Selected BMPs: Bioretention

Definition: Two general types of bioretention facilities exist: off-line areas and on-line areas. Off-line bioretention areas consist of sand and soil mixtures planted with native plants, which receive runoff from overland flow or from a diversion structure in a traditional drainage system. On-line bioretention areas have the same composition as off-line areas, but are located in grass swales or other conveyance systems that have been modified to enhance pollutant removal by quiescent settling and biofiltration.

Purpose: Bioretention is an efficient method for removing a wide variety of pollutants, such as suspended solids and nutrients. It can also be an effective means of reducing peak runoff rates and recharging groundwater by infiltrating runoff. However, not all bioretention facilities will necessarily bey optimized for all of these functions. The purpose of this practice is to filter contaminants out of runoff before it reaches receiving water.

Picture
(Adapted from Prince George’s County, 1993)

Application: Bioretention applications include:

Bioretention areas, consisting of sand and soil mixtures planted with native plants, which filter urban runoff, can be used in residential and nonresidential developments. Sources of runoffflow can be overland flow from impervious areas or discharge runoff from small storms diverted from a drainage pipe.

Onpipe. Also, on-line bioretention facilitiesconcepts are use incorporated in grass swales by including check dams or other barriers to retain flow in grass swalesto flow in residential and nonresidential developments.

Figure 2 shows a gabion used as a check dam to create a bioretention area.

Bioretention facilities are most effective if they receive runoff as close as possible to the source. A site designer needs to look for opportunities to incorporate bioretention facilities throughout the site and minimize the use of inlets, pipes, and downstream controls. Prince George’s County, Maryland, which initially developed the bioretention concept, reports saving as much as 50 percent on drainage infrastructure costs in developments that incorporate bioretention facilities.

Bioretention should not be used in areas with the following characteristics:

• The water table is within 6 feet of the land surface (the use of collector pipes may reduce this limitation).
• Mature trees would be removed for constructing the bioretention area.
• Slopes are 20 percent or greater.
• An unstable soil stratum is in the proximity.

Similar savings have been achieved in Prince William County, Virginia, in sections of developments with bioretention facilities.

Off-Line

Off-line bioretention facilities can be applied to most development situations. They are particularly applicable in urban areas where the opportunities and the land available for controlling stormwater reliably are scarce. Bioretention facilities may be installed in median strips, parking lot islands, or lawn areas of commercial developments. They also can be used in residential subdivisions with open drainage systems or in easements located around lots. Figure 1 shows a bioretention area receiving runoff diverted from a storm sewer.

Figure 1. Storm sewer diversion into bioretention area

On-Line

On-line bioretention facilities use check dams to "collect" the water in the bioretention area, as shown in Figure 2. Adding a bioretention area behind the check dam allows filtering and sedimentation to occur. Check dams should only be used in small open channels or in filter strips that drain 5 acres or less. Runoff from storms larger than the water quality design storm should safely flow over or bypass the bioretention area.

Figure 2. Bioretention cross section; bioretention facility incorporated in a grass swale with flat to mild slope.

Recommended Design Criteria:

Requirements for Regulatory Compliance

(none specified)

Performance-Based Guidelines

Bioretention facilities should be optimized to treat the runoff generated by the of a water quality design storm. If a water quality storm is not specified, then the 6-month return-frequency storm should be used. . The peak discharge from larger storms should be bypassed, if possible.

For off-line bioretention systems, the Prince George’s County’s design manual recommends using planting soil of at least ranging from 10 to 25 percent clay along with sandy loam, loamy sand, or loam texture. The soil pH should range between 5.5 and 6.5. The soil should be placed in lifts less than 18 inches and lightly compacted by tamping with a bucket from a bulldozer or a backhoe. A desirable planting soil would:

• Be permeable to allow infiltration of runoff
• Adsorb organic nitrogen and phosphorus

In areas where clay contents are higher and the soil is not conducive to infiltration, the bioretention facility can be modified with a collector pipe system installed beneath the basin to form a bioretention filter. The City of Alexandria has developed design guidelines for bioretention filters (City of Alexandria, 1995) and collector pipes for areas of clay soil.

