Bioretention is a terrestrial-based (up-land as opposed to wetland) water quality and water quantity control process. Bioretention employs a simplistic, site-integrated design that provides opportunity for runoff infiltration, filtration, storage, and water uptake by vegetation.
Bioretention areas are suitable stormwater treatment practices for all land uses, as long as the contributing drainage area is appropriate for the size of the facility. Common bioretention opportunities include landscaping islands, cul-de-sacs, parking lot margins, commercial setbacks, open space, rooftop drainage and street-scapes (i.e., between the curb and sidewalk). Bioretention, when designed with an under-drain and liner, is also a good design option for treating potential stormwater hotspots (PSHs). Bioretention is extremely versatile because of its ability to be incorporated into landscaped areas. The versatility of the practice also allows for bioretention areas to be frequently employed as stormwater retrofits.
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Function within stormwater treatment train
Unlike end-of-pipe BMPs, bioretention facilities are typically shallow depressions located in upland areas of a stormwater treatment train. The strategic, uniform distribution of bioretention facilities across a development site results in smaller, more manageable subwatersheds, and thus, will help in controlling runoff close to the source where it is generated (Prince George’s County Bioretention Manual, 2002). Bioretention facilities are designed to function by essentially mimicking certain physical, chemical, and biological processes that occur in the natural environment. Depending upon the design of a facility, different processes can be maximized or minimized depending on the type of pollutant loading expected (Prince George’s County, 2002).
MPCA permit applicability
One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. Standards for various categories of stormwater management practices must be applied in all projects in which at least one acre of new impervious area is being created.
For regulatory purposes, bioretention practices fall under the “Infiltration / Filtration” category described in Part III.C.2 of the CGP. If used in combination with other practices, credit for combined stormwater treatment can be given as described in Part III.C.4 of the permit. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the bioretention practice will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.
There are situations, particularly retrofit projects, in which a bioretention practice is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular bioretention practice, depending on where it is situated both jurisdictionally and within the surrounding landscape.
Retrofit suitability
The ability to use bioretention as a retrofit often depends on the age of development within a subwatershed. Subwatersheds that have been developed over the last few decades often present many bioretention opportunities because of open spaces created by modern setback, screening and landscaping requirements in local zoning and building codes. However, not every open area will be a good candidate for bioretention due to limitations associated with existing inverts of the storm drain system and the need to tie the underdrain from the bioretention area (for practices requiring an underdrain) into the storm drain system. In general, four to six feet of elevation above this invert is needed to drive stormwater through the proposed bioretention area.
Special receiving waters suitability
The table below provides guidance regarding the use of bioretention practices in areas upstream of special receiving waters. This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated. The corresponding information about other BMPs is presented in the respective sections of this Manual.
Design restrictions for special waters - bioretention
It is Highly Recommended that bioretention practices be designed off-line. off-line facilities are defined by the flow path through the facility. Any facility that utilizes the same entrance and exit flow path upon reaching pooling capacity is considered an off-line facility.
Cold climate suitability
Little research exists on the cold climate effectiveness of bioretention practices. Some believe that bioretention can be of marginal effectiveness for treating snowmelt runoff because of the dormancy of the vegetation during the cold season. However, the incorporation of some sump storage into the design of any bioretention system will provide an opportunity to route and collect snowmelt runoff and begin the filtration and infiltration processes. The incorporation of some storage as part of the system (for example, setting the outlet elevation 6 to 12 inches above the bottom of the bioretention practice) is necessary for this adaptation. Once relatively “warm” snowmelt runoff begins to accumulate in a bioretention system, some downward migration will usually begin and the system will activate. A system that is relatively dry when winter begins will respond more quickly and treat spring runoff more effectively once melt begins. To reduce freeze-up an 8 inch diameter (rather than the standard 6 inch) drain-tile is Recommended for practices requiring an under-drain.
Water quantity treatment
Bioretention practices are not typically suitable for providing water quantity control. It is Highly Recommended that bioretention practices be designed off-line. Off-line facilities are defined by the flow path through the facility. Any facility that utilizes the same entrance and exit flow path upon reaching pooling capacity is considered an off-line facility. However they may be designed to safely pass large storm flows while still protecting the ponding area, mulch layer and vegetation. In limited cases, a bioretention practice may be able to accommodate the channel protection volume, Vcp, in either an off-line or on-line configuration, and in general they do provide some (albeit limited) storage volume. Bioretention can help reduce detention requirements for a site by providing elongated flow paths, longer times of concentration, and volumetric losses from infiltration and evapotranspiration. Experience and modeling analysis have shown that bioretention can be used for stormwater management quantity control when facilities are distributed throughout a site to reduce runoff and maintain the pre-existing time of concentration. This effort can be incorporated into the site hydrologic analysis. Generally, however, it is Highly Recommended that in order to meet site water quantity or peak discharge criteria, another structural control (e.g. detention) be used in conjunction with a bioretention area.
Water quality treatment
Bioretention is an excellent stormwater treatment practice due to the variety of pollutant removal mechanisms including vegetative filtering, settling, evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption. Pollutant removal and effluent concentration data for select parameters are provided in the two tables below. Bioretention can also be designed as an effective infiltration / recharge practice, particularly when parent soils have high permeability (> ~ 0.5 inches per hour). Where soils are not favorable, a rock infiltration gallery can be used to promote slow infiltration / recharge of stored water.
Pollutant removal percentages for bioretention BMPs. Source Winer, 2000.
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| Practice | TSS | Total Phosphorus | Total nitrogen | Metals1 | Bacteria | Hydrocarbons |
|---|---|---|---|---|---|---|
| Bioretention | 802 |
|
50 | 95 | 352 | 80 |
1 Average of zinc and copper
2 Assumed values based on filtering practice performance
Typical pollutant effluent concentrations, in milligrams per liter, for bioretention BMPs. Source Winer, 2000..
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| Practice | TSS | TP | TN | Cu | Zn |
|---|---|---|---|---|---|
| Bioretention | 11 | 0.3 | 1.11 | 0.007 | 0.040 |
1 Assumed values based on filtering practices
Early bioretention facilities were designed to provide water quality benefits by controlling the “first flush” event. Using highly permeable planting soils and an underdrain, however, creates a high-rate biofilter, which can treat 90 to 95 percent (or higher) of the total annual volume of rainfall/runoff. Furthermore, monitoring has shown the pollutant removal rates to be significantly above the estimates presented previously in the Prince George’s County Manual (Prince George’s County, 2002).
Limitations
The following general limitations should be recognized when considering installation of a bioretention practice:
- Emerging stormwater management technique with limited long-term experience available;
- Maintenance personnel may need additional instruction on routine Operation and Maintenance requirements; and
- Minimal long-term performance, operation and management information.