Overview for bioretention
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Overview for bioretention

A rain garden in a residential development. Photo courtesy of Katherine Sullivan.

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 underdrain 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.

Function within stormwater treatment train

Unlike end-of-pipe BMPs, bioretention facilities are typically shallow depressions located in upland areas of a 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).

Green Infrastructure: bioretention facilities are designed to mimic a site's natural hydrology

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, bioinfiltration practices fall under Infiltration systems described in the CGP. Biofiltration practices fall under Filtration systems of the permit. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance 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, 4 to 6 feet of elevation above this invert or use of an upturned elbow 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. Note that the suitability of a bioretention practice depends on whether the practice has an underdrain (i.e. filtration vs. infiltration practice).

Infiltration and filtration bmp¹ design restrictions for special waters and watersheds. See also Sensitive waters and other receiving waters.
Link to this table

BMP Group	receiving water
BMP Group	A Lakes	B Trout Waters	C Drinking Water²	D Wetlands	E Impaired Waters
Infiltration	RECOMMENDED	RECOMMENDED	NOT RECOMMENDED if potential stormwater pollution sources evident	RECOMMENDED	RECOMMENDED unless target TMDL pollutant is a soluble nutrient or chloride
Filtration	Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments	RECOMMENDED	RECOMMENDED	ACCEPTABLE	RECOMMENDED for non-nutrient impairments

¹Filtration practices include green roofs, bmps with an underdrain, or other practices that do not infiltrate water and rely primarily on filtration for treatment.
² Applies to groundwater drinking water source areas only; use the lakes category to define BMP design restrictions for surface water drinking supplies

It is Highly Recommended that bioretention practices be designed offline. Offline 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 offline facility.

Cold climate suitability

Studies conducted since the 2008 version of this manual indicate the difference between summer and winter performance of bioretention systems is not substantial, even on sites with severe winters (Shanstrom, 2012; Davidson, et al., 2008; Dietz and Clausen, 2006; Kahn et al., 2012; LeFevre et al., 2009; Roseen et al., 2009; Toronto and Region Conservation (TRCA), 2008). Davidson et al. (2008) provide several recommendations for bioretention systems in cold climates. These recommendations are consistent with design recommendations in the Minnesota Stormwater Manual.

Water quantity treatment

High-flow bypass systems are utilized to safely discharge stormwater when bioretention cells fill and reach their maximum ponding depth. This will occur during storms exceeding the Water Quality Volume design storm. There are typically three types of high-flow bypass systems which are split into two categories: offline and online. Whenever possible, offline designs are preferable, as they reduce the potential for internal erosion in the bioretention cell. Offline facilities are defined by the flow path through the bioretention cell. Any facility that utilizes the same entrance and exit point upon reaching maximum ponding depth is considered an offline system. This is typically achieved with a curb cut set at the intended elevation of maximum ponding or through the use of some other upstream diversion, which results in flow bypass down the gutter when the cell has filled. This type of bypass is often simple to utilize in retrofit situations (commercial and transportation applications) where existing drainage infrastructure is present.

Where offline designs are not achievable, it is Highly Recommended that bioretention practices be designed to route high flows on the shortest flow path across the cell to avoid scour in the bioretention practice. The overflow location should be placed as close as practicable to the inlet(s). No matter the bypass design, energy dissipation should always be provided at the inlet(s) to avoid high flow velocity and associated turbulence that can re-suspended particulates and cause erosion in the bioretention cell.

Two types of online bypass systems may be used. The first option is to utilize an internal drainage inlet. Concrete box drop structures may be used to provide an overflow for bioretention cells; however, they should be located away from the inlet(s) to provide an elongated flow path and prevent short-circuiting. These internal drainage structures may be tied into the existing drainage infrastructure, which is an attractive benefit in commercial applications. When using these high-flow bypass devices, it is critical to set the brink-of-overflow elevation properly, otherwise the cell will not function properly when construction is complete. In a tree-shrub-mulch cell, the internal drainage inlets should have a system of screens to prevent loss of mulch. These overflow devices should be designed to safely pass the design discharge.

A second option is to use a broad crested or compound weir in the berm of the bioretention cell to convey overflow. This will typically be the best option in residential, institutional, and rural bioretention applications, where the overflow can tie in to an existing surface conveyance ( swale or ditch). Weir structures may be constructed of pressure-treated lumber, cast-in-place concrete, or precast concrete. The invert of the weir should be set at the intended brink-of-overflow elevation. This type of bypass structure should be designed to non-erosively bypass the design discharge.

In limited cases, a bioretention practice may be able to accommodate the channel protection volume, V_cp, in either an offline or online 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.

