Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring
Fact Sheet - Bioretention
Bioretention was developed as an innovative approach in the ultra-urban environment. Bioretention areas (BAs) are easy to construct and require less infrastructure maintenance than many other BMPs. In addition to their well-accepted aesthetic value, BAs can be tailored in design and location to fit into the ultra-urban landscape.
Water quality improvements result from sedimentation, filtration, soil adsorption, microbial decay processes, and the uptake of pollutants by plants. The use of vegetation in BAs is modeled from the properties of a terrestrial forest community-an ecosystem dominated by mature trees, subcanopy of understory trees, shrubs, and herbaceous plants. Plants are selected based on their tolerance to varying hydrologic conditions, soil and pH requirements, and general characteristics like aesthetics. An additional important feature of bioretention is the soil in the system, which contains a mixture of detritus, humus, and mineral and biological complexes. The soil layer and the microbes living in the soil enhance infiltration, groundwater recharge, and nitrogen and metals removal; provide valuable water and nutrients for plant growth; and provide oxygen for plant root metabolism and growth.
BAs consist of a flow-regulating structure that processes inflow passing through a shallow depressed planted area containing ground cover (low-lying plant growth or an organic mulch), a planting soil supporting a range of facultative plant types, and a bottom support soil layer. Each of these features has a specific role in stormwater pollutant removal (Figure 6).
BAs have unique features that make them attractive for use in the ultra-urban environment. They have the ability to fit in existing or proposed medians or grassy areas along streets and parking lots. In addition, by disposing of a significant volume of annual rainfall on-site, BAs may reduce the infrastructure costs required to collect and convey the runoff off-site. BAs can also provide benefits other than stormwater management, including creating green areas and natural habitat. For facilities placed in new developments, the land area requirement and cost can be minimized if the local jurisdiction considers BAs part of the required vegetated open space set-aside or if installed trees count against local landscaping and tree coverage requirements.
Limited monitoring of the effectiveness of BAs has been completed to date although there are ongoing monitoring efforts. Due to the similarity between bioretention technology and dry swales, however, the pollutant removal capability should be comparable (Claytor and Schueler, 1996). For planning purposes it is acceptable to anticipate BAs will remove 50 percent of total phosphorus (TP), 50 percent of total nitrogen (TN), between 75 and 80 percent of metals, and 75 percent of total suspended solids (TSS). Based on the nature of the planting soil and the facultative plants normally installed, BAs should be capable of managing some petroleum hydrocarbon concentrations commonly encountered in urban settings. Pretreatment is not considered crucial to the removal performance of BAs except where there is an atypically high level of pollutant loading, which can harm the planted growth (i.e., heavy commercial or industrial settings).
In variable climates, seasonal differences in removal performance should be anticipated for BAs, due to the growing and dormant periods of plants. Fall and winter temperatures force vegetation into dormancy, thereby reducing uptake of some runoff pollutants. However, carefully selected planting soil should provide significant storage capacity for many common urban pollutants during no/slow growth periods as long as soil infiltration can occur. Freezing temperatures greatly reduce infiltration in BAs and inactivate the most important pollutant removal mechanism.
BAs are intended to be water quality control practices, but they can be employed as either an on-line or off-line design. If BAs are employed as on-line facilities, design features must be incorporated to ensure nonerosive flow velocities exist within the BA. During these larger rainfall events, BAs should provide marginal treatment of the high flow volume (principally large-diameter suspended solids) even though the residence time in most facilities will be short.
Siting and Design Considerations
Bioretention is a relatively new technology being refined to achieve maximum water quality benefits. The basic design elements and major components of BAs are discussed below. For design examples and additional information, several good sources are available, including Design Manual for Use of Bioretention in Stormwater Management (Prince George's County, 1993), Design of Stormwater Filtering Systems (Claytor and Schueler, 1996), and Highway Runoff Manual (WSDOT, 1995).
