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.
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. 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. 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.
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.
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.
