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A sophisticated system to green up the Central Corridor

Running parallel to the construction of the Central Corridor Light Rail Transit line is a $5 million project to improve the quality of stormwater runoff along University Avenue in St. Paul.

It includes funding from Clean Water, the Capitol Region Watershed District, the city of St. Paul, Ramsey County, and the Metropolitan Council, according to project materials. 

The existing drainage system “conveys untreated stormwater runoff from paved surfaces to the Mississippi River, which is impaired for turbidity, nutrients and bacteria,” project materials state.

To change that, the project will use something called an “integrated tree trench system,” stormwater planters, rain gardens, and “infiltration trenches,” according to project materials.

Will Nissen, who writes for Hindsight on the Minnesota 2020 blog, explains in an online post that the state-of-the-art tree trenches will use “pervious pavers and structural soils to help trees grow and survive in extreme urban conditions.”

Additionally, “Strategically installed stormwater planters and rain gardens will help capture and filter contaminated water runoff that currently goes untreated into the Mississippi River,” he says.

The tree trench idea came out of various community meetings and a workshop, according to Mark Doneux, a representative of the Capitol Region Watershed District, which handles the aspects of the project that have to do with stormwater regulations.

In conjunction with the project, trees that have been lost in construction will be replaced, and 1,000 new trees will be added, too, which adds a challenging element. "Some of the commercial areas have a lot of pavement," says Doneaux, and it's tougher to maintain them. Often, urban trees only live for seven to eight years. "The city wants to find better practices for planting healthier urban trees." 

The challenge is that along the related portion of University Avenue in St. Paul, there’s “quite the web of utilities,” and the sidewalks need to be able to accommodate emergency vehicles, he says. To create a healthy rooting medium for trees, including pathways for air and water, the group went for a system that doesn’t use irrigation. “It was a bold step to say ‘let’s make this work,’” he says.  

The idea is to create a system that can serve as a template for other parts of the city and watershed. “This is a new practice. This isn’t tried and true,” Doneux says.   

In the future, he hopes that there might be some signage to describe the project. “No one knows there’s a pretty sophisticated effort [here] to have healthy urban trees and treat stormwater,” he says.  

Source: Mark Doneux, Capitol Region Watershed District  
Writer: Anna Pratt
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