Magazine article American Forests

Trees: The New Sewers: The City of Portland, Oregon, Is Planting Tens of Thousands of Trees, Adding Green Infrastructure to the Traditional Grey

Magazine article American Forests

Trees: The New Sewers: The City of Portland, Oregon, Is Planting Tens of Thousands of Trees, Adding Green Infrastructure to the Traditional Grey

Article excerpt

As the Willamette River winds through downtown Portland, Oregon, it flows past dozens of sewage outfall pipes. On almost any rainy day, you can stand on a riverside walkway and watch as these pipes spew raw sewage into the river.

Over 800 cities across the US, including Portland, have combined sewer systems, which means that they collect and move both sewage and rainwater. During storms, these cities' treatment plants, and the pipes leading to them, fill up with a combination of sewage and rainwater. That combination often empties into lakes, rivers, and oceans. Some of the measures Portland is taking to solve its problem include investing $1.5 billion in sewage storage, and investing in an older, cheaper, and greener stormwater management solution: trees.


The city aims to plant 83,000 trees as part of its five-year, $50 million Grey to Green program. When the program started in 2008, tree canopy covered 26 percent of the city; by 2013 it should be expanded to 33 percent. The idea is that Portland should invest in infrastructure that is literally green (green roofs, trees, and natural areas), along with the grey infrastructure (pipes, roads, and culverts) that cities traditionally build and maintain.

As Canopy Coordinator for the Grey to Green program, Jennifer Karps spends her time scheming ways to plant 83,000 trees. She's passionate about urban trees, and says the program will "reintroduce natural processes. It's public works for the twenty-first century." As with any public works project, Grey to Green presents challenges. Where wall the city plant 83,000 trees? And how will it decide what trees and sites will best manage stormwater?


The main part of Portland's sewer system was built in the early 1900s, when most cities simply dumped raw sewage into rivers, lakes, and oceans. Allowing rainwater to drain into wastewater sewers meant that cities could lay fewer pipes and the rainwater would clean them regularly.


However, Portland's early planners could not know that future generations would treat sewage. They also could not foresee how many parking lots, roofs, and streets would come to replace the city's forests and wetlands. Impervious surfaces now funnel in more rainwater than the sewer pipes can hold. Portland's main sewage treatment plant, built in the 1950s, does not have the capacity to treat the sheer quantity of water that comes with a rainstorm.

In 1991 the Environmental Protection Agency ordered Portland officials to stop raw-sewage overflows. At that time, about six billion gallons of untreated sewage and stormwater flowed into the Willamette and Columbia Rivers each year. The city decided to solve half of its sewage problem with traditional grey infrastructure, including a $1.5 billion wastewater storage tunnel, and the other half with projects like downspout disconnection, stream daylighting, and what would become Grey to Green.

83,000 TREES

Theoretically, it's not all that difficult to plant 83,000 trees in five years. A professional can plant more than a thousand trees in one day. The challenge is to find appropriate places for trees in the complex urban grid. Urban foresters contend with overhead and underground utilities, traffic visibility, growing space, and--perhaps the most complicated part of the equation--property owners. Portland has already planted its tree-worthy public spaces, and has the most to gain from tree planting on private property.

To encourage property owners to plant trees. Grey to Green's canvassers talk to residents about benefits like air quality and increased home value (up to $7,000 for a big, healthy tree). Canvassers also explain the city's rewards for tree planting: Property owners can claim a "Treebate" for trees they buy, and receive regular discounts on sewer bills. Property owners can get help at every step of the process from the Portland-area nonprofit Friends of Trees, an organization that helps property owners choose, buy, and plant trees. …

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