Managing stormwater with bioretention low impact development systems

by brittney_cutler_2 | April 4, 2022 4:00 pm

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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[2]. 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[3] 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.

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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.

Overcoming design challenges

Bioretention systems are best placed in areas that promote infiltration, unless otherwise specified by the design practitioner. The elements one ought to look for when siting a bioretention system include well draining soils and sufficient separation from groundwater and bedrock. These factors allow bioretention systems to drain through infiltration. Options can still exist where site conditions are not ideal.

For sites that have soils with low infiltration rates, subdrains can help prevent the bioretention system from becoming water-logged. Bioretention systems typically have stone layer between 0.3 and 1 m (1 and 3 ft) deep at the bottom of the system. Depending on the design, these subdrains may be located either at the bottom of the stone layer, so that all the runoff entering the system drains out through the subdrains, or be placed at the top of the stone layer, enabling a portion of the runoff to infiltrate. If subdrains are used, the design should specify a geotextile surrounding the stone layer or a filter sock around the subdrain to mitigate the system clogging.

For sites with shallow groundwater, a clay or synthetic liner can help prevent the filter media from becoming water-logged. Depending on the size of the system, the installation of this liner may need to be supervised by the site geotechnical engineer. Lined bioretention systems must include both a subdrain and an overflow outlet, so they drain between rainfall events. The plants in lined systems must be carefully selected, so root systems do not damage the liner. If plant substitutions are needed, they should be approved by the system designer.

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For sites with shallow bedrock, the construction challenges include both low infiltration rates and excavating around bedrock. Bioretention systems installed on sites with shallow bedrock could be designed with shallower stone and filter media layers. If the bedrock is fissured, a liner may be needed to keep enough water in the bioretention system to support the plants. If the bedrock is solid, a subdrain and overflow will be needed for the system to drain between rainfalls. Plants used in shallow bioretention systems must tolerate having shallow root systems. Any necessary substitutions should also be approved by the system designer to ensure the plants can thrive.

The requirement for LID systems in many jurisdictions lead designers to feel pressured to design bioretention systems in less-than-ideal locations. However, bioretention systems can be specified with a few design modifications to work on sites with low infiltration rates, or those with shallow groundwater or bedrock.

Communicating these design modifications to the construction team enables designers, contract administrators, site inspectors, and contractors to understand the design and be aware of the potential challenges when building a modified bioretention system. With innovative design and good communication within the project team and those in the field, bioretention systems can work well for years, even on sites with unfavourable conditions.

Construction challenges influencing bioretention performance

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The industry is learning to alleviate some of the challenges of installing bioretention systems by improving communications, enhancing specifications, and working together. When designers, contract administrators, and contractors work collaboratively from the design and specification stage all the way through final certification, it results in improved long-term performance of the bioretention system.

When preparing the tender documents, all design practitioners—including civil engineers, landscape architects, and geotechnical engineers—should not only discuss the specifications for the bioretention system and its components, but also the staging necessary for a bioretention system. Before any site work begins, the area of the bioretention system must be protected with erosion and sediment control measures to avoid sediment clogging the native soil or the construction activity compacting it. If earthworks are happening onsite, the bioretention system area must be filled with high permeability soils, which may need to be imported and placed in these areas. All trades onsite, including building and utility trades, should be aware of the bioretention system, so they do not inadvertently compact or otherwise damage the area after it has been landscaped and appears as a regular landscaped area.

Communication between all parties is also important when adapting to field conditions. Although every effort may be taken to have records of existing infrastructure and sub-surface soil conditions during the siting and design stage of the project, actual site conditions could be different or change once construction begins. Open communication and problem solving is imperative at this stage to minimize impacts to both costs and schedules.

Table 1 – Bioretention System Design Guidelines

Design Guidelines	Description
General Information of Bioretention Systems
Sustainable Technologies Evaluation Program (STEP)	This program is a multi-agency initiative developed to support broader implementation of sustainable technologies and practices within a Canadian context.[7]
STEP Low Impact Development WIKI	This website contains information on low impact development. Although managed by STEP, this site allows users to edit content; therefore, please verify information found on this website before implementing it in a design.[8] https://wiki.sustainabletechnologies.ca/wiki/Main_Page
Design and Planning Guides for Bioretention Systems
CSA W200-2018 Design Of Bioretention Systems, CSA Group, 2018	CSA Group has developed a standard for the design of bioretention systems in compliance with the Standards Council of Canada. This standard provides recommendations for the design of bioretention systems intended for the management of urban stormwater runoff.
Stormwater Management Planning and Design Guide, Ontario Ministry of the Environment, 2003	The Ontario Ministry of the Environment, Conservation, and Parks (formally Ministry of the Environment) is the final approval agency for stormwater infrastructure in Ontario. This manual is the foundation most of the agency’s design guidelines are based.
Low Impact Development Stormwater Management Planning and Design Guide, Credit Valley Conservation (CVC) and Toronto and Region Conservation Authority, 2010 (Now found on the STEP website)	A guide for designers who want to implement low impact development practices, it includes information on the design of many low impact development (LID) techniques, including bioretention systems. This guide also includes how to integrate LID systems into a design starting from the early planning stages of a development.
Fact Sheet on Low Impact Development Stormwater Management Planning and Design Guide – Bioretention Credit Valley Conservation and Toronto and Region Conservation Authority, 2010	This fact sheet provides a useful summary of the design of bioretention systems. A fact sheet has been developed for each low impact development (LID) system which now can be found on the STEP website.

