November 16, 2017
By Jennifer Drake
Urban flooding is a multibillion-dollar challenge for Canadians. (For more information, see the article, “Urban Flooding in Ontario: Toward Collective Impact Solutions,” published by RAIN Community Solutions in 2017.) Since the 1970s, approximately half of all natural disasters in the country have been caused by floods. (This statistic is derived from the 2016 publication, “The Road to Flood Resilience in Canada,” from Swiss Re.) In urban centres, pluvial flooding (i.e. caused by torrential rainfall) can be particularly destructive. The situation has been exacerbated by an increased area of hard surfaces as a result of urbanization, outdated and undersized stormwater infrastructure, and increased extreme weather associated with climate change.
In recent years, municipalities across North America have increasingly invested in green infrastructure to help mitigate the instances and severity of flooding. Both New York City and Philadelphia are implementing green infrastructure into city-owned property to reduce combined sewer overflow (CSOs), enhance city resiliency, and improve flood management.
Resilient cities have the capacity to survive, adapt, and grow regardless of any unexpected challenges. (This definition is derived from 100 Resilient Cities. For more, visit www.100resilientcities.org.) Green infrastructure builds resiliency by mitigating carbon pollution, managing flooding, reducing dependence on imported water, mitigating urban heat island effect, and—in coastal environments—reducing the impact of storm surges. (For more, see the U.S. Environmental Protection Agency’s [EPA’s] infographic on Green Infrastructure for Climate Resiliency at www.epa.gov/file/green-infrastructure-climate-resiliency-infographic.)
Green infrastructure are natural vegetative systems and ‘green’ technologies that replicate or mimic the functions of ecosystems. They include:
Toronto’s green roof industry
Over the past decade, the construction of vegetated roof systems (i.e. green roofs) has rapidly increased across Ontario. With the implementation of the Green Roof Bylaw in 2008, Toronto has quickly become a hotspot for the green roof industry in North America. The bylaw applies to all new building permits for residential, commercial, institutional, and industrial developments with a minimum gross floor area of 2000 m2 (21,528 sf). Its required coverage for a green roof ranges from 20 to 60 per cent of the gross floor area, increasing with a building’s footprint.
Thus, it is no surprise Toronto has the second-largest total amount of green roofs installed in North America, with more than 55,742 m2 (600,000 sf). (This information comes from Green Roofs for Healthy Cities’ [GRHC’s] “2016 Annual Green Roof Industry Survey Executive Summary.”) In high-density urban centres, these vegetated assemblies are an effective technology for mitigating urban heat island effects (for more information, see J.S. MacIvor, L. Margolis, M. Perotto, and J.A.P. Drake’s “Air Temperature Cooling by Extensive Green Roofs in Toronto, Canada” in volume 95 of Ecological Engineering, E. Del Barrio’s “Analysis of the Green Roofs Cooling Potential in Buildings” in volume 27 of Energy Build, and B. Raji’s “The Impact of Greening Systems on Building Energy Performance: A Literature Review” in volume 45 of Renewable and Sustainable Energy Reviews) and managing stormwater, (more information can be found in K.X. Soulis, N. Ntoulas, P.A. Nektarios, and G. Kargas’ “Runoff Reduction from Extensive Green Roofs Having Different Substrate Depth and Plant Cover,” in volume 102 of Ecological Engineering, V. Stovin’s “The Potential of Green Roofs to Manage Urban Stormwater” in volume 24 of Water Environment, and J. Hill, J. Drake, and B. Sleep’s “Comparisons of Extensive Green Roof Media in Southern Ontario,” in volume 94 of Ecological Engineering) while simultaneously creating multifunctional spaces with the opportunity for public green space.
A green roof can be any system that supports rooftop plantings. They range from intensive green roofs large enough to support mature trees to rooftop gardens supporting urban agriculture. Intensive green roofs, capable of supporting large vegetation, are best suited to new construction and publically accessible roof spaces. Across Canada, extensive green roofs are a popular system because they are lightweight, shallow, and cost-effective. As light and low-maintenance systems, extensive green roofs are easily integrated into both redevelopment projects and new construction. However, even within this category, there is a great range of variability, as roofs are designed with different combinations of plants, substrate, and irrigation.
