By Nick Mocan, M.Sc., P.Eng.
Bioretention low impact development (LID) systems are becoming increasingly popular across Canada to meet stormwater management objectives for new developments, and for retrofitting mature developments. Guidance for the design and use of bioretention systems to manage stormwater varies across Canada. This article explores bioretention systems and provides a reference for design practitioners, contract administrators, and contractors including the following topics:
∞ An overview of bioretention systems and how they are used to manage stormwater;
∞ The latest bioretention system design guidelines available to design professionals;
∞ Myths and design oversights;
∞ Overcoming design challenges on sites with unfavourable conditions;
∞ Installation challenges that influence bioretention performance; and
∞ Pre-construction and post-construction monitoring to guide maintenance activities.
Bioretention systems are somewhat like icebergs; they are almost entirely “submerged” below ground (Figure 1).
A bioretention system is a soil and vegetation filter designed to improve the quality of stormwater runoff. They filter out pollutants commonly found in stormwater, such as suspended sediments, nutrients, and oils, among others. Some bioretention systems also provide stormwater quantity control by allowing infiltration to the subsurface and decreasing the volume of stormwater entering a sewer or watercourse. In most cases, bioretention systems manage stormwater quality and quantity from small and frequent rainfall-runoff events up to a 25 mm (1 in.) rainfall from urban drainage areas of up to 5 ha (12 acres).
A typical bioretention system is illustrated in Figure 2. In this example, a depressed topsoil and mulch layer are underlain with bioretention soil media, including a mixture of 85 per cent sand, 10 per cent fines, and five per cent organics. The base layer is usually gravel with an underdrain if native soils are unsuitable for infiltration. Although most bioretention systems have a rectangular configuration, their designs are quite flexible and can be modified to suit site-specific constraints. In some cases, prefabricated bioretention systems can be specified and installed to facilitate quicker installation.
Design guidance for these systems is typically found locally with the agency having authority over watershed management. Local municipalities may also have design guidance for bioretention and other LID systems. Table 1 lists guidance documents primarily from Ontario agencies; however, the design principles, and construction and inspection techniques presented in these guidelines are applicable across Canada.
It is encouraging that several post-secondary institutions across Canada are conducting research to further understand and improve the design and performance of bioretention systems. One such research project with Western University assesses and optimizes bioretention soil media for enhanced removal of stormwater contaminants. Ongoing research will ensure the relevance of bioretention systems in protecting the environment for future generations as development pressures continue to increase.
Myths and design oversights
Bioretention systems are relatively new in civil and water resources engineering in Canada and, as a result, several myths exist about their performance. Some of these myths are:
∞ Bioretention systems cannot handle large rainfall events;
∞ They need more care and maintenance than a traditional stormwater management pond; and
∞ They take up more space than traditional end-of-pipe infrastructure, such as stormwater management ponds.
Traditional stormwater management ponds focus on controlling large infrequent rainfalls and bioretention systems focus more on treating smaller frequent rainfalls. Although bioretention systems are designed for smaller rainfall events, overflow pipes are included in their design to convey runoff from larger rainfall events to other stormwater storage systems. Any surface ponding on the bioretention system resulting from a larger rainfall event usually dissipates within 24 hours in systems with an overflow pipe.
Benefits aside, bioretention systems do need regular maintenance. This routine maintenance, however, is not labour intensive and does not involve heavy equipment. It involves removing accumulated sediment and debris, cleaning or replacing mulch, weeding, and trimming shrubs. Conversely, stormwater management pond maintenance may only be required every 10 years; however, it requires much more effort with pond dewatering and sediment removal using heavy equipment.
Space requirements are another reason why bioretention or other LID systems are sometimes overlooked as an option during the design process. Most bioretention and LID systems need adequate surface area to capture and filter runoff through the soil media before it discharges to the storm sewer or receiving watercourse. These systems require dedicated space on development sites to properly manage the stormwater.
Although bioretention systems do require space on site, they can often be incorporated in landscaped areas, or as a component of a landscape feature or product. If co-ordinated properly between the civil engineer and the landscape architect, bioretention systems can take up less space than traditional end-of-pipe stormwater management facilities.
Design oversights often relate to the soil media or plantings. Organizations such CSA Group and Credit Valley Conservation (CVC) have released design standards for bioretention systems that specify soil media composition. For best results, plantings should be drought resistant, and if the bioretention system will be receiving runoff containing road salt, the plants should be salt resistant as well. Correctly specifying these design elements will help ensure the biological treatment spans the design lifetime of the bioretention system and the plant life stays healthy. Designs should also include inspection ports for ongoing monitoring.
Depending on the size and location of the proposed bioretention system, prefabricated systems can be specified and delivered to site. These prefabricated systems typically include a concrete vault with the appropriate preinstalled soil media, drainage layer, etc. Although these prefabricated systems have a higher supply cost, they often reduce installation complexity, cost, and time.