By Kathleen Ferris
Green infrastructure is a vague name for a particular way of designing streets, sidewalks, plazas and parking lots to make better use of rainwater. Green infrastructure redirects more storm runoff into public landscaping instead of pooling on hard surfaces or rushing into underground storm-drain pipes. Its purpose is to help cool urban areas by encouraging more greenery and shade.
A city creates a heat island when the sun heats its parking lots, streets and rooftops. Around sunset these surfaces release heat into the air. This daily process creates a bubble of heat, making a hot city like Phoenix even hotter than the natural air temperature. Green infrastructure is designed to increase vegetation, which remoistens the air and could reduce the heat island’s impact. Public and private grants are funding a handful of small green infrastructure pilot projects around the Valley. There is little research available yet to confirm its impact on a city’s heat island, in particular on a heat island created by a desert city that receives 7 inches of rain a year.
Visitors can see green infrastructure on Arizona State University’s downtown Phoenix campus. There are cutouts in the curbs that allow storm water to drain and collect into recessed roadside landscaping. That means more storm runoff goes toward watering landscapes, instead of to the city’s underground storm-drain pipes. The city used permeable pavers and permeable pavement in parking spaces, in the Civic Space Park, and in parts of the sidewalks surrounding the downtown campus. These alternative surfaces trap moisture in the sand and gravel beneath them and give trees and plants more water and room to spread their roots.
Tucson doesn’t have much of an underground drainage system and uses its streets to carry off most of its storm water. Unlike Phoenix, the city does have a permitting process for some green infrastructure. Neighborhoods can ask Tucson to create cut outs or holes in their curbs to allow storm water into landscaping between the road and sidewalk. About 15 neighborhoods have taken advantage of the process. The city and homeowners have planted 500 trees in these spaces in the last three years. Tucson just added an ordinance requiring similar cuts and landscaping on larger streets.
Phoenix is studying a similar ordinance. Questions remain about the cost effectiveness of such an ordinance and the reliability of permeable surfaces to stand up to urban traffic and provide a safe surface for people traveling in wheelchairs. For more information about green infrastructure visit the Watershed Management Group and the U.S. Environmental Protection Agency.
For 45 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.