The basic design elements to be addressed are proper soils, vegetation, and drainage. For most ultra-urban applications designers should look for relatively flat areas where deep soils (1.68 m [6 ft] to bedrock) are present and where seasonal high groundwater elevations are at least 1.68 m (6 ft) below grade. Ideally, BAs will discharge collected stormwater into underlying in situ soils and then into the surficial groundwater aquifer. As an option, designers can employ an underdrain system to collect exfiltration from the BA wherever existing deep soil layers will prevent exfiltration. Underdrains are typically placed approximately 1.52 m (5 ft) below grade and must drain by gravity to either an outlet or a storm drain. Underdrain systems can also be used in BAs where they will be placed in close proximity of building foundations. A minimum 9.2 m (30 ft) offset is recommended for BAs without underdrains.
Bioretention facilities combine a number of physical, biological, and hydrologic components to provide complementary functions to improve water quality, control hydrology, and provide wildlife and aesthetic improvements. The major components of the BA are:
- Pretreatment area (optional).
- Ponding area.
- Ground cover layer.
- Planting soil.
- In-situ soil.
- Plant material.
- Inlet and outlet controls.
Some BA designs incorporate an upstream pretreatment area. Pretreatment is necessary where a significant volume of debris or suspended material will be conveyed by stormwater into the BA; for example, parking lots or commercial areas that are regularly sanded. In Figure 6, a grass buffer strip is used to reduce the runoff velocity and to filter large-diameter particulates from the runoff. Other pretreatment devices that can be employed are oil/grit separators, forebays, and stilling basins.
In BAs the ponding area is located over the planting soil and provides surface storage for stormwater runoff while it infiltrates and/or evaporates after the rainfall period. Major design parameters for the ponding area are the maximum ponding depth and the duration of ponding. In Prince George's County, Maryland, these parameters were established based on the type of planting soil used and the type of adjacent land use. The higher the infiltration rate of the planting soil, the greater the maximum ponding depth (up to 0.3 m [12 in]). Applications in residential areas are permitted ponding for less than 24 hours; all other applications are permitted 36 hours of ponding (Prince George's County, Maryland, 1993).
Ground Cover Layer
The surface of the BA is covered with an organic ground cover layer. The organic layer provides a medium for biological growth and provides the carbon source needed for biological activities at the air/soil interface. It also helps to maintain a sufficient organic percentage in the surface soil horizon, in a sense simulating the leaf litter in forest communities. It is recommended that designers of BAs either use a mature mulch (maximum depth of 76.2 mm [3 in]) or establish permanent growth (e.g., grasses) within one growing season (Prince George's County, Maryland, 1993).
BAs contain a thick layer of planting soil, located below the ground cover layer and supported by the underlying in situ soils. This thickness also provides for deep root plant growth. Planting soil must have a high infiltration rate, support healthy plant growth, adsorb nutrients and pollutants, and provide additional storage capacity for stormwater. These objectives can be met by using a planting soil containing a clay content of 2.5 to 10 percent and an organic content between 1.5 and 3 percent.
Prince George's County permits BAs with higher infiltration soils to have a greater ponding depth, which resulted in a smaller surface area of the BA. Based on this approach, designers might have to choose between using less expensive existing onsite soils or replacing existing soils with imported highly permeable soils to permit a smaller BA. To provide the infiltration necessary to remove ponded stormwater it is recommended that the soil texture be sand, loamy sand, sandy loam, loam, or silt loam. In addition it is recommended that the planting soil thickness be 1.22 m (4 ft) to ensure significant contact time between infiltrating stormwater and the soil. This soil depth will also help deeply rooted plant growth become well established (Prince George's County, Maryland, 1993).
In Situ Soil
As shown in Figure 6, the in situ soil layer provides a foundation for planting soils and drains the infiltrated stormwater from BAs. Experimental BAs have shown that in situ soils are crucial to the success of the facility; if a location drains in a poor manner, the BA will fail unless another means of drainage is established. Prince George's County, Maryland, recommended percolation tests be performed to demonstrate that in situ soils possess at least 12.7 mm/h (0.5 in/h) infiltration capacity. Where poorly drained in situ soils are encountered, it is still feasible to install bioretention but only with the aid of an underdrain system. Additional information on investigating in situ soils and designing underdrain systems is provided in the Prince George's County Design Manual for Use of Bioretention in Stormwater Management (Prince George's County, Maryland, 1993).
