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Green Infrastructure vs "Grey"

by Thomas R. Tavella, FASLA, President, Tavella Design Group, LLC, Orange, CT

Green infrastructure offers benefits not available with typical "grey" infrastructure (e.g., storm drains), such as combining permeable pavers and bioswales to infiltrate stormwater to recharge groundwater supplies.

A recent survey by the American Society of Landscape Architects detailed the reasons why more and more stormwater professionals are selecting green infrastructure over traditional stormwater management practices, often referred to as "grey infrastructure," because they rely heavily on heavily engineered and structural solutions.

The survey found that green infrastructure offers benefits not available with grey infrastructure; that long-term costs associated with operation and maintenance can be less, particularly when combined with efficiencies that correspond with LEED certification; and that green infrastructure was cheaper than grey when large new equipment capacity was necessary, when new conventional equipment would require more space than available.

This article will detail how landscape architects are implementing green infrastructure in stormwater projects, and how the result has been long-term fiscal savings, a reduction in energy usage, and various aesthetic and social benefits.

Green vs. Grey: Cost Comparison
The most easy-to-understand assessment of green infrastructure cost-effectiveness--value in terms of relatively low costs for the benefits--relies on direct cost comparisons with grey infrastructure. For example, Portland, Ore., integrated green and grey approaches to stormwater management in its combined sewer overflow (CSO) abatement program. Using the marginal cost per gallon removed from the CSO system as a metric, an analysis of green and grey infrastructure options showed the former, which involved downspout disconnections, curb extensions that included vegetated swales, and parking lot infiltration, was among the most cost-effective, as the approaches ranged from $0.89 to $4.08 per gallon removed.

New York City officials have proposed a plan to use both grey and green investments, rather than relying on traditional infrastructure alone. The approach, aimed at reducing discharges into the city's combined sewer system, could save $1.5 billion over 20 years.

Green streets in Seattle required less pavement, which reduced pavement costs by 49 percent. Meanwhile, when condominium developments use porous asphalt and other green infrastructure elements, the overall construction costs turn out to be less than the conventional approach, particularly in the areas of drainage infrastructure, erosion control and curbing.

A Cost-Effective Approach to Rebuilding Right-of-Ways
By replacing portions of aging public streets, improving amenities for vehicles and pedestrians, and incorporating drainage features to improve the quality and reduce the quantity of stormwater, many stormwater projects have proven green infrastructure can be a cost-effective approach to rebuilding municipal right-of-ways. One example is the natural drainage projects in Seattle. Data from this project shows designs incorporating green infrastructure cost $217,253 less than a conventional street design, and can yield a cost savings equivalent to $329 per square foot.

Also, when compared to conventional approaches, green infrastructure can be more cost-effective over time. Take Chicago's Green Alleys program, for example. (Editor's note: See "Green Alleys: How Chicago's Pilot Program Seeks to Alleviate Flooding," by Erin Flegel, ASLA, Nov. 2008 LASN This program has demonstrated that when compared to conventional methods, green infrastructure--in this case permeable pavements, downspout disconnection, rain barrels and tree planting--is estimated to be three to six times more effective in managing stormwater per $1,000 invested. Of course, the cost estimates vary depending on the type of technology deployed. The cost for permeable pavement in green alleys is $0.10 to $6.00 per square foot with a service life of seven to 35 years, depending on the material employed. However, when these costs are offset by avoiding the costs of local flooding and stormwater capture and treatment, the benefits of a green streets or alleys program frequently outweigh upfront costs.

An analysis of green and grey infrastructure options in Portland, Ore. using the cost per gallon removed from the CSO system as a metric, showed that green infrastructure, which involved curb extensions, vegetated swales and parking lot infiltration, was among the most cost-effective.

Less Expensive Operations and Maintenance
Since green infrastructure uses natural processes rather than built systems, it will increase in resilience and function as vegetation matures and adapts to local resource cycles. Of course, it will require periodic maintenance for high-level functionality. This is in stark contrast to grey infrastructure, which requires increased operations and maintenance (O&M) over time as equipment and materials wear down, meaning it can be significantly more expensive over time than green infrastructure.

Simply put, using green infrastructure requires a change not just in stormwater systems, but in the approach to the O&M of these systems. This means that to keep green infrastructure working, stormwater managers will need to implement maintenance programs that involve regular, less costly approaches. Green infrastructure's promotion of sustainability and its aesthetic value will also give stormwater managers opportunities for maintenance partnerships. In a survey of Portland, Ore. residents, over half reported they would be willing to spend one to three hours per month (unpaid) maintaining green infrastructure vegetation.

Another example of how green infrastructure can lead to a change in how O&M programs are conducted: In New Hampshire porous pavements are reducing the winter salting and plowing costs, since freeze-thaw cycles cause melting and infiltration that would form ice layers on conventional pavements.

Reductions in Energy Consumption and Spending
Stormwater projects that feature green infrastructure practices can also help municipalities reduce energy consumption and spending. One California study found the adoption of widespread green infrastructure practices in the state could save over 1.2 million megawatt-hours of electricity per year. These energy savings represent enough electricity to annually power more than 102,000 single-family homes.

Green infrastructure practices that capture and infiltrate rainwater can also save energy by reducing the amount needed to pump and treat drinking water and wastewater. For example, infiltrating stormwater to recharge groundwater supplies will reduce the energy required to transport water from distant sources. Water harvesting and reuse (including practices such as rain barrels and cisterns) can reduce energy usage by saving on the need for highly-treated drinking water for outdoor and other non-potable uses.

Also, in many instances, water used in industrial processes, landscaping, and for toilets can be replaced by harvested rainwater. A feasibility study for the use of rainwater harvesting systems at Texas A&M University, featuring 43 rainwater harvesting systems on 113 buildings on campus, brought a potential $406,000 in savings per year.

Another common green infrastructure practice that can reduce runoff and cut energy costs is the planting of street trees. These can lower surface and air temperatures through shading and evapotranspiration. Shaded surfaces are likely 20 to 40?F cooler than those that are nonshaded, which reduces electricity demand for cooling in summer. It's estimated that Washington, D.C.'s urban forest, the parks, and street trees (which cover 28.6 percent of the district) reduce building energy consumption costs by $2.65 million per year.

Finally, there's the green roof, which in addition to keeping polluted runoff out of local waterways, can also increase building energy efficiency (when compared to traditional roofing techniques). The vegetation on green roofs lowers absorption of solar radiation and thermal conductance, which substantially reduces annual energy consumption for interior heating and cooling. The green roof on the Target Center Arena in Minneapolis encompasses 113,000 square feet. It captures nearly one million gallons of stormwater a year, and has cut annual energy costs by $300,000. These energy savings are consistent with modeled results and other experiences. A Canadian model estimated that a green roof could provide savings of six percent in total cooling and 10 percent of heating energy usage. In the warmer climate of Santa Barbara, Calif., the same model estimated a 10 percent increase in savings from avoided cooling costs.

An Increasingly Expanding Role
Over the past few decades, many U.S. communities have learned that including green infrastructure in stormwater projects can result in long-term fiscal savings, as well as reduce energy usage and provide aesthetic and social benefits. Surely, green infrastructure's role in stormwater management projects will continue to expand in the coming years.

Thomas Tavella, FASLA can be reached at

Thomas R. Tavella, FASLA, President, American Society of Landscape Architects

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August 23, 2019, 1:31 pm PDT

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