 

Construction and Inspection of Bioretention Systems
Low Impact Development Certification Protocols: Bioretention Practices Credit Valley Conservation, 2017	This guide help municipalities and private landowners in Ontario formalize a system for certifying low impact development (LID) systems after they are constructed. LID systems are mostly underground and, therefore, operation and maintenance personnel need a procedure to ensure the systems are working as specified.
Low Impact Development Construction Guide, Credit Valley Conservation, 2012	The Low Impact Development (LID) Construction Guide bridges the gap between the design and construction of LID systems. Written for design consultants, municipal engineers, plan reviewers, and construction project managers, this guide highlights common LID construction failures and how to avoid them.
Contractor’s & Inspector’s Guide for Low Impact Development,Credit Valley Conservation, 2014	This guide complements the CVC Low Impact Development Construction Guide (2012). Contractors and inspectors can use this guide daily to ensure LID projects are installed properly through all phases of the project.
Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide Toronto and Region Conservation Authority, 2016	Municipalities and property managers in Ontario can use this guidance document to design an effective inspection and maintenance program for low impact development (LID) systems.
Inspection and Maintenance of Stormwater Best Management Practices—Fact Sheet – Bioretention Toronto and Region Conservation Authority, 2016	This fact sheet summarizes the inspection and maintenance practices for bioretention facilities along with cost estimates for common maintenance activities. Similar fact sheets are also available for other low impact development (LID) practices on the STEP website.

Even with bioretention systems becoming more popular with a shift to green infrastructure and the introduction of pre-manufactured systems, installing a system is challenging. Filter media mixes may require more testing to ensure the soil media mix meets the specifications. Additionally, if the system is not pre-manufactured, its performance relies on several contractors. For example, a civil contractor may install the geotextile, piping, and granular material; a landscape contractor may install the filter media; and a curb contractor would install any curb cuts or inlets around these systems. Each of these components are critical to the overall performance of the bioretention system. With each additional contractor involved, more complexity is introduced, which further reinforces the need for care during the installation process.

It is frustrating for contractors when inconsistencies exist in design drawings and contract documents. Designers must recognize the need to have consistent contract documents and problem solve with contractors when issues arise. This collaborative approach helps the long-term performance of bioretention systems, because it results in a better-quality installation that aligns with the design intention. Projects do better when good working relationships and open communication exists between designers, contract administrators, and contractors.

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Pre- and post-construction monitoring

Although bioretention systems are low-maintenance features by design, they do require some maintenance to properly function over time. Neglecting bioretention systems can lead to poor infiltration, clogged media, flooding, overgrown vegetation, and leaks which could lead to dangerous sinkholes[10]. Sinkholes are depressions or holes on the surface caused due to a collapse of a ground surface layer. Pre-construction and post-construction monitoring can guide maintenance activities to mitigate these issues.

Before construction of the bioretention system, infiltration testing of the native soils can confirm if the proposed location is suitable or if an underdrain is required. Choosing the best soil filter media and plant material helps minimize the potential for maintenance issues that may arise over the lifespan of the system.

During construction, inspections must take place before the sewers are installed and backfilled to confirm the depths, slopes, and elevations. Erosion and sediment control measures as well as flow diversion devices used during construction need to be maintained, especially after large rainfalls. Bioretention systems can easily get clogged during construction if the sediment material from construction activities is not properly managed.

After construction, system maintenance can help sustain the bioretention system’s longevity. Over time, sediment can enter the system and cause water to backup and flood the site. Twice a year, bioretention systems should be maintained by regularly removing trash, trimming vegetation, cleaning out sediment inlets and underdrains, and checking for slide slope erosion[11]. If persistent standing water is observed, it may indicate the filter media is clogged. Equipment should never be driven over a bioretention system, because it can compact filter materials and cause drainage issues. Periodically inspecting the systems every five to 15 years will determine if rehabilitation or replacement of the system is warranted. Performance monitoring options such as monitoring wells or piezometers can be installed to confirm if the system is operating as designed[12].

Bioretention systems must be maintained. Completing pre-construction and post-construction monitoring, together with ongoing maintenance, can influence the long-term success of these systems. The long-term maintenance of bioretention system remains open to further research for input on its frequency and the signs that rehabilitation of the system is required.

Conclusion

The design and performance of bioretention systems to manage stormwater continues to evolve. Designers and specifiers can stay on top of evolving guidance by visiting the agencies who continue to publish design guidance listed in this article, as well as by staying up to date with the ongoing research conducted by institutions to improve the future of stormwater management.

Author’s note: This article was developed in close collaboration with Crozier engineering staff including Rebecca Archer, P.Eng.; Rebecca Alexander, P.Eng.; Brendan Hummelen, P.Eng.; and Amanda Pinto, EIT.

[13]Nick Mocan, M.Sc., P.Eng., is the president of C.F. Crozier & Associates Inc., a consulting engineering firm that focuses on land development across Canada. Mocan’s expertise in water resources for land development and municipal infrastructure projects has inspired him to lead research projects with Wilfrid Laurier University and Western University aimed at improving the future of stormwater management. He has presented his research findings at national conferences and was recognized as a Community Fellow by Wilfrid Laurier University for his ongoing research collaborations. He can be reached at nmocan@cfcrozier.ca.

Source URL: https://www.constructioncanada.net/managing-stormwater-with-bioretention-low-impact-development-systems/

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