Rain Gardens Boost Wash. Stormwater Control

Clark County residents in Washington State were initially skeptical of adding more than a dozen new rain gardens in one neighborhood, which are meant to ease the burden on a nearby stormwater pond that failed in 2009. Improvement to local water quality, however, combined with the aesthetic value of the streetside plantings, is making believers out of the locals.
Credit: The Columbian

While public officials in Washington State pursue the best methods for monitoring and controlling stormwater runoff – and the pollutants it can carry into local waterways – Clark County is making progress, having installed 15 rain gardens in the Mt. Vista neighborhood earlier this year.

The gardens sit in concrete basins between the sidewalk and street, and 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 eventually rebuilt, but the facility is still considered undersized. The hope is the rain gardens will mitigate the flow into the pond enough to prevent undue pressure.

"We are hoping to get some infiltration," Clark County Clean Water Program Manager Ron Wierenga told The Columbian. "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."

Construction cost $285,000. An $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 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."