Study Puts Dollar Value On Benefits Of Building Green

Cities are working to find ways to ease the impact of rising temperatures. Many are exploring green building techniques, also called Low Impact Development, which allow more rainwater to stay in place and to sink into the landscape. These green building techniques can reduce floods, restore washes and wetlands, improve air and water quality, and help cities cool down faster during evening hours. 

In Arizona, the City of Mesa used Low Impact Development techniques to redesign the streetscape near Fiesta Mall, along Southern Avenue at Alma School Road. The City of Glendale used permeable paving in the Park and Ride lot at 99th and Glendale avenues. The City of Tempe is developing computer models that show how green infrastructure would impact its neighborhoods. Now, the City of Phoenix has examined the benefits of these Low Impact Development techniques using similar projects within Valley cities and the City of Tucson, along with data collected by the U.S. Environmental Protection Agency (EPA). Phoenix’s goal is to start a conversation with builders and others about the benefits of Low Impact Development that is based on experience, data and dollars. Here’s a simple summary of the study.

Objectives: Phoenix examined green building techniques to answer three questions.

• What are the costs verses benefits, including initial expenditures and avoided costs?
• What are the social benefits, such as the impact on the heat island, flood risks and property values?
• What are the environmental benefits, including the impact on water quality, carbon emissions and air pollution?

Techniques Studied: Phoenix researchers included six Low Impact Development techniques in their study and compared each to traditional concrete.

1. Rain garden with bio-retention basin: These are shallow earthen depressions that collect storm runoff into loosely compacted basins with native soil and planted with native trees and plants.

2. Swale: These are open shallow rock or vegetated channels that slow runoff toward downstream discharge points. In Phoenix, most of these discharge points empty into the Salt River bed. Phoenix has a demonstration project at Taylor Mall on the Arizona State University Downtown Phoenix Campus.

3. Infiltration trench: This is an underground channel filled with gravel to provide spaces where water from storms can easily infiltrate. Phoenix examined infiltration information from Pima County and the EPA.

4. Pervious Pavers: The spaces between these pavers make it easier for storm runoff to infiltrate and can reduce peak runoff. Phoenix examined its own Taylor Mall project and Glendale’s Park and Ride project.

5. Porous Concrete: This type of concrete allows rainfall to penetrate and directly sink into the soil beneath. Phoenix examined its project at Central Station, the light rail hub at Van Buren Street and Central Avenue, and in Glendale. 

6. Porous Asphalt: This allows rainfall to drain through the surface into a stone bed and infiltrate the soil.

Conclusions: When examining all three questions, swales and rain gardens with bio-retention basins generated the most value. Rain gardens with bio-retention basins are cheaper to build and both swales and rain gardens provide the most social and environmental benefits. Overall, traditional concrete, pervious pavers, and porous asphalt offered fewer benefits. Traditional concrete is not expensive, but provides the least social and environmental benefits. The study cautions that the benefits of green infrastructure methods can differ among projects and the benefits should be evaluated early in the planning process. Phoenix calls the study an important first step because it measures the benefits of Low Impact Development methods in dollars and makes the study easy for everyone to understand, including businesses and builders. You can read the entire study here .

For 49 years, Arizona Municipal Water Users Association has worked to protect our member cities’ ability to provide assured, safe and sustainable water supplies to their communities. For more water information visit www.amwua.org .