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Bioretention Basins

Bioretention basins are landscaped depressions or shallow basins used to slow and treat on-site stormwater runoff. Stormwater is directed to the basin and then percolates through the system where it is treated by a number of physical, chemical and biological processes. The slowed, cleaned water is allowed to infiltrate native soils or directed to nearby stormwater drains or receiving waters.

Photo: Bioretention Basin at Ann McCrary Park,
Wilmington, NC (source: http://www.ncgreenbuilding.org)

 

These systems normally are composed of seven elements, each element with a different function:

  1. Grass buffer strip- reduces runoff velocity and removes suspended solids.
  2. Vegetation –Help remove water through process of evapotranspiration and remove excess nutrients through nutrient cycling.
  3. Shallow ponding area –provides storage of excess stormwater flows and its subsequent evaporation, also aids in the additional settlement of particulate matter.
  4. Mulch –an organic layer that encourages micro biological degradation of petroleum-based pollutants, aids in pollutant filtration and reduces soil erosion.
  5. Engineered soils –to support vegetation growth along with nutrient uptake and provision for water storage. Soils should include some clay to adsorb pollutants such as hydrocarbons, heavy metals and nutrients.
  6. Sand bed –provide drainage and aeration of planting soil as well as an aid in flushing pollutants.
  7. Underdrain system –removal of excess treated water to storm drain system or receiving waters.


Schematic of a Typical Bioretention Area
(Source: http://www.georgiastormwater.com)

Bioretention basins are typically associated within small areas of land with residential usage or with parking lots where the islands become visually pleasing stormwater treatment centers.

Excessive stormwater from large storms is normally diverted to stormwater drains once the basin’s ponding area is full.

 

Applications

Bioretention systems can be adapted through minor design adjustments to meet a wide range of climate and geological conditions found in the United States. For colder climate detail refer to State of Minnesota Stormwater Manual-2005, Chapter 12-5: Bioretention [pdf, 1.5 MB]. Additional information on the use of bioretention basins can be found in this manual referring to development of an Operation and Maintenance Check List [pdf, 215 KB], Bioretention Basin concept drawings [pdf, 1.7 MB], cold climate runoff management [pdf, 2.6 MB], guidance for their development in areas of surface bedrock and soils with low infiltration rates [pdf, 3.2 MB] and a list of suitable plants [pdf, 2.2 MB] to use.

Typically bioretention practices are best suited to small sites and highly urbanized spaces. The use of bioretention practices is possible given adaptations to specific site usage conditions are followed, these include:

  • Areas where little pervious surface exist, such as parking lots, large buildings or sheds, are ideal candidates for use of bioretention practices such as a bioretention basin. These systems require a relatively large area of land-about five percent of area drained-however, they can be fit into existing parking lot islands and adjoining landscaped areas.
  • Areas with highly contaminated runoff, like gas stations and convenience store parking lots, must have the bottom of bioretention basin lined with impermeable liner to prevent egress of contaminated water to nearby stormwater drains, groundwater sources and receiving waters.
  • Areas where existing developments are being required to retrofit with stormwater management practices to improve on negative impacts of stormwater will find bioretention a suitable option that can be implemented by modifying present landscape or adding to a parking lot that is being resurfaced. Remember bioretention is best employed for small sites and becomes expensive (land and development costs) when trying to apply to large areas.
  • Areas near cold water/trout streams are a good option for inclusion of bioretention basins in a stormwater management plan. Ponding water exists for only short periods and thus not likely to warm-up thereby reducing storm warming.
Additional advantages of using bioretention basins include:
  • Aesthetically pleasing if properly designed and maintained.
  • Reduces amount of runoff from drainage areas.
  • Effective at removal of sediment loads, nutrients, traces of heavy metals, bacteria (possible) and organics found in stormwater. Find out more about these pollutants and their impact on water quality.
  • Allows for a flexibility of design layout so able to fit this practice into most landscapes.
  • Relatively low maintenance requirement
Examples
Glensheen Historic Estates Project
Duluth, MN
“Dan (Mcclelland) recognized the connection between runoff and shoreline protection, and wanted to protect the integrity of this historic property. Dan wanted to clean up the water, use native plants, and try out a new approach” –R.C. Boheim, South St. Louis Soil and Water Conservation District Manager

