Central Corridor light-rail system will reduce runoff pollution with tree trench system


When completed, the Central Corridor light rail will be known as the green line.

Sustainability measures used in construction of the project as well as infiltration trenches will help the project live up to its name.

The project team is using recycled materials throughout construction, but the big sustainability focus is the incorporation of a $5 million tree trench system.

Tree trenches are urban infiltration systems used to filter runoff and ease street flooding and environmental issues.

Eight miles of filtration papers will lie beneath the sidewalks lining University Avenue in St. Paul.

Storm water will seep through permeable brick pavers passing through several layers of soil and drainage rock. The runoff will in turn water 1,200 newly planted trees along the light rail’s route in St. Paul.

“The mechanics are elaborate, but the idea is simple,” said Nick Landwer, manager of engineering and design at the Metropolitan Council. “Basically, the trenches will collect water, filter pollutants and nourish trees.”

Landwer said the trenches will capture and breakdown pollutants like gasoline, oils, phosphorus and other contaminants.

“The system won’t eliminate street flooding in low-lying areas of University Avenue, but it will lessen the impact,” Landwer said.

Tree trenches are becoming more popular in urban construction, said Metropolitan Council spokeswoman Laura Baenen.

“Ours is catching the attention of other cities planning similar public service projects due to the significant positive effect on the environment,” Baenen said.

The city tested the infiltration system this summer, and it was successful, she said.

But Landwer said the real results won’t come for many years as researchers monitor the long-term reduction of pollutants entering the Mississippi River.

“This is a huge sustainability effort that we are really proud of,” Baenen said. “We really hope people embrace these environmentally friendly strategies … during the entire construction and operation of the light rail.”