Bioretention areas can be used successfully in a wide range of drainage areas. Median strips, ramp loops, and parking lot islands are examples of small drainage areas (less than 1 acre). In large drainage areas (less than 10 acres), diversion structures and energy dissipation devices need to be incorporated into the design to preserve the integrity of the bioretention area.

The Prince George’s County’s design manual recommends that the size of the bioretention area be 5 percent to 7 percent of the drainage area multiplied by the c coefficient of the rational formula. Smaller and larger ranges are being constructed in Virginia. Ongoing Mmonitoring data will provide better guidance on the design of these facilities. The land required for bioretention facilities can be reduced by partially substituting vertical-extended detention storage for horizontal storage.

Check dams, as shown in Figure 2, reduce the velocity of concentrated stormwater flows, promoting sedimentation behind the dam. If properly anchored, railroad ties, gabions, or rock filter berms may be used as check dams. The use of railroad ties is shown in Figure 3. The use of gabions as a drop structure is shown in Figure 4. These types of structures can be used in swales with moderate slopes.

Figure 3. Bioretention cross section; bioretention facility incorporated in a grass swale with mild to moderate slope.

Check dams must be sized and constructed correctly and maintained properly, or they will be either washed out or contribute to flooding. The relationship between ponding depth and discharge rate can be computed by using the critical-depth formula, which accounts for a generalized weir profile. The relevant equation is:

Q = ( (A³ x g) /T) ^½

Where: Q = discharge rate

A = area subtended by top of check dam and ponding elevation
T = width of check dam
g = gravitational constant

Check dams can be constructed of either rock or logs. The use of other natural materials available on the site that can withstand the stormwater flow velocities is acceptable. Check dams should not be constructed from straw bales or silt fences, because concentrated flows quickly wash out these materials.

Maximum velocity reduction is achieved if the toe of the upstream check dam is at the same elevation as the top of the downstream dam. The center section of the dam should be lower than the edge sections to minimize the potential for erosion of the abutments during frequently occurring storm events.

Figure 4. Bioretention cross section; bioretention facility incorporated in a grass swale with mild to moderate slope.

Bioretention facilities can be incorporated in an overall site plan for capturing runoff and recharging groundwater. Comparatively small, frequently occurring storms are most appropriate for establishing design criteria where ground recharge occurs (see Appendix F___,, Runoff Capture Design). The appropriate design criterion is the maintenance of the total annual runoff volume below a fixed value (usually the predevelopment runoff volume). Additional information on how to use this criterion for designing bioremediation facilities is in "Specifications and Methodology."

Operation and Maintenance: Monthly inspections are recommended until the plants are established. Annual inspections should then be adequate.

Accumulated sediment behind check dams should be removed when it reaches one-half the sump depth.

Considerations: Collector pipe systems, if used, in bioretention areas can become clogged by underlying clay soil. Pipe cleanouts are recommended to facilitate unclogging of the pipes without disturbing the bioretention areas.

References:

Bitter, S.D. and J.K. Bowers. "Bioretention as a Water Quality Best Management Practice." Watershed Protection Techniques. Vol. 1, No. 3. 1994.

Pennsylvania State University. The Agronomy Guide. Collete of Agricultural Sciences. (most recent)

City of Alexandria (Virginia). Alexandria Supplement to the Northern Virginia BMP Handbook, Section XIII, Bioretention and Bioretention Filters. Department of Transportation and Environmental Services. Alexandria, VA. February 1994.

Pennsylvania State University. The Agronomy Guide. College of Agricultural Sciences. (latest edition.).

Prince George’s County Government. Design Manual for Use of Bioretention in Stormwater Management. Prepared by Engineering Technologies Associates, Inc. and Biohabitats, Inc. 1993.

Reeves, E. "Performance and Condition of Biofilters in the Pacific Northwest." Watershed Protection Techniques, Vol. 1, No. 3. Fall 1994.

Schueler, T. R. "How to Design Urban Stormwater Best Management Practices," Seminar Notebook. 1995.

