Bioretention basins are shallow landscaped depressions used to slow and treat storm water runoff. Storm water is directed to the basin and then percolates through the soil or designed growing medium, where it is treated by a number of physical, chemical, and biological processes. The slowed, cleaned water infiltrates into native soils or is directed to nearby storm water drains or receiving waters.
Bioretention is a terrestrial-based (upland as opposed to wetland) water-quality and water-quantity control process. Bioretention employs a simple, site-integrated design that provides opportunity for storm water runoff infiltration, filtration, storage, and water uptake by vegetation.
Bioretention areas are storm-water treatment practices suitable for all land uses as long as the contributing drainage area is appropriate for the size of the facility; the basin floor is typically 5% to 10% of the impervious catchment area. When designed with an underdrain and liner, bioretention is also an option for treating potential storm water hotspots.
Healthy vegetation planted in a bioretention basin increases infiltration of storm water runoff. Plants can lose 30% of their root structures annually, which produces macropores in the substrate. Macropores increase the infiltration rate of the soil so that more runoff is infiltrated. Additionally, dense vegetation reduces overland flow velocities and therefore reduces erosion and re-suspension of captured solids. Vegetation also aids in the breakdown of pollutants and uptake of nutrients in storm water runoff. Typically, the basins are planted with a mix of native deep-rooted perennials and/or shrubs and trees that are adapted to growing in periodically wet and dry conditions. A developing dominance of wetland plants can be an indication that the area is not draining properly.
Bioretention basins are often amended with imported soil medium and/or native subsoils that are conducive to infiltration. Once the soil pore space capacity of the medium is exceeded, storm water begins to pool at the surface but is drained down within 48 hours if the basin is properly designed, implemented, and maintained. Pollutants are removed by a number of processes including adsorption, filtration, volatilization, ion exchange, and decomposition. Absorbed runoff can either be allowed to infiltrate into the surrounding soil or can be filtered to an underdrain system and then discharged.
Bioretention basins can be broken into the following subcategories:
Simple, small excavated basins with no soil replacement, located on native soils that are conducive to infiltration.
Engineered basins with soil replacement, located on native subsoils that are conducive to infiltration.
Engineered basins with soil replacement, located on subsoils with insufficient infiltration rates (or other subsurface restrictions), designed to achieve 48-hour maximum drawdown time and relying on an underdrain for filtration.
Function within Storm-Water Treatment Train
Unlike end-of-pipe practices such as storm water ponds, bioretention areas are typically shallow depressions located in upland areas of a storm-water treatment train. The strategic, uniform distribution of bioretention areas across a site results in smaller, more manageable catchment areas and thus will help in controlling runoff close to the source where it is generated. Bioretention areas are designed to function by essentially mimicking certain physical, chemical, and biological processes that occur in the natural environment. Depending on the design of an area, different processes can be maximized or minimized depending on the type of pollutant loading expected (Prince George's County, 2002).
Bioretention areas benefit from pretreatment to prevent clogging by the sediment carried in the storm water runoff. Pretreatment devices, such as vegetated filter strips, dry filter boxes, sump catch-basin structures, and other underground treatment devices, trap sediment where it can be relatively easily removed prior to its entering a treatment practice.
Overflow structures provide a controlled outlet for storm water volumes above the design volume. Ideally a bioretention basin is designed so that when the basin has reached the design volume during a storm event, additional runoff flows past the inlet and basin. Ideally, bioretention basins receive runoff from storm events of 1 inch or less and are able to safely bypass the flows from any larger storm events.
Bioretention is extremely versatile because it can be incorporated into landscaped areas. Common bioretention opportunities include parking lot islands, perimeters, medians, and other suitable open space.
To minimize the possibility of drainage seeping under the pavement section and creating "frost heave" during winter months, a buffer may be necessary along the outside curb perimeter. Alternatively, the installation of a geotextile filter fabric "curtain wall" along the perimeter of the bioretention island will accomplish the same effect.
Curbless Parking Lot Islands
A "ribbon curb" can be installed around the perimeter and around islands of a parking lot. This application should only be attempted where shallow grades allow for sheet flow conditions and a filter strip application. Boulders or posts installed around the perimeter of the island may be necessary on PWA sites to prevent vehicles from driving over the bioretention basin.
Parking Lot Islands with Curb Cut
Curb cut situations with concentrated flow may require the installation of a dry filter box, or at least a concrete apron as a pretreatment to capture accumulating sediment before it can enter the bioretention area.
