Rain gardens bulk up

15 installed in Mt. Vista neighborhood are more sophisticated, aimed at cleaning stormwater before it reaches Mill Creek




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Susan Tredick likes having an extra garden feature in front of her house, even if it is used primarily to clean up dirty stormwater.

“I really do like it,” Tredick says as she rakes leaves from the spillway to the new rain garden in front of her home. “I think there was a good reason to do this, and I’m really thrilled with how it turned out. It’s actually very attractive. I think these really add to the neighborhood.”

Clark County installed 15 rain gardens in the Mt. Vista neighborhood earlier this year. The gardens sit in concrete basins between the sidewalk and street.

Construction cost $285,000. A $184,300 grant from the Washington State Department of Ecology paid the bulk of the cost, while the county’s clean water fund paid the rest.

The garden tubs are filled with soil, plants and trees, which catch and clean runoff water from the roadway.

The gardens should help to ease the strain on a nearby stormwater detention pond that failed back in 2009.

The pond was later rebuilt, but the facility is still considered a bit undersized. The hope is the rain gardens will mitigate the flow into the pond just enough to keep it from being strained.

“We are hoping to get some infiltration,” said Clark County Clean Water Program Manager Ron Wierenga. “Looking at the soils, we expect a five- to seven-percent reduction (in flows to the pond). But that’s conservative. We think we will get more than that.”

Wierenga points out that mitigating the volume entering the stressed pond is only one part of the equation. More important, he says, is cleaning the water.

“Our focus wasn’t primarily on reducing water volume,” he said. “It’s on improving quality.”

The soils and vegetation in the gardens help clean the water naturally before it makes its way to nearby Mill Creek. The creek has been degraded over time, but it still serves as a tributary to other waterways that support salmon and steelhead populations. The hope is that the rain gardens will help to filter out the grime and pollution collected by water off county roads before it hits the watershed.

It’s a process that is becoming more popular in public works development. In fact, the science behind them is getting so involved these really can’t be called rain gardens anymore.

“What we ended up putting in is more technically termed a bioretention facility or bioretention cell,” Wierenga said. “It’s a rain garden on steroids. They’re much bigger.”

And as the gardens become more sophisticated, there becomes a higher likelihood they will be included in additional county projects.

Ken Lader, Clark County design engineer, said when the county began the project in the Mount Vista neighborhood, there was some consternation among residents.

“When we first started the project, I definitely had some push-back,” Lader said. “But as we went forward, it got better and better. Most of the project now has received pretty good comments. Mostly, I think, because the final result didn’t take up as much of the road as people thought.”

Tredick says she’s seen a change in the way her neighbors view the gardens now that they’re completed. But she admits she has been talking with them about the benefits of the addition from the start. She likes the way they look, she likes that they clean the water and she hopes they will keep sitting water off the street during heavy rains.

“I think I may have changed a few opinions around here,” Tredick says with a smile. “I think people just have to get used to the change.”

Erik Hidle: 360-735-4547; http://twitter.com/col_clarkgov; erik.hidle@columbian.com.