Understanding green roofs
The University of Toronto’s Green Roof Innovation Testing Laboratory (GRIT Lab) was created by the John H. Daniels Faculty of Architecture, Landscape, and Design in 2010, with the goal of investigating the environmental performance of green roofs specifically for the Toronto climate. GRIT Lab brings together researchers and students from many different backgrounds, including landscape architecture, engineering, biology, forestry, and planning. It functions as an educational and research facility. Each year, the lab provides hands-on training for undergraduate and graduate students. Since 2010, it has trained three PhD students, 12 master’s degree students, and numerous undergraduate students. The lab has hosted international students from Brazil, France, and Israel.
GRIT Lab partners with the industry and regulators to produce Ontario-based data on vegetated roof performance, as well as to test innovative and new green roof designs. Data produced by the lab is shared to inform and improve green roof design.
Not all green roofs are equal, and decisions made during the design process have long-term implications on a roof’s as-built performance and function. The lab has investigated the role of four design parameters:
The GRIT Lab facility includes 33 green roof modules, each instrumented with sensors to measure temperature, soil moisture, and stormwater discharge. Tipping buckets at the base of each module measures stormwater as it flows out of the green roof, while thermocouples measure ambient temperature conditions above the roof surface. Sensors operate all year long and record measurements every five minutes. Data collected from each green roof module is analyzed to determine the influence of each design parameter (soil, planting, depth, and irrigation) on stormwater and thermal characteristics.
The lab was expanded in 2014 to investigate the performance and benefits of integrating photovoltaic (PV) arrays with a green roof system. Two full-size green roofs and four arrays of solar panels were installed. Temperature sensors were installed on the back side of the solar panels to measure surface temperature, as well as above the green roof to measure ambient temperature. Small and large tipping buckets at the base of each module were also connected in series to measure low and high stormwater flows from the green roof.
Designing for stormwater management
GRIT Lab researchers have found irrigation is a critical operational practice that influences green roof performance, including stormwater management, temperature, and plant growth. Green roofs with irrigation have healthier plants and create cooler local conditions, but have a smaller capacity to store rainwater. Even so, an irrigated green roof intercepts, stores, and evapotranspirates 50 per cent of summer rainfall (May to October). Green roofs with on-demand irrigation or no irrigation retain 70 per cent of summer rainfall. (For more, consult J. Hill, J. Drake, B. Sleep, and L. Margolis’ “Influences of Four Extensive Green Roof Design Variables on Stormwater Hydrology,” in volume 22 of Journal of Hydrologic Engineering.)
Irrigation has proved to be an essential practice for green roofs with grass and herbaceous flowing plants. Test modules with these plant types and no irrigation have, over time, lost up to 100 per cent of their vegetative cover. Sedum plants have an evolutionary adaptation allowing them to regulate transpiration and, as a result, survive water-limited conditions more easily than the grass and herbaceous plants.
The mineral-based growing media used in the test modules follows the German Landscape Research, Development, and Construction Society’s (FLL’s) Green Roof Guidelines. Within the vegetated roof industry, it has been debated by designers and green roof installers whether growing media blends that do not follow FLL guidelines (like the organic-based media tested at GRIT Lab) may be more susceptible to wind erosion, compaction, and/or breakdown. However, GRIT Lab researchers have observed minimal to no loss of media depth on both the FLL-compliant mineral-based and the non-FLL-compliant organic-based vegetated roofs in the beds tested in the lab and roofs surveyed around the Greater Toronto Area (GTA). (See D. El Helow, J. Drake, and L. Margolis’ “Testing the Potential Synergy of Green Roof Integrated Photovoltaics at the University of Toronto Green Roof Innovation Testing [GRIT] Laboratory,” published in 32nd RCI International Convention and Trade Show, pages 229 to 235.) The organic-based media provides additional performance benefits for green roofs—plants were generally healthier and stormwater retention was increased relative to test modules with the mineral media.
