A concrete and grass walking bridge allows pedestrians to cross rain gardens that run adjacent to the street at the Harrison and Ives Apartments in Centralia.
Water from a hose falls onto a pervious concrete sidewalk adjacent to a rain garden on Center Street in Centralia on Thursday, Oct. 25. The water passes directly through the concrete preventing runoff onto the streets which reduces the strain on stormwater retention ponds while also limiting pollutants that can mix with the runoff and enter into local streams, ponds and rivers.
Turning on a garden hose full bore, Kim Ashmore, aimed it at a stretch of sidewalk on Center Street between Tower Avenue and Pearl Street.
Instead of running across the pavement and into the street as expected, the water drained straight into the sidewalk.
It’s not magic. It’s the future of dealing with stormwater, and Ashmore said he hopes in the near future everyone will be as excited about it as he is.
“We’re really trying to demonstrate to people what these things can do,” Ashmore, street, stormwater and fleet reserve manager for the city of Centralia, said.
Low Impact Development, also called LID, is going to become the state requirement within the next several years for dealing with stormwater. Once that time arrives, all new development and remodels will be required to use LID to mitigate the impervious surfaces on the development.
“You’ll have to treat it on site,” Ashmore explained. “You can’t just let it run out into the street and into the catch basins and into China Creek anymore.”
The reason for the emphasis on LID is environmental. Pollutants from vehicles, garbage and other wastes picked up by rainwater now currently run straight from catch basins into fish-bearing bodies of water. Requiring on-site retention means the water will be slowly filtered into the ground, where the pollutants will be filtered out before the water reaches wildlife habitat.
But for areas such as Centralia and Chehalis where urban flooding is an issue, rain gardens may do good for more than just salmon. The city recently built the demonstration area on Center Street to give residents an idea, not only of what will soon be required, but what is possible. Ashmore said the potential benefits to urban flooding are astronomical considering that virtually overnight entire streets’ worth of runoff can be taken out of the water entering catch basins and rivers.
“What I can do is say, ‘Consider low-impact development for stormwater,’” Ashmore said of the current state of LID. “They have to weigh the pros and cons and decide if it’s what they want to do.”
There are a number of ways, from retention ponds to bioswales, that can accomplish this on-site treatment. Pervious asphalt and pervious pavers used for sidewalks, parking areas and other typically impervious areas are also helpful. But one of the most dynamic methods currently being suggested is the rain garden.
A rain garden is an area that is dug out and planted to create a pond-like area into which all stormwater runoff is channeled. In a home use, gutter downspouts might be channeled into the rain garden. In an urban use, pipes channel water from the street away from catch basins and into the rain garden. Rain gardens should not be located above septic systems or in locations where water pools during rainy periods because the goal is to choose a place with good drainage so the water will filter back into the ground. Because they filter so much water, rain gardens should also be sited at least 10 feet from a basement or home foundation.
Many free resources are available to help homeowners choose both the location as well as the size of a rain garden. But no matter the size, every rain garden is broken into three zones of plants: the bottom of the garden contains plants that love a lot of water; the sides of the hole will have plants that require some but not a large amount of water; and the rim has plants that are drought tolerant. Choosing a location with at least partial sun will provide a wider variety of plants from which you can choose.
Soil called a “rain garden mix” is used for the first 18 inches of soil in the rain garden. Ashmore explained it is a more porous mix of soil containing sand and rocks to allow the water to drain away
Every eight to 10 years the top layer of soil, where all the pollutants have been filtered out, will need to be dug up and replaced with new soil.
If you are interested in trying out a rain garden in your own home, one way to start is by testing the soil by digging a hole six inches deep and completely filling the hole with water. If all of the water drains within 24 hours, the area can support a rain garden.
“Putting in your test hole where you may want your rain garden is best this time of year even if they may not be able to actually put the entire rain garden in right now,” said Debbie Burris of the WSU Lewis County Extension.
