Rooftop stormwater management technologies: Climate change adaptation and resilience

by brittney_cutler | April 14, 2022 5:50 pm

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By Sasha Aguilera and Karen Liu

Americans[2] experienced many climate disasters in 2021. Much of Texas was covered in snow in February with more than 4 million households lost power. In June, the Pacific Northwest suffered from a heat dome, which set all-time heat records in seven states and caused hundreds of deaths. In October, three atmospheric rivers caused mudslides, flooding, and power outages on the California coast. Scientists predict extreme weather events will become more intense and more frequent with climate change.

In many major American cities, a sizable percentage of rainfall occurs in fewer days, during larger and more intense storms. For example, the atmospheric rivers brought 138 mm (5.44 in.) of rain over 24 hours in Sacramento and 102 mm (4 in.) in San Francisco in October 2021. According to a study by Princeton University, new flood maps show ‘100-year’ floods may now occur annually along the southeast and Gulf of Mexico coasts, or every one to 30 years[3].

Urban centers have high concentrations of impervious areas and outdated sewer infrastructures, which post a challenge in managing stormwater during extreme rainstorms. Heavy rain events or back-to-back storms cause flash flood, damage property and infrastructure. Large volumes of stormwater can also lead to combined sewer overflow in older cities across the U.S., which pollutes drinking water, harms aquatic habitats and causes beach closures.

Green infrastructure builds climate resilience

Green infrastructure mimics a site’s natural hydrological cycle to manage runoff close to its source. As land is expensive real estate, designers are turning to the many rooftops, which make up 20 to 25 percent of the land area in major North American cities. There are growing interests in using green roofs, blue roofs, and blue-green roofs to manage stormwater in urban centers such as Washington DC’s Green Area Ratio (GAR) environmental sustainability zoning regulation, NYC’s Climate Mobilization Act–Local Laws 92 & 94 and San Francisco’s Better Roofs Ordinance.

Sustainable roof colors—blue or green?

Rooftop stormwater management solutions are classified as source control tools, capturing rainfall right where it lands before it runs off into the storm sewer network. They are the first line of defense when working in combination with other at or below grade options onsite, such as bioswales or underground storage tanks. They manage stormwater through two distinct mechanisms: retention and detention.

Green roofs

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Green roofs are specialized roofing systems which support vegetation growth on rooftops (Figure 1). The vegetation and growing medium capture rainfall, reduce runoff, and delay peak flow. The rainwater is retained in the growing medium, which is either taken up by the vegetation or returned to the atmosphere through evapotranspiration, completing the natural hydrological cycle.

Green roofs reduce runoff volume, thus lessening the burden on storm sewers and wastewater treatment plants, particularly those located in areas where storm and sanitary sewers are combined. They work well most of time, except during back-to-back rainstorms because fully saturated green roofs cannot retain more water until they are dried out or “recharged” before the next rain event.

Green roofs are multi-taskers. They offer multiple benefits to buildings and the environment. They create amenity space, extend roof membrane longevity, reduce a building’s energy demand, improve air quality, and enhance biodiversity in the urban areas. Currently, more than 30 municipalities across North America have some form of policies or programs to encourage green roof adoption.

Blue roofs

Blue roofs pond rainwater on the roof and slowly release it over time. Runoff is controlled at the roof drains through a flow restrictor or a mechanical valve that opens and closes using smart technology. Water is detained on the roof and released slowly to prevent overwhelming the storm sewers during heavy rainstorms to avoid flash floods. The International Building Code (IBC) requires all water must drain off the roof within 48 hours of precipitation, although local jurisdictions may have different maximum drain time.

Blue roofs are often the most economical rooftop stormwater management tool. While effective, this technology has some inherent disadvantages that prevent it from gaining popularity in North America. First, dirt picked up by the runoff and wind-blow debris tend to collect at the control flow drains. Clogging affects the operation and effectiveness of blue roofs.

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Also, blue roofs require zero percent slope to maximize water storage as even a one to two percent slope can prevent the surfaces further from the drain to be fully utilized. However, IBC requires a minimum slope of two percent to promote positive flow to the drain (with exception for recovering or replacement), which can greatly impact the storage efficiency of blue roofs (see Figure 2, page 33).

In addition, ponding water exerts hydrostatic pressure, which can force water to leak through small defects in the waterproofing. Consequently, special attention must be paid to the membrane type, installation method, and workmanship to ensure the roofing system warranties are valid. Lastly, standing water may pose a risk of disease and safety issues to maintenance personnel.

Rooftop stormwater management options

With increasing interests in using rooftops for stormwater management, green roofs and blue roofs have evolved to combine enhanced retention and advanced detention to provide greater stormwater management capabilities. We have summarized the advantages and limitations of a few options below. Selection will depend on the design intents and site constraints of the specific projects.

Enhanced retention green roofs

An enhanced retention green roof consists of highly absorbent materials to increase water storage capacity while reducing system weight. Water retention fleeces and horticultural mineral wool are lightweight and highly absorbent materials that can retain seven to 14 times their own weight in water. They have a long history in the hydroponic industry and are increasingly being incorporated in green roofs.

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Water retention layers can boost the water storage capacity while keeping the weight low for green roof systems (e.g. a 25-mm [0.98-in.] thick mineral wool mat retains about 24 l/m² [0.59 gal/sf] of water compared to 12 to 15 l/m² [0.29 to 0.37 gal/sf] for a typical green roof growing medium of the same thickness).

Water retention fleece and horticultural mineral wool are designed under the growing medium in a green roof system (Figure 3a). Additional water retention lowers irrigation needs, increases resilience of the plants, and reduces annual runoff. Enhanced retention green roofs are particularly attractive on buildings where structural capacity is limited, such as on retrofitted buildings.