Road Medians and Perimeter Areas
A multifunctional landscape with small catchment areas can be created by using ribbon curbs along road medians and perimeter areas to allow sheet flow into mowed filter strips.
Cold Climate Suitability
A three-year research study conducted simulated snowmelt events to measure the infiltration responses of four existing bioretention cells under Minnesota winter conditions. The study revealed a dramatic range of performance, including rapid infiltration during varying cold climate conditions, and proved that three of the four bioretention cells were hydrologically active during winter most of the time. A fact sheet is also available that translates the study findings into practical design, installation, and maintenance recommendations that storm water professionals can easily apply to optimize the hydrologic performance of bioretention cells for successful operation under cold (and warm) climate conditions. (Hydrologic Bioretention and Performance and Design Criteria for Cold Climates, Dakota County Soil and Water Conservation District, 2008).
Bioretention areas should not be used as dedicated snow disposal areas. Excessive snow can cause soil compaction and may contain sand and salt, which can damage the vegetation. Thought should be given to planting salt-tolerant species if excessive road salt will be an issue.
Water Quantity and Water Quality Benefits
Bioretention is a terrestrial-based water-quantity and water-quality control process. Bioretention employs a simple, site-integrated design that provides opportunity for runoff infiltration, filtration, storage, and water uptake by vegetation.
Bioretention is an excellent storm-water treatment practice due to the variety of pollutant removal mechanisms, including vegetative filtering, settling, evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption.
Inspection activities, maintenance, and warranties should be written into the construction contract to assure that the bioretention area is properly installed. Construction Inspection Checklist
If the native soils are not conducive to infiltration, an "engineered" soil mixture comprised of sand and compost is used to promote rapid infiltration and good plant growth. Underdrains may be added to compensate for poorly suited subsoil (e.g., clay/tight soils, shallow groundwater or bedrock proximity, lined basin over karst geology). Adjustable gate valves, cleanouts, and inspection/observation wells are frequently part of the underdrain system.
Performance Specifications for All Bioretention Media (Minnesota Storm Water Manual 2014)
- The growing media must be suitable for supporting vigorous growth of selected plant species.
- The pH range (soil/water 1:1) is 6.0 to 8.5.
- Soluble salts (soil/water 1:2) should not exceed 500 parts per million.
- All bioretention growing media must have a field-tested infiltration rate between 1 and 8 inches per hour. Growing media with slower infiltration rates may clog over time and may not meet drawdown requirements. Target infiltration rates should be no more than 8 inches per hour to allow for adequate water retention for vegetation and for pollutant removal. The following infiltration rates should be achieved if specific pollutants are targeted in a watershed:
- Total suspended solids: any rate is sufficient; 2 to 6 inches is recommended
- Pathogens: any rate is sufficient; 2 to 6 inches is recommended
- Metals: any rate is sufficient; 2 to 6 inches is recommended
- Temperature: slower rates are preferable (less than 2 inches per hour)
- Total nitrogen (TN): 1 to 2 inches per hour; 1 inch per hour is recommended
- Total phosphorus (TP): 2 inches per hour
The following additional bioretention growing media performance specifications are required to receive phosphorus reduction credit:
- Option A: use bioretention soil with phosphorus content between 12 and 36 milligrams per kilogram when using the Mehlich-3 test
- Option B: include a soil amendment that facilitates adsorption of phosphorus
Guidance for Bioretention Media Composition
- Mix B, enhanced filtration blend, is a well-blended, homogenous mixture of 70% to 85% construction sand, and 15% to 30% organic matter.
- Sand provided should be clean construction sand, free of deleterious materials: AASHTO M-6 or ASTM C-33 washed sand.
- MnDOT specifications 3890 Grade 2-certified compost is recommended for organic matter (this compost shall not contain any biosolid/mixed municipal compost/animal manure components).
- It is assumed this mix will leach phosphorus. When an underdrain is utilized, a soil phosphorus test is needed to receive water quality credits for the portion of storm water captured by the underdrain. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram when using the Mehlich-3 test. This is enough phosphorus to support plant growth without exporting phosphorus from the cell.
Bioretention areas may have an initial period of approximately 3 to 5 years during which more frequent inspection and maintenance are required while the plants establish. All maintenance activities need to be undertaken in a manner that does not lead to soil compaction. Such compaction will reduce vertical water flow rates and can cause damage to an underlying drain tile system. Operation and Maintenance Checklist