East Ridge Community Church
Duluth, MN

east ridge church
Regulatory or zoning ordinances didn’t force the church to decide on this method. They just wanted to do the right thing. They welcome attention to the rain garden-retention basins and welcome folks to stop by and take a look

Materials and Installation
  1. Siting:
    • Drainage areas should not exceed five acres. A half to two acres is preferred. If used to treat larger than recommended areas rapid clogging of the filter is an expected outcome Multiple bioretention basins are recommended for larger sites
    • Approximately five percent of the impervious area to be drained must be dedicated to bioretention basin development.
      • A minimum size of 200 square feet is required.
      • Minimum dimensions are 10 feet wide by 20 feet long.
      • A length to width ratio of at least 2:1 should generally be maintained.
    • Slope of area landscape should be a maximum of five to six percent and sufficient enough to allow filtered storm water to flow into the stormwater drainage system.
    • Minimum elevation or head from the point of inflow into the basin, through the filter material and to the outflow should be five feet.
    • A minimum separation distance of three feet between the bottom of the bioretention basin and the depth of the seasonally high water table is required.
    • Soil type is not an important consideration since soils used in bioretention basins are engineered or blended soils.
    • Depth of bedrock needs to be surveyed to establish and facilitate fitting of basins within required dimensions of site.
    • Bioretention basins need to drain reasonably quickly to function correctly, they should not be sited in areas where there is continuous flow from groundwater, sump pumps or other sources.
    • Bioretention basins need to be integrated into a site plan [pdf, 357 KB] to ensure that their full aesthetic potential is captured and they are located correctly within the sites elevation plan to function properly.
  2. Design Considerations:

    Design of bioretention basins can vary widely due to site conditions [pdf, 5.9 MB], preferences of the designers and wishes of the community where the practice is being installed. However, the following features will help ensure success:

    • Successful design of bioretention basins is a technical matter. The design and subsequent construction should be completed by those with expertise and experience in the field. Selected consultants must work closely with those responsible for developing site plans and landscape architecture. Good working rapport must be established between all parties to ensure successful instillation.
    • The most important design features for bioretention basins include:
      • Pretreatment – a practice that helps remove much of the heavy sediments and associated pollutants in stormwater prior to entering the filter basin, this reduces clogging of the filter bed and reduces cost of maintenance. Pretreatment should be successful at removing 25 to 30 % of the sediment load. The use of grassed swales or filter strips meets these criteria. Addition of a pea gravel flow spreader also helps capture sediments
      • Treatment areas – should be sized between five and 10% of the impervious drainage area entering the basin for treatment and should include [pdf, 1.7 MB]:
        • A mulch layer above the soil bed. This layer should be composed of one to two inch sized shredded hardwood or chips laid to a depth of two to four inches. The layer reduces erosion, helps maintain moisture levels for plants and aids in filtration and decomposition of organics
        • An engineered soil bed containing a sand-soil matrix. This bed provides most of the basins filtration capacity as well as providing water, nutrients and support of the plant community.
        • A ponding area to store a small quantity of water to a depth of six to nine inches. This area allows for:
          • for surface storage of stormwater before filtration occurs.
          • some evaporation thereby reducing water quantity.
          • settling–out of heavy sediments.
        • Further discussion on mulches, engineered soil beds and ponding areas can be found in the State of Minnesota Stormwater Manual-2005, Chapter 12-BIO [pdf, 1.8 MB].
      • Water conveyance – design should ensure that stormwater flows do not cause erosion prior to or post treatment and if possible be subject to other treatment practices during conveyance. Additional conveyance recommendations:
        • If treated water is not to be allowed to infiltrate into native soils, the use of an underdrain system [pdf, 1.7 MB] is needed to convey treated water to the storm drain system. Further discussion on bioretention underdrain systems can be found in the State of Minnesota Stormwater Manual-2005, Chapter 12-BIO [pdf, 1.8 MB].
        • An overflow structure to the storm drain system is needed to convey storm flows larger than can be treated by the designed system.
      • Maintenance – design should ensure easy access is possible for maintenance personnel and any associated machinery.
      • Landscaping – choice of correct landscaping materials is critical to functioning and aesthetics of bioretention basins. Plants help reduce water quantities through evapotranspiration, remove pollutants and nutrients and their root systems increase water percolation.
        • Selection of plant materials [pdf, 2.2 MB] should:
          • Utilize native plants where possible.
          • Include a mixture of trees, shrubs and herbaceous materials. Such combinations are more visually pleasing and provide a variety of habitats for wildlife.
          • Withstand periods of inundation and drought. Edge plants may experience longer periods of dryness.
          • Tolerate climate conditions and extremes of region
    • Some design variations to increase effectiveness of Bioretention basins:
      • Partial Exfiltration – is used to recharge groundwater [pdf, 1.7 MB]. The underdrain is installed in only part of the basin. Some level of infiltration occurs throughout the remainder of the basin, recharging ground water. The partial underdrain acts more as an overflow. The variation is only suitable to apply if soils and other conditions are appropriate to encourage infiltration. Further discussion on groundwater recharge considerations can be found in the State of Minnesota Stormwater Manual-2005, Chapter 12-BIO [pdf, 1.8 MB].
      • In Arid Climates – plant selection should focus on choosing drought-tolerant species.
      • In Cold Climates [pdf, 2.6 MB]– Bioretention basins can be used for snow storage. However, if the use of chloride based deicers and sand is not managed, maintenance will be complicated and more costly. Also plant material selection [pdf, 2.2 MB] must be limited to non-woody, salt tolerant species (see Tips and Wisdom section for more advice in dealing with Cold Climate conditions).
  3. Maintenance:
    • Routine inspection and attention to maintenance needs are required if bioretention basins are to continue to function correctly. High maintenance levels are required for new systems, but once established and correctly operating maintenance requirements are expected to decline. The property’s normal landscaping contractor, when provided with appropriate training, can be expected to successfully maintain an established bioretention basin.
    • Scheduled maintenance [pdf, 1.8 MB] tasks include:
      • Project completion: Water plants daily for at least two weeks
      • As Needed
        • Re-mulch void areas
        • Mow turf areas
        • Treat plant diseases
        • Water plants throughout periods of persistent drought.
        • Removal of top two to three inches of discolored planting medium and its replacement with fresh mix, when ponding of water lasts for more than 48 hours.
      • Monthly
        • Inspect basin to evaluate condition and problems needing maintenance attention.
        • Remove litter and plant debris.
        • Repair eroded areas.
      • Twice per year: Remove and replace dead and diseased plants.
      • Once per year
        • Add new mulch.
        • Replace tree stakes and wires if needed.
    • Refer to the State of Minnesota Stormwater Manual-2005, Appendix D-OMCHK [pdf, 215 KB] for examples of maintenance checklists.
  4. Costs
    • Bioretention basins are relatively expensive to build ranging from $5,000 to $10,000 per acre drained (dated-2000). Another suggested approach is to use a cost factor of $3 -$15 per square foot of bioretention surface area.
    • Some variables impacting costs include:
      • Landscape and type of soil found at proposed site (site conditions).
      • Regional land prices. Bioretention practices require the use of relatively larger areas compared to other stormwater management practices.
      • Permit costs.
      • Regional design and construction costs.
    • When examining cost of bioretention basins, remember that:
      • Area used by the basin would most likely be landscaped in a traditional manner. These landscaping costs should be deducted from basin instillation costs to reflect actual cost of installing a bioretention practice.
      • Area utilized as a basin should not be regarded as lost area since basin landscaping can and should be comparable or better in aesthetic appeal to traditional landscaping.
      • Maintenance of bioretention basins should not be substantially greater than that required of an equally sized traditional landscaped area.


