandy clay loam
      2. Corresponding Unified Soil Classification
        1. ML - Silts, very fine sands, silty or clayey fine sands
  4. Hydrologic Soil Group D
    1. Infiltration Rate [inches/hour] - <0.2””
      1. Soil Textures - Clay loam, silty clay loam, sandy clay, silty clay or clay
      2. Corresponding Unified Soil Classification
        1. GC – Clayey gravels, clayey sandy gravels
        2. ''''SC – Clayey sands, clayey gravelly sands
        3. CL – Low plasticity clays, sandy or silty clays
        4. OL – Organic silts and clays of low plasticity
        5. CH – Highly plastic clays and sandy clays
        6. OH – Organic silts and clays of high plasticity

Source: Thirty guidance manuals and many other stormwater references were reviewed to compile recommended infiltration rates. All of these sources use the following studies as the basis for their recommended infiltration rates: (1) Rawls, Brakensiek and Saxton (1982); (2) Rawls, Gimenez and Grossman (1998); (3) Bouwer and Rice (1984); and (4) Urban Hydrology for Small Watersheds (NRCS). The rates presented in this infiltration table use the information compiled from these sources as well as eight years of infiltration rates collected in various infiltration practices located in the metro area.

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