The Energy Benefits of Low Impact Development and Green Infrastructure

As town's across the country adopt low impact development and green infrastructure strategies to manage their stormwater, many are finding that the benefits of these approaches go far beyond clean water. A report produced by the Bureau of Environmental Services in Portland, OR has quantified some of these benefits, including the direct energy savings resulting from the city's innovative Grey to Green Initiative, illustrating the significant energy and greenhouse gas emissions reductions that can be achieved through low impact development.

Low Impact Development (LID) refers to comprehensive land planning and engineering design approaches that seek to maintain or enhance the pre-development hydrologic regime of urban and developing watersheds. Green infrastructure – which is defined as “strategically planned and managed networks of natural lands, working landscapes and other open spaces that conserve ecosystem values and functions and provide associated benefits to human populations” – is a major component of LID. In fact, the two terms are often used interchangeably.Green roofs, rainwater harvesting, bioretention areas (or rain gardens, bioswales), permeable pavement, tree planting and riparian habitat protection are among the most commonly used LID and green infrastructure techniques.

While most cities adopt LID to manage their stormwater and improve water quality, people are beginning to find out that LID strategies can also reduce the energy required for stormwater treatment, avoid the carbon emissions associated with building traditional infrastructure, prevent groundwater levels from dropping, and provide a “new” local, low energy water supply through aquifer storage or rainwater harvesting. While the full extent of energy savings attainable through LID techniques is currently unknown, early studies indicate that LID is an effective approach to addressing both water supply and energy challenges.

For instance, a study conducted by the Natural Resources Defense Council and University of California Santa Barbara’s Bren School of Environmental Science and Management found that LID practices can save up to 1.2 million megawatt hours of electricity per year by creating low-energy local water supplies, thus obviating the need for energy-intensive imported or desalinated water. These energy savings represent enough electricity to power more than 102,000 single-family homes, with corresponding carbon emissions reductions of 535,500 metric tons per year – equivalent to taking nearly 100,000 cars off the road.

The city of Portland, OR is among a handful of cities leading the country in implementing LID techniques through its Grey to Green Initative. Portland describes its Grey to Green Initiative as, “expanding the city’s green infrastructure to sustainably manage stormwater runoff, stop the spread of invasive plants, restore native vegetation, protect sensitive natural areas, and replace culverts that impede fish passage.” The five year Grey to Green goals include:

• Add 43 acres of ecoroofs (green roofs)
• Plant 33,000 yard trees and 50,000 street trees
• Restore native vegetation
• Construct 920 Green Street facilities
• Control the spread of invasive plants
• Replace 8 culverts that block fish passage
• Purchase and protect 419 acres of high priority natural areas

To achieve these goals, the city is investing $50 million dollars over the next five years to plant trees, construct green street facilities and protect natural areas. Portland is also funding up to $5 per square foot for 50 ecoroof projects through the Ecoroof Incentive Program. Another innovative green infrastructure program that Portland adopted to control stormwater is the Downspout Disconnection Program, which provided incentives for homeowners to prevent stormwater from flowing into the sewers by disconnecting their rain gutter downspouts. The program removes an estimated 1.5 billion gallons of stormwater from the sewer system every year.

Although energy was not an initial consideration when Portland adopted its Grey to Green Initiative, the city has realized significant energy benefits and recently attempted to quantify them in a report called Portland’s Green Infrastructure: Quantifying the Health, Energy and Community Livability Benefits (PDF).

Based on internal data on the energy required to treat stormwater in the city’s conventional system, it is estimated that every acre of ecoroof installed saves 1,470 kilowatt hours per year, every green street or bioretention facility saves 550 kilowatt hours per year and every tree planted saves 1.4 kilowatt hours per year. The report also identified potential energy savings from reduced urban heat island effect, as well as space heating and cooling energy savings in buildings with ecoroofs, since the ecoroofs can provide insulation in the winter and absorb heat in the summer.

As significant as the energy savings are, they fail to take into account the full extent of greenhouse gas emissions reductions resulting from LID and green infrastructure. In addition to the GHG emissions reductions associated with the energy savings mentioned above, there are two more ways that LID can reduce global warming pollution: 1) by minimizing the use of carbon-intensive concrete and piping required for “grey” infrastructure, and 2) through carbon sequestration from the tree plantings and other vegetation found in ecoroofs and bioswales.

As the title implies, in addition to looking at energy benefits, the report also contains some great information on health and community livability improvements resulting from the city’s green infrastructure efforts. One big benefit that the report’s authors mention but were unable to include in their analysis is that Portland’s green infrastructure has become a major tourist attraction, attracting curious visitors (and their wallets) to the city to learn about and witness the beauty of Portland’s green streets program.

Over the last decade or so, towns all across the country have started to embrace LID and green infrastructure strategies as a way to manage their stormwater, and it is becoming increasingly clear that there are numerous benefits to LID besides just clean water. As more towns begin to quantify the value of their LID programs, it will be exciting to see just how much energy and greenhouse gas emissions are saved by these cost-effective, common sense strategies.