Operation and maintenance of bioretention

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

• standing water;
• clogged filter surface; and
• inlet, outlet or under-drains clogged.

Recommendations described below are aimed at preventing these common problems.

Design phase maintenance considerations

Implicit in the design guidance for bioretention 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.

Warning: It is required that the contributing drainage area has been fully stabilized prior to bringing the practice on line

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 here.

BIORETENTION - Construction inspection checklist

Post-construction operation and maintenance

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 but will improve aesthetics and property value.

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 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 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. It is recommended that the practice be bonded.
• 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 below.

Recommended maintenance activities for bioretention areas (Source: adapted from EPA, 1999)

• First year after planting
  • Adequate water is crucial to plant survival and temporary irrigation will be needed unless rainfall is adequate until plants mature
• As needed
  • Prune and weed to maintain appearance
  • Stabilize or replace mulch when erosion is evident
  • Remove trash and debris
  • Mow filter strip
  • Renew mulch to replace that which has broken down into organic matter
  • Replace vegetation whenever percent cover of acceptable vegetation falls below 90 percent or project specific performance requirements are not met. If vegetation suffers for no apparent reason, consult with horticulturist and/or test soil as needed.
• Semi-annually
  • Inspect inflow points for clogging (off-line systems) and remove any sediment
  • Inspect filter strip/grass channel for erosion or gullying and sod as necessary
  • Herbaceous vegetation, trees and shrubs should be inspected to evaluate their health and replanted as appropriate to meet project goals
  • Remove any dead or severely diseased vegetation
• Annually in fall
  • 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.
  • Cut back and remove previous years plant material and remove accumulated leaves if needed (or controlled burn where appropriate).

For proper nutrient control, bioretention cells must not be fertilized unless a soil test from a certified lab indicates nutrient deficiency. The one exception is a one-time fertilizer application during planting of the cell, which will help with plant establishment. Irrigation is also typically needed during establishment.

Pre-treatment devices need to be maintained for long-term functionality. Accumulated sediment in the forebay will need to be cleaned out at a minimum when it is half-full, which should be approximately every 10 to 20 years. In an especially dirty watershed, the frequency may be increased to every 2 to 3 years. Sediment should also be cleaned out of rip rap and sumps. A vacuum truck is typically used for sediment removal. If a grassed filter strip or swale is used as pre-treatment, they should be mowed as frequently as a typical lawn. Depending on the contributing watershed, grassed BMPs may also need to be swept before mowing. All grassed BMP’s should also be swept annually with a stiff bristle broom or equal to remove thatch and winter sand. The University of Minnesota’s Sustainable Urban Landscape Series website provides guidance for turf maintenance, including mowing heights.

Maintenance of vegetation after establishment is similar to adjacent gardens (except for application of fertilizer). Weeding is especially important during the plant establishment period, when vegetation cover is not 100 percent yet, but some weeding will likely always be needed. It is also important to budget for some plant replacement (at least 5 to 10 percent of the original plantings) during the first few years after planting, in case some of the plants that were originally planted die. Rubbish and trash removal will likely be needed more frequently than in the adjacent landscape, since the hydraulic loading ratio is high. Trash removal is important for prevention of mosquitoes. Mulch renewal will be needed two or three times after establishment (first five years). After that, the plants are typically dense enough to make it difficult to mulch, and the breakdown of plant material will provide enough organic matter to the infiltration/filtration device. It is recommended that bioretention performance evaluations follow the four level assessment system in Stormwater Treatment: Assessment and Maintenance (Gulliver et al., 2010). More detailed information about maintenance procedures, a maintenance schedule, and estimated maintenance costs are also available in Gulliver et al. 2010.

Owner’s Representatives may wish to consider deducts and liquidated damages for bad construction practices. Regulating authorities may wish to consider fines for bad construction practices.

Related pages

• Overview for bioretention
• Types of bioretention
• Design criteria for bioretention
• Construction specifications for bioretention
• Operation and maintenance of bioretention
• Cost-benefit considerations for bioretention
• External resources for bioretention
• References for bioretention
• Requirements, recommendations and information for using bioretention BMPs in the MIDS calculator