Specifications and Methodology:

Planting Plan

The use of plants in bioretention areas is intended to replicate a terrestrial forest community ecosystem. The components of this community include tress, shrub layer, and a herbaceous layer. Native plants selected from Appendix H (tables H-2 and H-3) should be able to tolerate typical stormwater pollutant loads, variable soil moisture, and ponding fluctuations (Prince George’s County, 1993). Designers are encouraged to check other sources, such as tThe Agronomy Guide, the Field Office Technical Guide, and local nurseries to identify plants that can adapt to specific site conditions.

The layout of the plant material should resemble a random and natural placement of plants rather than a standard landscaped approach with trees and shrubs in rows or other orderly fashion. The location of the plan material should provide optimal conditions for plant establishment and growth (Prince George’s County, 1993).

Off-Line Bioretention Areas

There are six major components to the bioretention area:

• Grass buffer strip or energy dissipation area
• Ponding or treatment area
• Planting soil
• Sand bed (optional)
• Organic layer
• Plant material

The grass buffer strip or energy-dissipation area filters particles from the runoff and reduces its velocity. The sand bed further slows the velocity of the runoff, spreads the runoff over the basin, filters part of the water, provides positive drainage to prevent anaerobic conditions in the planting soil, and enhances exfiltration from the basin.

The ponding area functions as storage area for runoff awaiting treatment and as presettling basin for particulates that have not been filtered out by the grass buffer. The organic or mulch layer acts as a filter for pollutants, protects the soil from eroding, and is an environment for microorganisms to degrade petroleum-based compounds and other pollutants.

The planting soil layer nurtures the plants with stored water and nutrients. Clay particles in the soil adsorb heavy metals, nutrients, hydrocarbons, and other pollutants. The plant species are selected on the basis of their documented ability to cycle and assimilate nutrients, pollutants, and metals through the interaction among plants, soil, and organic layer (Bitter and Bowers, 1994). The minimum depth of the planting soil layer should be 3 to 4 feet.

The number of tree and shrub plantings may vary, especially in areas where aesthetics and visibility are vital to site development, and the density should be determined on an individual site basis. The minimum and maximum number of individual plants and spacing recommended by Prince George’s County are shown in the following table. A minimum of three species of trees and three species of shrubs should be selected to ensure diversity.

As with any experimental BMP, sizing rules are continually changing. Although the site requirements will determine the actual dimensions, the following dimensions are recommended for bioretention areas:

• Minimum width is 10 to 15 feet.
• Minimum length is 30 to 40 feet.
• The ponded area should have a maximum depth of 6 inches. If collector pipes are used, the maximum pond depth can be increased to 12 inches.
• The planting soil should have a minimum depth of 4 feet.

Figures 5 and 6 show a profile and plan of a typical bioretention area. A curb diversion structure that can be installed to divert gutter flow to a bioretention area is shown in Figure 7.

Figure 5. Bioretention cross section: Runoff from large storms (greater than a 1-year storm) is bypassed through the main drainage system. Runoff from small storms is diverted at the control structure (manhole). The energy of the stormwater flow is dissipated by the splash block or the rip rap. The stormwater is filtered through an open sand filter. Excess stormwater is treated in the bioretention area.

Figure 6. Bioretention plan view

Figure 7. Plan and section views of a curb diversion structure (Prince George’s County, 1993)

On-Line Bioretention Areas

A bioretention area upstream of a check dam is constructed with similar specifications as the off-line bioretention areas. The depth of the planting soil zone can be reduced (1 to 2 feet) if the drainage area is small (less than 2 acres).

Rock check dams usually are constructed of approximately 8- to 12-inch rock The rock is placed either by hand or mechanically, but never just dumped into the swale. The dam must completely span the ditch or swale to prevent being washed out. The rock used must be large enough to stay in place, given the expected design flow through the channel.

Log check dams usually are constructed of 4- to 6-inch-diameter logs and are illustrated in Figure 3. The logs should be embedded into the soil at least 18 inches. Gabion applications are illustrated in Figures 2 and 4.