No matter the type of overflow device used, it is important that the designer provide non-erosive flow velocities at the outlet point to reduce downstream erosion. During the 10-year or 25-year storm (depending on local drainage criteria), discharge velocity should be kept below 4 feet per second for grassed channels. Erosion control matting or rock should be specified if higher velocities are expected.

Water quality treatment

Bioretention can 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.

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.

Caution: The information in the tables below will be updated in summer of 2014. Ranges will be provided rather than a single number because the data are highly variable.

Median pollutant removal percentages for several stormwater BMPs. Sources. More detailed information and ranges of values can be found in other locations in this manual, as indicated in the table. NSD - not sufficient data. NOTE: Some filtration bmps, such as biofiltration, provide some infiltration. The values for filtration practices in this table are for filtered water.
Link to this table

Practice	TSS	TP	PP	DP	TN	Metals¹	Bacteria	Hydrocarbons
Infiltration²	³	³	³	³	³	³	³	³
Biofiltration and Tree trench/tree box with underdrain	80	link to table	link to table	link to table	50	35	95	80
Sand filter	85	50	85	0	35	80	50	80
Iron enhanced sand filter	85	65 or 74⁶	85	40 or 60⁶	35	80	50	80
Dry swale (no check dams)	68	link to table	link to table	link to table	35	80	0	80
Wet swale (no check dams)	35	0	0	0	15	35	35	NSD
Constructed wet ponds^{4, 5}	84	50 or 68⁵	84	8 or 48⁵	30	60	70	80
Constructed wetlands	73	38	69	0	30	60	70	80
Permeable pavement (with underdrain)	74	41	74	0	NSD	NSD	NSD	NSD
Green roofs	85	0	0	0	NSD	NSD	NSD	NSD
Vegetated (grass) filter	68	0	0	0	NSD	NSD	NSD	NSD
Harvest and reuse	Removal is 100% for captured water that is infiltrated. For water captured and routed to another practice, use the removal values for that practice.

TSS=Total suspended solids, TP=Total phosphorus, PP=Particulate phosphorus, DP=Dissolved phosphorus, TN=Total nitrogen
¹Data for metals is based on the average of data for zinc and copper
²BMPs designed to infiltrate stormwater runoff, such as infiltration basin/trench, bioinfiltration, permeable pavement with no underdrain, tree trenches with no underdrain, and BMPs with raised underdrains.
³Pollutant removal is 100 percent for the volume infiltrated, 0 for water bypassing the BMP. For filtered water, see values for other BMPs in the table.
⁴Dry ponds do not receive credit for volume or pollutant removal
⁵Removal is for Design Level 2. If an iron-enhanced pond bench is included, an additional 40 percent credit is given for dissolved phosphorus. Use the lower values if no iron bench exists and the higher value if an iron bench exists.
⁶Lower values are for Tier 1 design. Higher values are for Tier 2 design.

Typical pollutant effluent concentrations, in milligrams per liter, for bioretention BMPs. Source Winer, 2000..
Link to this table

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 creates a high-rate biofilter, which can treat 90 to 95 percent (or higher) of the total annual volume of rainfall/runoff, depending on the design.

Limitations

Bioretention practices have been widely utilized for the past decade. Data suggests that these practices, when properly designed, constructed and maintained, perform well over long periods of time. However, design, construction and maintenance of these practices can be complex. In particular, maintenance personnel may need additional instruction on routine Operation and Maintenance requirements.

References

Davidson, J.D., M. Isensee, C. Coudron, T. Bistodeau, N.J. LeFevre, and G. Oberts. 2008. Recommendations to Optimize Hydrologic Bioretention Performance for Cold Climates. WERF Project 04-DEC-13SG.
Dietz, M.E. and Clausen, J.C. 2006. Saturation to improve pollutant retention in a rain garden. Environmental Science and Technology. Vol. 40. No. 4. pp. 1335-1340.
Kahn, U.T., C. Valeo, A. Chu, and B. van Duin. 2012. Bioretention cell efficacy in cold climates: Part 2 — water quality performance. Canadian Journal of Civil Engineering. 39(11):1222-1233.
LeFevre, N.J., J. D. Davidson, and G. L. Oberts. 2009. Bioretention of Simulated Snowmelt: Cold Climate Performance and Design Criteria. Proceedings of the 14th Conference on Cold Regions Engineering.
Roseen, R.M., Ballestro, T.P., Houle, J.J., Avelleneda, P., Briggs, J., Fowler, G., and Wildey, R. 2009. Seasonal Performance Variations for Storm-Water Management Systems in Cold Climate Conditions. Journal of Environmental Engineering. Vol. 135. No. 3. pp. 128-137.
Toronto and Region Conservation (TRCA). 2008. Performance Evaluation of Permeable Pavement and a Bioretention Swale, Seneca College, King City, Ontario. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.

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