Plant type and green roof depth have been found to not affect stormwater processes. Similarly, none of the tested variables affect peak flow reduction values. On average, GRIT Lab test modules reduced peak discharge rates by 88 per cent. (For more information, check out D. Chemisana and C. Lamnatou’s “Photovoltaic Green Roofs: An Experimental Evaluation of System Performance,” in volume 119 of Applied Energy, as well as J. Breuning, K. Tryba, and R. Miller’s “Vegetated Roofs [Green Roofs] Combined with Photovoltaic Panels,” in Green Roof Technology.) This observation is particularly exciting, as it suggests green roofs provide an ‘existence’ value—in other words, the existence or presence of a green roof, regardless of its design, automatically provides a known reduction in stormwater flow rates. These findings give landscape architects and green roof designers a great deal of flexibility. As green roofs provide similar stormwater benefits regardless of the selection of plant type or depth, planting layout and selection can also be governed by other considerations, such as esthetics, biodiversity, or cost.
Integrating green technologies
To integrate PV panels with a green roof, additional structural analysis is required during the design phase. PV panels, racking, and associated infrastructure such as invertors must be secured to the roof and capable of resisting wind loads. The building structure must also be sufficient to support the weight of both the vegetated roof and PV structure. Roof loading requirements are easily accommodated in new-build applications, but for retrofit projects, integrated PV/green roof systems are significantly more complex to design.
Data collected from the combined PV/green roof system has allowed researchers to understand better the tradeoff and benefits of integrating multiple green technologies into the same roof space. Small-scale studies have suggested cooling provided by plant evapotranspiration may increase the efficiency of PV panels, extending their lifespan and increasing the rate of energy production of individual panels. Unsurprisingly, shading effects from the PV panels affect plant health and reduce evapotranspiration rates. However, through proper irrigation, shaded green roof plants can continue to thrive.
Side-by-side temperature measurements collected at GRIT Lab have shown the back-surface temperature of PV panels, as well as ambient temperatures, are cooler above a green roof when compared to temperatures above a ‘cool’ roof with a white high-reflective membrane. These promising results suggest with good design, integrated PV/green roof systems have tangible environmental benefits mitigating urban heat island effects and improving the operating conditions for PV arrays.
In 2017, the Daniels Faculty relocated to the Historic One Spadina building. Along with this move will come a new wave of expansion for GRIT Lab in 2018. Design work is underway to create more lab space to examine new research questions and innovated approaches to green roof design. One of the highest-priority questions involves re-examining options for irrigating green roofs.
From a sustainability perspective, it can be counterproductive to rely on treated municipal water for irrigation. Treated water is costly to produce, and treatment and distribution processes require embedded energy and generate carbon emissions. The new lab facility is serviced with two irrigation lines: treated municipal and recycled stormwater. Stormwater that discharges from the base of the building’s green roofs (both in the lab and on surrounding vegetated roof surfaces) is collected in a cistern and pumped back to the lab as irrigation water.
Developing new blends for green roof growing media may be able to reduce or eliminate the need for irrigation. Over the next few years, researchers at GRIT Lab will investigate soil amendments that increase the water retention capabilities of media while maintaining the lightness of existing media mixes.
Cities across North America continue to seek innovative solutions to the challenges of population growth, climate change, and aging infrastructure. Green roofs and infrastructure are important tools —from reducing storm flows to mitigating urban heat island effect and supporting urban wildlife. (For more information on how design/construction professionals can benefit from and contribute to GRIT research, visit grit.daniels.utoronto.ca.) The ongoing work at GRIT Lab provides architects with science-driven design guidance for green roofs that optimizes performance, function, sustainability, constructability, and longevity.
Jennifer Drake is a professional engineer and assistant professor of civil engineering, cross-appointed with the John H. Daniels Faculty of Architecture, Landscape, and Design. She joined the University of Toronto faculty in January 2013. Drake is a researcher with the Green Roof Innovation Testing Laboratory (GRIT Lab) and teaches hydrology and hydraulics, water resources engineering, and stormwater management. Since 2015, she has served as a member of the board of directors for the Toronto and Region Conservation Authority. Drake is an expert in urban flood management and green infrastructure. Her research group specializes in emerging technology, including vegetated roofs, rain gardens, and permeable pavements. Drake can be reached via e-mail at firstname.lastname@example.org.
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