Although these systems have significantly higher water storage capacity than regular green roofs, like any retention-based systems, the enhanced retention layer will become saturated eventually and cannot retain more water. It still needs to dry out before it can retain more water, so it is not effective in managing back-to-back rainfall events or large intense storms.

Blue-green roofs

A blue-green roof consists of a green roof installed on top of a blue roof basin—a reservoir formed by a geo-cellular unit (Figure 3b). The upper vegetated portion filters and retains the rainwater and provides all the benefits of a green roof. Excess water infiltrates through the green roof and ponds in the blue roof underneath, which is detained and slowly released through control flow drains.

This lower portion enables the blue-green roof to manage back-to-back rainfall events and large intense storms regardless of the antecedent weather conditions (i.e. even when the upper green roof is fully saturated). Water is released slowly, so the blue roof basin is emptied or ‘recharged’ for the next storm. Blue-green roofs offer reliable controlled release of runoff like traditional detention-based systems.

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At first glance, blue-green roofs offer the best of both worlds, combining retention and detention to maximize stormwater management potentials. Unfortunately, as detention is achieved through control flow drains, it also inherits many of the blue roof’s weaknesses such as the dependence on zero-degree slope for water storage efficiency and clogging at the drains can affect the operation and effectiveness. As a result, blue-green roofs work best on dead flat roofs, preferably on large regular-shaped roofs to minimize the number of flow restrictors required for best economics.

Friction-detention green roofs

A friction-detention green roof consists of an enhanced retention green roof on top of a friction detention mat (Figure 3c). The retention of the upper green roof is enhanced by a mineral wool and an optional reservoir cell. The detention mat consists of thousands of fine vertical fibers sandwiched between two geotextiles. These fibers create friction to slow down runoff traveling to the drain to achieve detention. Water is backed up, filling up the reservoir layer and saturating the retention layer and growing medium.

The friction-detention mat allows small amounts of runoff to flow through unimpeded but slows down large volumes of runoff. As a result, runoff comes out at a slower rate over a longer time, which prevents overloading the storm sewers. The detention mat enables the system to manage back-to-back rainfall events and large intense storms regardless of antecedent weather conditions (i.e. even when the green roof is fully saturated). The water is released slowly within 24 hours and the system is ‘recharged’ for the next storm.

It is important to note because detention happens at the drainage level across the entire green roof, this avoids clogging issues associated with single-flow restrictors such as control flow drains. It also enables the system to be implemented on low-sloped roofs without losing efficiency and sloped roofs effectively. Lastly, the friction detention system is particularly economical on irregular shaped roofs where roof drains would require flow restrictors.

A successful friction-detention green roof design requires collaboration between several disciplines—architects, landscape architects, civil, and mechanical engineers—to provide project-specific details such as size, location, design storm, maximum allowable outflow rate, etc. Using this input data, a proprietary detention modeling program accurately predicts performance and calculates the appropriate green roof profile to meet detention and retention requirements of the project.

The research behind vegetated systems with friction-detention technology

Figure 4 compares the water retention and detention performance of three distinct green roof systems. Water fills and exists differently given a “traditional” system using simple drainage cups to expedite fast drainage, an “enhanced retention” system with a high performing water retention layer, and a “friction-detention” green roof with both retention and friction-detention layers. A short explanatory video can be viewed at:

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Figure 5 illustrates the effectiveness of the friction-detention green roof by comparing hydrographs from the same three systems in an actual rain test.

Conclusion

While green roofs and blue roofs are effective low-impact development tools, enhancing retention capacity and adding detention elements can take their stormwater management potentials to the next level. Enhanced retention green roofs not only reduce runoff, but they also lower irrigation needs and increase resilience to heat and drought.

Blue-green roofs and friction-detention green roofs incorporate detention to reliably manage back-to-back rainfall events and large intense storms, regardless of antecedent weather conditions when the roof is already saturated. While blue-green roofs work best on large dead flat roofs, friction-detention green roofs also work well on low-slope and sloped roofs, as well as irregular shaped roofs with many roof drains.

These advanced systems store a large amount of rainwater to be evaporated back to the atmosphere and delay runoff reliably during heavy downpour to minimize flood risks. Adding detention to retention on green roofs better manages stormwater and increases climate resilience in our cities.

Sasha Aguilera, B.Arch, GRP, is a design ambassador for Next Level Stormwater Management (NLSM), which provides tech support innovative stormwater management technology for projects in Canada (only).  With nearly 15 years of green roof experience, Aguilera is recognized as a leading green roof design consultant. Aguilera has considerable experience working on vegetated roofs of various complexities—from retrofits to new construction—across the country. She was honored with the F. Ross Browne Award in 2017. Aguilera can be reached via email at sasha@nlsm.ca.

Karen Liu, PhD, is a green roof specialist at Next Level Stormwater Management (NLSM). Before joining the private sector, Liu was a lead researcher of the green roof programs at the National Research Council Canada (NRC) and the British Columbia Institute of Technology. In recent years, Liu was a key participant in the research consortium that developed the first national wind testing standard for vegetated roofing, the Canadian Standards Association (CSA) A123.24-15, Standard Test Method for Wind Resistance of Modular Vegetated Roof Assembly. Additionally, she has vast practical experience having worked on hundreds of green roof projects across North America, Europe, and Asia. At NLSM, Liu works on special projects and wind and stormwater calculations. She can be reached at karen@nlsm.ca.

Source URL: https://www.constructionspecifier.com/rooftop-stormwater-management-technologies-climate-change-adaptation-and-resilience-2/

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