29th July 2022
...Continued from Part 1
Smog and heat mitigation
A city’s many miles of hot, dark pavement absorb and radiate heat into the surrounding atmosphere at a far greater rate than a natural landscape would. Known as the urban heat island effect, this phenomenon can significantly increase ambient air temperatures. The EPA estimates that the average air temperature of a city with one million people or more can be one to seven degrees Fahrenheit warmer during the day than surrounding areas. Higher temperatures reduce air quality by increasing ground-level ozone (also known as smog). In Los Angeles, for example, a one-degree temperature increase alone makes the air roughly 3 percent smoggier. Green roofs, green streets, and other forms of green infrastructure help improve air quality and reduce smog through their use of vegetation. Plants not only provide shade but also absorb pollutants such as carbon dioxide (CO2)—which is valuable for carbon sequestration—and help reduce air temperatures through evaporation and evapotranspiration.
By reducing air temperatures and pollution, improving water quality, and creating more natural spaces, green infrastructure provides myriad public health benefits. Cooler and cleaner air can reduce heat-related illnesses—vital when you consider that 210 million Americans currently live in places where high summer temperatures put them at risk for heat-related problems such as exhaustion and heatstroke. Risks for respiratory issues, such as asthma, can also be reduced. Healthier waterways mean less illness due to less exposure to polluted water and fewer contaminants in drinking water and seafood. Greener areas also promote physical activity and can boost mental health while improving neighborhood livability.
Green infrastructure is often far cheaper than more conventional water management strategies. Philadelphia found that its new green infrastructure plan will cost $2.4 billion over 25 years, compared with the $9.6 billion that a gray infrastructure would have cost. The capital expenses associated with green infrastructure are often smaller—planting a rain garden to deal with drainage costs less than digging tunnels and installing pipes. But even when it isn’t cheaper, green infrastructure still represents a good long-term bet. The life expectancy of a green roof is twice that of a regular roof, while the low maintenance costs of permeable pavement can make it a solid long-term investment. The Iowa town of West Union, for example, determined it could save $2.5 million over the lifespan of a single parking lot by using permeable pavement instead of asphalt.
Green infrastructure saves money in other ways as well. Water from municipal drinking water systems is typically treated and then transported to homes. This alone can account for up to 40 percent of electricity use in municipal governments and about 2 percent of total U.S. use. But green infrastructure provides a source of water on the cheap for many non-potable uses that the EPA says presents “little human health risk.” That’s significant, given that only an estimated 19 percent of household water needs require purified water, and commercial users require even less. In other words, there is no reason to use expensive drinking water to flush the toilet or wash your car when rainwater works just as well.
Green infrastructure also improves the quality of water drawn from rivers and lakes for drinking, which reduces the costs associated with purification and treatment—in some cases by more than 25 percent. And green roofs can cut heating and cooling costs, leading to energy savings of as much as 15 percent.
Green infrastructure creates jobs in several ways. People can find direct employment opportunities, such as in the design, construction, and maintenance of green sites, but also indirectly. For example, when those workers have more money to spend, their spending fuels additional economic activity. Green infrastructure also raises local property values: A 2020 meta-analysis found that property values can increase by as much as 20 percent. New research suggests green infrastructure may even help reduce crime: Pleasant, verdant areas encourage people to gather outdoors (increasing safety); open spaces with grass and tall trees are especially effective.
Types of Green Infrastructure
From flowering rooftop gardens to absorbent pavement to tree-lined streets, green infrastructure comes in many forms and can often hide in plain sight. Here are some common examples.
A living landscape of vegetation (think hardy grasses, succulents, and wildflowers), green roofs provide a verdant oasis for birds, butterflies, and the people who have access to them. By providing an extra layer of insulation to a home or building, green roofs slash cooling- and heating-related energy usage and costs. Whereas a conventionally dark roof basically bakes on a hot summer day (transmitting the heat it absorbs to the building below), a green roof can remain cooler than the ambient air temperature around it. According to the EPA, green roofs can be up to 40 degrees Fahrenheit cooler than conventional roofs and even reduce citywide temperatures by as much as five degrees.
Green roofs generate other pluses: They also sequester rain and carbon pollution. Somewhere between 40 and 80 percent of the total volume of rain that falls on green roofs can be retained. Moreover, the water that is released from a green roof is released slowly, reducing the amount of runoff that rushes into a watershed all at once, which curbs flooding and erosion. The vegetation found on green roofs also captures CO2, which gets stored in plants and soil. One study found that if the Detroit area greened all of its commercial and industrial rooftops, the carbon sequestered over two years would be equal to taking more than 10,000 midsize SUVs or trucks off the road for a year.