    Cost Components for Bioretention Practices
    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
    Topsoil salvage Clearing cost ($/area) x Affected Area
    Site Formation Excavation / grading 4-ft Depth Excavation Cost ($/ acre) x Area (acre) Soil & rock fill material, tunneling
    Hauling material offsite Excavation Cost x (% of Material to be hauled away)
    Structural Components Under-drains Under-drain cost ($/lineal foot) x length of device Pipes, catch-basins, manholes, valves
    Inlet structure ($/structure) or ($/curb cut)
    Outlet structure ($/structure)
    Liner Liner cost ($/square yard) x area of device
    Site Restoration Filter strip Sod cost ($/square foot) x filter strip area Tree protection, soil amendments, seed bed preparation, trails
    Soil preparation Topsoil or amendment cost ($/ acre) x Area (acre)
    Seeding Seeding Cost ($/acre) x Seeded Area (acre)
    Planting / transplanting Planting Cost ($/acre) x Planted Area (acre)
    Annual Operation, Maintenance, and Inspection Debris removal Removal Cost ($/acre) x Area (acre) x Frequency Vegetation maintenance, cleaning of structures
    Sediment removal Removal Cost ($/acre) x Area (acre) x Frequency
    Weed control Labor cost ($/hour) x Hours per visit x Frequency
    Inspection Inspection Cost ($) x Inspection Frequency
    Mowing Mowing Cost ($) x Mowing Frequency
    Source: Minnesota Stormwater Manual 2005,
    Chapter 12-BIO, Table 6 [pdf, 1.8 MB] (Pg 18 of 26)

     

Suggested References –Guidebooks, websites and pamphlets:

[ = pdf file; it will be opened in a new window]

  1. Post-Construction Storm Water Management in New Development & Redevelopment - Bioretention (2002) by USEPA.
    A good overall review of use of bioretention practices. Strong reference section for pursuit by the more curious.
  2. The Urban Small Sites Best Management Practices (BMP) Manual (2003) [ 2.5 MB]
    by the Metropolitan Council of Minnesota’s Twin Cities.
    Detailed information on 40 BMPs for stormwater pollution management with comments on their application in a cold-climate setting; find information on bioretention and a list of suitable plants.
  3. Stormwater Practices for Cold Climates [ 82 KB]
    by Deb Caraco and Richard Claytor of the Center for Watershed Protection for the USEPA-Region 5 in 1997; focuses on adjustments needed to make traditional stormwater management practices work in cold climates; find information on Filter Strips in the chapter on bioretention and biofiltering.
  4. Managing Stormwater: Best Management Practices
    Website produced by Perkiomen Watershed Conservancy and the Environmental Planning Section, Planning Commission Norristown, PA. A nice simple description of parking lot bioretention islands accompanied by a visually descriptive 5.5 minute long video.
  5. Bioretention.com (last updated 2005)
    produced by T.E. Scott & Associates, Inc, consultants for design of stormwater management practices. This is an elaborate site for the more technically inclined, land planners, civil engineers, landscape architects, and environmental professionals. Materials found on the site meet Wisconsin Department of Natural Resource’s published Conservation Practice Standard 1004 for bioretention.
    and (technical note-see link to Bioretention for infiltration)
  6. Georgia Stormwater Management Manual – Volume 2/3.2.3 Bioretention Areas (2001). [ 2 MB]
    An example of one state’s approach to stormwater management. A good overview from a technical viewpoint of design for bioretention areas. Presents examples of design schematics and required design calculations.
  7. State of Minnesota Stormwater Manual (2005) is a valuable tool for stormwater managers. Chapter 12 [ 2.5 MB] of the manual provides details on Bioretention Basins and other stormwater filtration practices applicable to Minnesota that help conserve, enhance, and restores high-quality water in our lakes, rivers, streams, wetlands, and ground water.
  8. Plants for Stormwater Design: Species Selection for the Upper Midwest - a book providing detailed descriptions of plants useful in stormwater management; published by the Minnesota Pollution Control Agency.

