Bioretention Systems: Stormwater Management Facilities for Urban Areas

In cities, bioretention systems help reduce the volume of runoff caused by impervious surfaces. When bioretention is implemented on a large scale across a city, it can significantly reduce the volumes of water conveyed through pipes and lower peak flows during heavy rainfall events. These nature-based stormwater management facilities support sustainable and resilient water management.

What is a bioretention system?

With the emergence of many types of green stormwater infrastructure, it can sometimes be difficult to understand the differences between them. This is particularly true of bioretention, which is used in various types of green infrastructure such as rain gardens and vegetated swales.

Essentially, bioretention areas are depressions containing plants and a filter media that filter runoff from small drainage areas. Bioretention systems play the same role as natural environments: they allow precipitation to infiltrate and evaporate.

Bioretention areas have slopes of less than 2% and are not intended to convey stormwater and runoff.

Designing bioretention areas

The key point is that bioretention systems must be carefully designed to meet a specific need and the constraints of a particular site.

An expert will be able to select the plants and type of filter media suited to the site based on:

• The characteristics of the pollutants present
• The soil infiltration rate
• The maintenance required
• Road traffic

The design of the bioretention system is also influenced by:

• The depth of the water table
• The available space
• Potential conflicts with other utilities

The CSA W200:18 standard specifies that bioretention design must be carried out by an engineer and a landscape architect, although other experts may also be involved.

A design standard for bioretention systems

The CSA W200:18 standard on the design of bioretention systems states that a single bioretention cell can manage a drainage area of 0.8 ha or less. To cover a larger area, several cells can be installed in series, as is sometimes done with vegetated swales with bioretention along streets. The CSA W200:18 standard also states that an area equal to approximately 10% to 20% of the watershed’s impervious surface should be provided.

Pretreatment components

Bioretention areas must treat the pollutants that may be found in runoff, namely total suspended solids (TSS), fertilizers, heavy metals, de-icing salts, pathogenic microorganisms and hydrocarbons.

When the pollutant load is too high, a pretreatment component, such as a filter strip, can be added upstream of the inlet to reduce the sediment load. The pretreatment component must be maintained regularly, since excessive sediment in the bioretention cell can cause it to clog.

Types of bioretention systems

There are generally two types of bioretention: full infiltration systems and systems with an underdrain.

Full infiltration bioretention systems

A full infiltration bioretention system must include:

• A surface ponding zone that allows water to accumulate in the cell before infiltrating into the soil. The size of the ponding zone depends on the volumes of water to be treated. A hydrological analysis is used to determine the design volume and flow.
• Carefully selected plants to ensure a diverse root network, dense vegetation cover with several vegetation layers, and a diversity of taxa and growth forms.
• A surface layer made of mulch, stones, coarse pebbles, gravel or a fast-growing seed mix.
• A filter media composed of different soils (sand, silt, clay and organic matter) that allows water to infiltrate while preventing clogging, which occurs when too many fine particles are used. According to an ASCE study, organic matter should not exceed 5% of the total weight of the media or 10% of its total volume.
• A granular filter layer placed beneath the filter media.
• A storage reservoir placed at the bottom of the cell.
• An overflow system that discharges excess water to the sewer system. The bioretention cell must drain in less than 72 hours.

Optional components of a bioretention system include:

• Mulch, which protects the filter media from erosion and sequesters hydrocarbons.
• An underdrain installed at the bottom of the cell to meet the system’s objectives and to comply with design criteria for drawdown time and water quality.
• An impermeable liner that prevents groundwater contamination when necessary or when an underdrain is installed.

Bioretention systems with an underdrain

According to the Toronto and Region Conservation Authority (TRCA) guide, systems with an underdrain should be used when the infiltration rate is below 15 mm/h or when runoff poses a risk of contaminating the water table. When the infiltration rate is high, water infiltrates too quickly to be adequately treated before reaching the water table.

According to the CSA W200:18 standard, underdrains must be placed in a gravel bed at least 300 mm thick. An impermeable liner must also be installed at the bottom of the cell when an underdrain is used.

Advantages and disadvantages of bioretention systems

The main advantages of bioretention are that it:

• Slows the rate at which runoff is discharged to the piped system
• Reduces peak flows during heavy rainfall events
• Reduces the volumes of water conveyed directly to natural environments. According to the TRCA, a full infiltration bioretention cell can reduce the volume of water conveyed to the piped system by 85%, while a system with an underdrain reduces this volume by 45%
• Recharges the water table
• Reduces the pollutant load in runoff
• Offers a good cost-benefit ratio
• Captures CO₂ through plants
• Helps reduce urban heat islands
• Contributes to urban greening

The main disadvantages of bioretention are that it:

• Carries a risk of groundwater contamination
• Requires regular maintenance
• Requires sufficient space
• Must be adapted to cold climates, snowmelt and road salt

Consult Avizo Experts-Conseils’ drainage specialists for the development of your urban bioretention areas.

References

• CSA Group. (2019). Design of bioretention systems (CSA W200:18). CSA Standard. https://www.csagroup.org/store/product/W200-18/
• Société québécoise de phytotechnologie. (2018). Les aires de biorétention. Online. https://www.phytotechno.com/fiches-techniques/
• Toronto and Region Conservation Authority and Credit Valley Conservation. (2010). Low Impact Development Stormwater Management Planning and Design Guide. https://cvc.ca/wp-content/uploads/2014/04/LID-SWM-Guide-v1.0_2010_1_no-appendices.pdf
• MDDEP. (2011). Guide de gestion des eaux pluviales. https://www.environnement.gouv.qc.ca/eau/pluviales/guide-gestion-eaux-pluviales.pdf
• Société québécoise de phytotechnologie. Les aires de biorétention. https://www.phytotechno.com/wp-content/uploads/2018/04/Fiche-bior%C3%A9tention-finale_LHEb-ilovepdf-compressed.pdf