Although not technically green infrastructure, blue roofs collect and store rainfall to reduce the influx of runoff into municipal sewer systems. They use detention ponds, basins, or trays to collect stormwater before draining it at a controlled rate into sewers or waterways. In addition to stormwater management, blue roofs can reduce the urban heat island effect when coupled with light-colored or reflective roof material, as well as provide energy savings in the form of reduced cooling expenses. The water from a blue roof can be reused too.
Even a few inches of rain falling on a house can result in thousands of gallons of stormwater runoff; just a single one-inch rainfall on a 1,300-square-foot rooftop can generate 832 gallons or 12 gallons per minute. Often channeled into storm drains by gutters and downspouts, this runoff increases the risk of sewer system overflows. Downspout disconnection is the practice of redirecting rooftop runoff from storm drains to a permeable surface, such as a lawn, or to rain barrels or cisterns, which capture and hold the water for later use. Downspout disconnection can also significantly reduce the amount of stormwater that municipalities must manage. The city of Portland, Oregon, disconnected the downspouts of more than 26,000 properties from the city’s combined sewer system between 1993 and 2011. Now, an estimated 1.2 billion gallons of stormwater get diverted from the sewer system each year, alleviating combined sewer system backups in city neighborhoods.
Beyond helping to rein in runoff, the capture, storage, and usage of rainfall (a practice known as rainwater harvesting) has the potential to meet 21 percent to 90 percent of a city’s annual non-potable water needs, effectively supplying enough water for up to hundreds of thousands of residents. What’s more, rainwater harvesting (which typically uses cisterns or rain barrels to collect runoff from impervious surfaces like rooftops) provides a practical way to meet municipal water needs as climate change, population growth, and increased demand from industries such as agriculture and energy strain the water supplies of many regions.
Rain gardens—which can be used in a variety of settings, from street medians to small yards—typically feature native shrubs, perennials, and grasses planted in a shallow basin. They are designed to trap and absorb rooftop, sidewalk, and street runoff. In addition to allowing rainfall to evapotranspire or slowly filter into the ground, rain gardens help recharge underground aquifers, keep stormwater from reaching waterways, provide habitat for wildlife, and beautify a street or yard. They are hardworking too: A typical rain garden is 30 percent more absorbent than a conventional lawn. In an analysis of the Seattle area, researchers estimated that a single rain garden can filter as much as 30,000 gallons of stormwater a year, and 12,000 rain gardens can absorb up to 160 million gallons.
Planter boxes, a type of rain garden, are often used in the space between a sidewalk and street. They feature elevated sides that allow rain to fall and be absorbed by vegetation and soil while small drainage openings help curb the flow of the runoff that exits from the planters.
Consisting of long, relatively deep channels of native plants, grasses, flowers, and customized soils that run parallel to parking lots or roads, bioswales can handle large quantities of runoff from large impervious surfaces. Not only do they absorb and retain runoff from small storms, but they also filter and slow the release of water from heavier rains to sewers or surface waters, limiting floods and providing a first filtration of pollutants. It is estimated that effective bioswales can capture and filter out as much as 90 percent of solids (like sediment); 80 percent of trace metals, oil, and grease; and about 65 percent of phosphorus from the runoff they collect.
Urban tree canopy
In addition to cleaning and cooling the air, trees provide a natural stormwater management system. Tree canopies intercept rainfall before it hits the ground. By providing surface area—branches and leaves—for raindrops to land on and evaporate from, a mature deciduous tree can capture as much as 700 gallons of rain a year, while a mature evergreen can absorb up to 4,000 gallons annually. The root system of a tree plays a large role in the management of runoff as well. In addition to drinking up water, which is then released via transpiration, roots create channels and open up space in soil, which enhances the ground’s ability to soak up water.
A pavement system often used for sidewalks, parking lots, or driveways, permeable pavement (also known as porous pavement) allows rainfall to seep through to underlying layers of pollutant-filtering soil before making its way to groundwater aquifers. Once installation costs are factored in, it can cost as much as 20 percent less up front than conventional pavement systems, and it can be cheaper in the long run to maintain.
Green parking lots
Designed to curb rainfall runoff, green parking lots often feature permeable pavement, vegetated areas around or within their perimeter, and shade trees that can help reduce the urban heat island effect.
...Continued in Part 3