Natural Infrastructure Solutions

Rain Gardens

A rain garden is a garden designed to temporarily hold and absorb rainwater and snowmelt run-off that flows from roofs, driveways, patios and lawns.

Rain gardens remove up to 90% of nutrients and chemicals and up to 80% of sediments from runoff. Compared to a conventional lawn, rain gardens allow for 30-40% more water to soak into the ground.

A rain garden is not a water garden, pond or wetland. It only holds water during and following rainfall or snow melt. And because it drains within 12-48 hours, it prevents the breeding of mosquitoes.

Rain gardens also help make neighbourhoods cleaner and more beautiful, and provide habitat for pollinators.

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In Action

Ajax Ontario: Rain Garden Performance Monitoring. A recent study explored the effectiveness of different rain gardens installed near the waterfront. This project monitored the performance of three adjacent Rain Gardens with differing bio-medias and evaluated runoff reductions and water quality to assess the performance of the rain gardens.

See The Report
See The Report

Rain Garden #1 (100 m2)

Rain Garden #1 used a typical soil medium of shredded hardwood mulch layered over and engineered mixture of soil, sand, and organic material + 5% iron filings. Iron filings increase phosphorus filtration.

Rain Garden #2 (100 m2)

Rain Garden #2 used a typical soil medium with no additives.

Rain Garden #3 (50 m2)

Rain Garden #3 used a typical soil medium + 5% Imbrium Systems Sorbitive Media


Of the three rain gardens, #2 had the highest overall runoff volume reduction (avg 79%). Garden #2 had 100% Runoff Volume Reduction in rain events up to 20 mm. Gardens #1 and #3 showed lower levels of phosphorus and dissolved phosphorus, however none of them exceeded the recommended levels of phosphorus. In this study, the Phosphorus Sorbtive Materials (PSMs) were mixed uniformly into the biomedia. Their application may also be done in layers.

Based on the results, it is recommended that PSMs be applied on a case by case basis where there is a need for the prevention/minimization of eutrophication in nearby bodies of water.

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Trees are among the most versatile and important pieces of natural infrastructure. Urban forests, tree canopies and street trees absorb stormwater runoff, reduce the urban heat island effect, and improve city air quality.

Urban trees play an important role in absorbing stormwater runoff, as mature trees intercept 25 to 35 percent of the rainwater that falls on them. According to the Chief Economist of the TD Bank no less, Toronto’s urban forest reduces 25 million cubic meters of rain water from going into the sewer system, saving the City $108 million every couple of years. That’s a lot of free work being done by Toronto’s 10 million trees.

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Bioswales are linear channels designed to concentrate and convey stormwater runoff while re-moving debris and pollution.

Bioswales improve water quality by infiltrating the first flush of storm water runoff and filtering the large storm flows they convey. The majority of annual precipitation comes from frequent, small rain events: much of the value of bioswales comes from infiltrating and filtering nearly all of this water. They are also beneficial in recharging groundwater.

According to the National Association of City Transportation Officials, of which several Ontario cities are members, bioswales are the most effective type of green infrastructure facility in slow-ing runoff velocity and cleansing water while recharging the underlying groundwater table.

In Action

Brampton, Ontario: The City of Brampton installed its first bio-filter swales along County Court Boulevard as part of planned road resurfacing in 2014. The primary role of the bio-filter swales is to collect and clean storm water run-off from County Court Boulevard before it enters the Etobicoke Creek.

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New York City: In Brooklyn, bioswales around the Gowanus Canal are helping to control stormwater and reduce water pollution caused by the legacy of industrial activities in the area. It is estimated that the curb-side installations will keep eight million gallons of stormwater runoff out of the city-wide sewer system each year.

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Green Roofs

A major source of stormwater runoff in urban areas comes from rooftops. A green roof is a rooftop surface with plants and a soil medium that helps to capture rainwater from storms before it can enter the stormwater system. Green roofs can also absorb air toxins, provide habitat and food, cool the building and save energy, and extend the roof lifespan by two to three times.

A number of municipalities have enacted by laws to reduce the impacts of rooftop runoff, including downspout disconnect by laws, rain barrel programs, and green roofs. The City of Toronto has a Green Roof By-law that requires the installation of a green roof on most new buildings and some additions, and mandates standards for construction and maintenance of green roofs.

In Action

ESRI Canada: This business occupies two floors of a 9-storey, 18,794-square metre suburban commercial office building overlooking Toronto’s Don Valley Parkway. As the result of a Ryerson University study that showed widespread greening of building roofs in Toronto could reduce Toronto’s ambient air temperatures up to 2° Celsius, Esri oversaw the transformation of a 704 square metre roof that now

— diverts an estimated 393,353 litres of of stormwater from the municipal system per year

— reduces an estimated 88kg of CO2 equivalent per year

— reduces heating and cooling costs, while providing better weather and sound insulation





Permeable Pavement

Standard forms of asphalt and pavement are impermeable surfaces, meaning that water cannot seep through them. Permeable pavement is a specific type of pavement with a high porosity that allows rainwater to pass through it into the ground below.

Through this movement, the pervious surface mimics the natural process that occurs on the ground’s surface, reducing runoff and returning water to underground aquifers. It also traps suspended solids and pollutants, keeping them from polluting the water stream.

Permeable pavements are somewhat limited in their use: for example, you could not replace a busy roadway with permeable pavement due to the daily wear and tear. However pedestrian throughways, parking spaces, bike lanes, and other low-traffic paved areas represent an opportunity to reduce the impact of the urban settings on the environment.

Permeable Pavements include:

Permeable interlocking pavers
Popular, precast modular units made of concrete, pervious concrete or rubber/plastic composite, with open joints between pavers, filled with fine, washed gravel.

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Permeable interlocking grids
These precast concrete or manufactured plastic grids have open cells that can be filled with fine gravel or a mixture of gravel, sand and topsoil and planted with grass or low-growing ground covers.

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Pervious concrete
Rigid pavement is installed as pre-cast blocks or poured in place, using a binder to adhere aggregate together, similar to conventional concrete — except that the fine aggregate component is minimized or eliminated, resulting in the formation of connected pores throughout.

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Porous asphalt
This flexible pavement uses a binder to adhere aggregate together, similar to conventional asphalt, except that the fine aggregate component is minimized or eliminated — resulting in the formation of connected pores throughout.

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Permeable articulating concrete block
These precast concrete blocks are designed with open joints that require no gravel joint fill mate-rial, linked together into mats by cables.

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In Action

Chicago, Illinois: One particularly creative application of permeable pavements, and natural infrastructure more, is Chicago’s Green Alleyways project. Chicago’s 3000 km of alleyways take up a total land surface of 14 km². Many lack proper connection to the stormwater system, causing alleyway drainage issues. Chicago addressed a number of issues that intersect with stormwater management, and adopted a comprehensive strategy to “green” its alleyways through the use of permeable pavements, bioswales, and rain gardens.

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