Bioretention

Bioretention facilities are shallow, landscaped depressions that capture and treat stormwater runoff using the chemical, biological, and physical properties of plants, microbes, and soils. Often referred to as rain gardens, bioretention cells are designed to mimic natural forested ecosystems by filtering runoff through a layered system of mulch, engineered soil media, and vegetation. As stormwater ponds on the surface, it slowly percolates through the filter media, where pollutants are removed through processes like filtration, adsorption, and biological uptake.

These systems are typically designed to treat runoff from smaller, frequent storm events. The filtered water, or effluent, is usually collected by a perforated underdrain pipe at the bottom of the facility and discharged to the conventional storm drain system. In areas with suitable native soils, bioretention can be designed without an underdrain to allow for full infiltration of the treated runoff, which helps recharge local groundwater. During larger storms that exceed the system’s capacity, an overflow structure safely conveys excess flow downstream.

Bioretention is a versatile and aesthetically pleasing stormwater control measure (SCM) that can be integrated into various site layouts, such as parking lot islands, residential yards, and campus open spaces. Its effectiveness at pollutant removal and runoff reduction makes it a popular choice for low impact development (LID) and green infrastructure projects.

Applicability

Bioretention is a highly adaptable practice, but its successful implementation depends on site-specific conditions. Proper siting is essential for long-term performance and can be guided by tools like a comprehensive BMP selector to evaluate alternatives.

Regional and Climatic Suitability

Bioretention systems can be deployed in most regions of the United States with minor design adjustments. In cold climates, facilities can serve as snow storage areas, though plant selection must account for salt tolerance. The underdrain should be placed below the frost line to prevent freezing. In arid climates, plant palettes must consist of drought-tolerant native species, and irrigation may be necessary during establishment.

Ultra-Urban and Space-Constrained Areas

Bioretention is ideally suited for densely developed ultra-urban environments. While the footprint is typically about 5% of the impervious area it treats, these facilities can be creatively integrated into required landscaping, parking lot islands, and streetscapes. Their ability to be distributed across a watershed in small, decentralized installations makes them an effective tool where space for larger, centralized facilities is unavailable.

Stormwater Hotspots

Land uses that generate highly contaminated runoff, known as stormwater hotspots, can be treated with bioretention. Examples include vehicle service stations and commercial loading docks. To protect groundwater from potential contamination, bioretention facilities at hotspot locations must be designed with an impermeable liner at the bottom and sides of the filter bed.

Retrofit Applications

Bioretention is one of the most common practices used for stormwater retrofitting in previously developed areas. Existing landscaped areas can be excavated and converted into functional bioretention cells, or they can be incorporated into parking lot resurfacing projects. While retrofitting an entire watershed with bioretention can be costly due to the distributed nature of the practice, it is often one of the few feasible options in highly urbanized catchments.

Design Criteria

A successful bioretention design balances hydrologic performance, pollutant removal, and long-term maintainability. The key components include feasibility assessment, conveyance, pretreatment, treatment media, and landscaping.

Feasibility

Several site conditions must be verified before selecting bioretention. The practice is most effective for drainage areas between 0.5 and 2 acres, with a maximum recommended limit of 5 acres to prevent clogging and flow concentration issues. The surrounding land should have a slope of less than 5% to promote sheet flow into the facility. A vertical separation of at least two feet is required between the bottom of the bioretention facility and the seasonally high water table to ensure proper drainage and prevent groundwater interaction.

Conveyance

Runoff must be safely conveyed into and out of the bioretention cell. For larger storms, an overflow structure, such as a raised inlet or weir, is required to pass flows exceeding the design capacity to a stabilized downstream outlet. Most designs include an underdrain system, typically a 4- to 6-inch diameter perforated pipe in a gravel layer at the bottom of the facility, to collect and discharge filtered water. Bioretention is generally designed as an off-line system, where a flow splitter diverts the target water quality volume into the facility while allowing larger flows to bypass it.