Design Methodology for Controlling Runoff Volume

The runoff capture volume is the minimum volume of rainfall that must be retained and completely infiltrated on site during every storm. It is also equal to the rainfall quantity associated with the runoff capture design storm. Runoff capture criteria are discussed in Section 5.3 of the Handbook and in Appendix F, (Runoff Capture Design).

The runoff capture volume is conveniently stated as a rainfall depth, in inches, over the area of the site. For example, to achieve a suitable level of groundwater recharge, the determination may be that a minimum of 0.75 inches of rainfall from every storm should be detained and infiltrated. In this example, all rainfall events with less than 0.75 inches of rainfall should be completely infiltrated.

Analysis of the site using the approach described in Appendix F will establish the total runoff capture storage that must be provided by infiltration BMPs at a particular siteThe criterion for controlling runoff volume generally is stated as a minimum volume of storm capture that must be completely infiltrated on the site during every storm. Although the methodology for developing the criterion may be involved, the approaches for complying generally are simple (see Appendix ___, Runoff Capture Design). For example, to achieve a suitable level of groundwater recharge, the determination may be that a minimum of 0.75 inches of rainfall from every storm should be retained and infiltrated. All rainfall events of less than 0.75 inches should be infiltrated completely.

The criterion may include a design storm that shows how the 0.75 inches of rainfall should be distributed in the design calculation. If no design storm is used, a conservative basis for the analysis is to distribute the storm capture volume over a 30-minute period. In this example, the design storm would have an average intensity of 1.5 inches/hour and a duration of 30 minutes. By using TR-55 (or a similar runoff simulation algorithm), the amount of surplus runoff that is generated from the site during the design storm can be determined. Alternatively, the approach described in Appendix ___ (Runoff Capture Design) can be used for evaluating the runoff capture requirements for the site.

Bioretention facilities are effective measures for increasing the runoff capture capability of the site. In general, the retention storage volume of appropriately located bioretention areas can be applied todirectly in satisfying the runoff capture storage requirementminimum storm capture volume for the site. The combined storage volume requirement for bioretention can be determined directly from this analysis..

Bioretention facilities are effective measures for increasing the runoff capture capability of the site. Other methods that can be used to improve runoff capture and infiltration include:

• Installing permeable pavement
• Installing infiltration trenches or dry wells
• Modifying the site design to decrease imperviousness

Design Methodology for Runoff Peak Attenuation

Only bioretention facilities with large retention storage capacities will be effective in controlling runoff peak discharge rates. To predict a change in peak runoff, the Natural Resources (formerly Soil) Conservation Service’s (NRCS) methodology (USDA, 1986) can be used. This methodology includes the so-called soil cover complex and nondimensionalized unit hydrograph techniques and is implemented in a variety of computer simulation packages. Alternative methodologies, including kinematic wave runoff routing and synthetic unit hydrograph generation, also are available in various computer software packages.

By retaining runoff during the initial stages of a storm, bioretention these facilities practices can significantly reduce peak runoff rates. With these measures implemented, runoff from the site will be delayed until the storage capacity of the facilities is exceeded. When using the NRCS methodology, TR-55 this effect can be accounted for as an increase in the initial abstraction, Ia, for the drainage subarea in which the facility is located. The relationship can be expressed as follows:

where:

V = Rainfall volume (inches, over the drainage area)

R = Runoff volume (inches, over the drainage area)

Ia = InitialIntial abstraction (inches, over the drainage area)

S = Potential maximum retention after runoff begins (inches, over the drainage area)

CN = NRCS runoff curve number

Ia can be approximated as the combined runoff capture storage divided by the surface area of the drainage subarea. An increase in Ia results in an increase in a coefficient in TR-55. The coefficient describes the ratio of Ia to the potential maximum retention, S. The effect will be more important for small runoff peak attenuation design storms than larger storm..

Some bioretention facilities may also include peak attenuation storage. The effectiveness of these facilities in attenuating runoff peak rates must be evaluated according to procedures described in DRY POND. In addition, the impacts of large flows and velocities on the plant material need to be carefully evaluated before using bioretention facilities as peak attenuation facilities. Bioretention facilities with small drainage areas (i.e., less than 0.25 acres) may be effective for peak attenuation if they are installed throughout a subdivision or non-residential development.