Last week, as leaves were plugging storm drains across Portland, I suggested flooded streets were a downside of urban trees.
Some people didn’t like that suggestion at all, and I was impressed by the responses.
Clearly, there are other ways to think about the relationship between flooded streets and urban trees.
Mary Vogel, Evelin and Tualatin Riverkeepers all saw the situation differently. They blame the streets and storm drains, which can be reconfigured to absorb more storm water.
“It’s the impervious surfaces that are to blame for flooding. Not the trees,” Vogel wrote. “For about half of the year, trees actually mitigate flooding by capturing some of the rain before it hits the surface and slowing down the rest.”
So, I talked with Tim Kurtz in the City of Portland’s sustainable storm water management department, to see if he knew how much storm water urban trees soak up.
Kurtz said of course urban trees and green infrastructure such as bioswales and ecoroofs help absorb storm water and slow its flow into the storm drains and sewers.
However, during a deluge like we had last week, he said, they can’t stop the streets from flooding.
“Typically an infiltrating green street not lined will retain 85 percent of the annual rainfall,” he said. “But when we get a storm like we did on Monday, with 2 to 3 inches in 24 hours, that typically exceeds what they were intended to do.”
The city hasn’t studied whether green streets flood less, he said. But they are designed to intercept most of the water from the area they’re managing. The city has found ecoroofs absorb 50 percent of annual rainfall and green streets absorb 85 percent. But in bigger storms, he said, they are designed to overflow.
“I would definitely say they help. But they’re not going to eliminate it,” Kurtz said. “They also capture some of those leaves that are on the streets. Leaves that would flow down the street might get caught in the facility.”
It’s plants like rushes and sedges that do most of the work in those facilities, Kurtz said, but trees help too.
“Trees alone soak up a lot of storm water,” he said. “There’s definitely more benefit than harm in big storm events. No question they absorb a lot. But there’s only so much they can handle. If there’s a minor nuisance with leaves plugging up drains, it is greatly outweighed by the benefits they provide.”
There were also some responses to the idea that urban residents should clean out their neighborhood storm drains and pick up after urban trees.
Both Evelin and Jory pointed out that homeowners who live near city trees spend their own money to maintain the trees and pay a fee to have the city clean up their leaves.
“I am expected to pay $30 a year to have the leaves from the city’s trees removed,” Jory said, “but my neighbor doesn’t pay a cent because he doesn’t have city trees adjacent to his property. … I am all for city trees, but the cost of caring for them needs to be shared by all of the residents, not just those unfortunate to live near city trees.”
I think that raises a question: Do you think the costs of maintaining urban trees are fairly distributed among all of the residents who benefit from them? Do they become more of a burden on the people who live near them, or do the benefits of living next to urban trees outweigh the costs of maintaining them?