Pretreatment

Pretreatment is essential for removing coarse sediment and debris that could otherwise clog the filter media. Unlike other filtering systems that use a separate sediment forebay, bioretention integrates pretreatment into its design. Common pretreatment measures include a grass Filter Strip at the inflow point, a stone or pea gravel diaphragm to spread flow and capture coarse particles, and the surface mulch layer, which provides initial filtering and prevents erosion of the soil media below.

Treatment

The primary treatment mechanism is the filter bed. A typical bioretention design includes several key treatment components:

• Ponding Depth: A shallow surface ponding area, typically 6 inches deep, temporarily stores runoff and allows for sediment to settle before filtration.

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• Mulch Layer: A 2- to 3-inch layer of aged, shredded hardwood mulch protects the underlying soil, retains moisture for plants, and provides a carbon source for denitrification.
• Planting Soil Bed: The core of the system is a 3- to 4-foot deep bed of engineered planting soil. This media is generally a mix of sand (85-90%), topsoil (5-10%), and organic compost (3-5%) to provide adequate infiltration capacity while supporting robust plant growth. The permeability (k) of bioretention soil is typically around 0.5 ft/day.
• Underdrain System: A gravel layer surrounding the perforated underdrain pipe provides a final filtration step and a storage reservoir.

The required surface area of the filter bed can be calculated using a variation of Darcy’s Law, which considers the water quality volume, filter depth, media permeability, and desired drawdown time (typically 48 hours). The principles behind this calculation are similar to those used for other filtration practices, and a filtering practice design calculator can be adapted to size bioretention areas.

Landscaping

Landscaping is a functional requirement for bioretention, not just an aesthetic one. A dense and diverse plant community is critical for maintaining soil structure, enhancing pollutant uptake, and promoting evapotranspiration. A landscaping plan should specify native vegetation adapted to local conditions and tolerant of both periodic inundation and dry periods. Plants are often selected based on zones of hydric tolerance, with wetland species in the center and upland species along the drier edges. A combination of trees, shrubs, and herbaceous perennials creates a resilient, multi-layered plant community.

Pollutant Removal

Bioretention facilities are effective at removing a wide range of urban stormwater pollutants. Performance is influenced by design factors such as filter media composition, depth, and drawdown time. Early monitoring provided foundational data on expected performance, though results vary by site and study. More recent performance data for this and other practices can be found in the comprehensive pollutant removal database.

The table below summarizes typical removal efficiencies based on early, influential research.

Source: Adapted from Davis et al., 1998.

The negative removal rate for bacteria in this early study may reflect wash-off from the mulch layer or other site-specific factors. Subsequent research has shown that mature, well-designed bioretention systems can achieve positive bacteria removal. The moderate removal of nutrients, particularly phosphorus, has led to ongoing research into optimized soil media amendments to enhance nutrient capture.

Construction and Cost Considerations

The construction cost of bioretention is moderate compared to other stormwater practices, typically estimated at around $7 per cubic foot of water treated (Brown and Schueler, 1997, adjusted for inflation). Costs can vary significantly based on site constraints, plant selection, and the need for underdrains or liners.

It is important to consider that bioretention costs are often partially offset by savings in other areas. The facility can fulfill site landscaping requirements, reducing the need for separate landscaping budgets. During construction, proper sequencing is critical. The bioretention area should be protected from compaction and sediment-laden runoff from unstabilized portions of the site. The filter media should be installed in lifts of 12 to 18 inches and lightly compacted.

Maintenance

Bioretention requires regular maintenance to ensure proper function and aesthetic appeal, similar to any landscaped area. An initial period of more intensive care is needed to establish vegetation. Long-term maintenance can often be handled by standard landscaping contractors.

Limitations

While highly effective, bioretention has some limitations. The primary constraint is the small drainage area that a single facility can treat, making it less suitable for regional stormwater control without being implemented on a wide, distributed scale. In high-density commercial settings, dedicating space to bioretention may reduce the number of available parking spaces, which can be a significant concern for developers. Finally, the construction cost can be higher than for simpler practices like swales, although this is often balanced by the added amenity and ecological value.

Frequently Asked Questions