The role of plant species is to use nutrients and other pollutants and remove water from the planting soil through evapotranspiration. Plants must be a low-maintenance, aesthetically pleasing variety that is tolerant of urban stormwater pollutants. They must have the ability to adapt to conditions of drought and inundation. Key design parameters for optimum plant material function include species diversity, density, and morphology, and the use of native plants. Ideally, the community structure will be similar to that of a forest community, providing diversity to reduce susceptibility to insect and disease infestation. The intention is to create a microclimate that is resistant to urban stresses. The plants selected must be able to prosper even when flooded to a depth of 0.15 m (0.5 ft) or more at frequent intervals.
Inlet and Outlet Controls
The specifics of inlets and outlets of BAs are highly dependent on whether the BA is an on-line or off-line design. An on-line facility is one that does not have a bypass that diverts excess stormwater around the BA once it becomes full.
Because all stormwater will pass through an on-line bioretention facility, both inlets and outlets must be designed to ensure that the runoff rate does not damage the BA. Prince George's County states that designers must ensure nonerosive flow velocities exist within the BA for the 10-year postdevelopment event (Prince George's County, Maryland, 1993). On-line facility designs usually include protection such as riprapped inlets and outlets, which are designed through an in-depth hydraulic evaluation. Possible outlets for on-line areas include drop inlets or overflow weirs that feed downstream swales or pipe systems.
Off-line BAs generally require smaller inlets than on-line facilities because inlets are usually designed to convey the runoff from the first 12.7 mm (0.5 in) of runoff from the site. All other runoff must be diverted around the BA and downstream to subsequent swales or pipe systems without passing through the BA. This diversion can be established by creating a ponding area in the BA, which causes backwater conditions and a resulting shift in discharge direction.
Designers must be careful not to undersize entrances into BAs and to keep entrance velocities in excess of 0.15 m/s (0.5 ft/s) to help prevent clogging of the inlet area. Debris (e.g., sand) on the parking area can be washed toward the bioretention inlet and form a small dike, blocking the inlet.
BAs require routine, low-cost maintenance, similar to conventional landscaping maintenance, to ensure the system functions well as a stormwater BMP and remains aesthetically pleasing. Routine inspections of the bioretention facility, semiannually for the first year and annually thereafter, along with spot inspections after major storms the first year to verify the BA has not been significantly disturbed, aid in ensuring the performance of the BA. Other maintenance considerations include:
- Planting soil bed - check the pH of the soils, correct erosion, cultivate unvegetated areas to reduce clogging from fine sediments over time.
- Ground cover layer - mulch or replant bare spots annually.
- Planting materials - replace dead or severely distressed vegetation, perform periodic pruning, etc.
- Inflow/outflow - inspect for clogging, remove sediment build-up, repair eroded pretreatment areas, remove accumulated trash and debris.
Initial estimates from engineers designing BAs suggest project costs will be approximately $24,700 per impervious hectare ($10,000 per impervious acre), exclusive of real estate costs (Bell, 1996).
Bell, W. 1996. BMP Technologies for Ultra-Urban Settings. In Proceedings of Effective Land Management for Reduced Environmental Impact, Tidewater's Land Management Conference on Water Quality, August 22, 1996.
Claytor, R.A., and T.R. Schueler. 1996. Design of Stormwater Filtering Systems. The Center for Watershed Protection, Silver Spring, MD.
Prince George's County. 1993. Design Manual for Use of Bioretention in Stormwater Management. Department of Environmental Resources, Prince George's County, Landover, MD.
Washington State Department of Transportation (WSDOT). 1995. Highway Runoff Manual. Washington State Department of Transportation.