Tips and Wisdom

  1. Bioretention basins vary in their effectiveness. Proper design, construction and a sufficient maintenance effort have been found to lead to more successful outcomes. Hire consultants and construction/landscapers that have appropriate technical knowledge backed by past successful experience at installing and maintenance of bioretention basins.
  2. Contact regional agencies involved in stormwater management to determine latest permit requirements for installing bioretention basins.
  3. Research based knowledge continues to accumulate for design and functionality improvements to bioretention basins. Before proceeding with design contact regional agencies involved in stormwater management for latest updates to current practices.
  4. Allow sufficient time for plantings to exhibit vigorous growth and dense cover. If certain plant species don’t appear to be doing well, try another species that exhibits similar properties. Refer to Appendix E: Minnesota Plant List and Application [ 2.2 MB] in the State of Minnesota Stormwater Manual and Plants for Stormwater Design for more information.
  5. Remember to select plants based on the site's climatic regime. Check that the plant choices are appropriate for the site’s USDAs hardiness zone and the level of expected sunlight exposure. Follow the links in the previous tip for more information.
  6. Frozen ground limits the effectiveness of bioretention systems since the filter bed freezes and becomes impervious throughout the winter months. However, other stormwater management systems may also not be practical, but coupling bioretention basins with other Best Management Practices such as snow removal to pervious infiltration areas allows for a successful year round stormwater treatment program. Suggested adjustments [ 1.8 MB] to design can maximize bioretention basin function by preventing freezing of the conveyance system and the filter in cold northern climates, these include:
    • Modification of pretreatment area:
      • Grassed strips should be a minimum of 25 feet long (in direction of flow) this will help remove sand which can quickly clog the filter.
    • Modifications to the underdrain system:
      • Perforated underdrain pipe should have a minimum of an eight inch diameter, this encourages more rapid drainage.
      • Underdrain pipe should have a slope of greater than one percent; this increases the flow of water, decreasing the likelihood of freezing.
      • Underdrain gravel should be a minimum of 18 inches deep; this promotes drainage and also combats frost heave.
    • Couple bioretention basin with other downstream stormwater management practices; this creates redundancy in treatment, but also increases cost.
    • Other practices to improve operation of bioretention basins in cold climates include:
      • Spring maintenance inspections with the purpose of removing excessive sand washed into the basin and repairing other winter damage.
      • Plant salt tolerant plants that are climate appropriate.

Limitations
The uses of bioretention basins have few limitations. However, the following concerns exist:

  1. Bioretention as a stormwater BMP is a relatively new idea and there is minimal long-term performance, operation and management information available.
  2. Use is limited to small drainage areas; these practices are not applicable to larger areas such as Regional Stormwater Control.
  3. In regions with Karst topography (limestone and caves) or in contaminant hot spots the use of an impermeable liner is required to seal the bottom of the basin to prevent drainage into stormwater drains and native waters.
  4. Susceptible to clogging, thus some form of pretreatment is required to help remove suspended solids prior to their deposition on the top of the filter media –mulch and soil.
  5. Take up a lot of land area.
  6. When used in parking lots, these practices may reduce the number of parking spaces.
  7. Relatively costly construction compared to other stormwater management practices.
  8. In cold climates a number of adjustments to design must be made to help facilitate wintertime operation, these will add cost to installing these types of systems.
  9. To discover more information on limitations refer to sections of the State of Minnesota Stormwater Manual dealing with Bioretention practices.
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