Downtown Tacoma is set to become one of Washington’s larger urban settings to install rain gardens – rock and plant sculptures that filter stormwater – but designers are certain they won’t turn into sidewalk swamps.
The specter of ditches full of stagnant water comes from the unfortunate experience of the Ballard neighborhood in Seattle, whose attempt to use an environmentally friendly stormwater treatment technique resulted in “miniponds” that are “like bathtubs without drains,” according to a SeattlePI.com report this spring.
Seattle installed the gardens in an attempt to reduce the amount of stormwater flowing into its system by helping it absorb into the ground. It didn’t work. The city now faces spending half a million dollars to fix the problem.
The goal in Tacoma is different: No soaking, just filtering. Tacoma’s 14 planned rain gardens will be filled with native plants and have a drain. In fact, they’ll have three ways to avoid swampiness, said Doreen Gavin, a principal at design firm AHBL Inc. Its Tacoma office is working for the City of Tacoma, designing an $8 million renovation of Pacific Avenue scheduled to begin next year.
“We know that the soils under downtown aren’t very good,” Gavin said. “We know that we would not have success trying to infiltrate.
“We’re filtering through 18 inches of special soils, then it’s picked up in a pipe” and carried away, she said. The pipe is perforated and surrounded by about 6 inches of drain rock. The filtered water then flows into the city’s stormwater system. If the water isn’t filtering fast enough, it flows into a grate on the surface. Finally, during a gully washer, water will flow out of the garden and into the gutter, heading to the standard storm basins under the road.
Why is Tacoma installing rain gardens now? It’s part of a promise made a few years ago when civic leaders were putting together an incentive package for Russell Investments and DaVita, both of whom were considering leaving. Tacoma’s offer included an upgrade in downtown’s main street, and leaders said they’d do it regardless of the companies’ decisions.
“We’re doing this because (Pacific Avenue) is the heart and life of downtown,” city public works director Dick McKinley said at an October open house about the plans. “And we have to do something with stormwater. All that’s collected now shoots straight into the bay. It’s 2011. That’s not really acceptable.”
Rain gardens “are one of the most effective techniques” to filter stormwater, Gavin said. Other cities agree. They’re popping up across the country, from Lansing, Mich., to St. Louis, which recently finished one in its downtown. And they’re all over Portland, a city with so many different types of rain gardens that they almost blend into the landscape.
Gavin estimates the Tacoma gardens will pull in 91 percent of the stormwater on Pacific Avenue and remove more than 80 percent of typical urban pollutants.
PROJECT DETAILS
This phase of the Pacific Avenue renovation involves the area from South 7th to 17th streets. The price tag is about $8 million, and the city has $5 million of it in hand, much of it related to stormwater upgrades.
The sources are about $1.8 million in the Federal Highway Administration grants; $1.5 million from the Environmental Protection Agency; $1.5 million of local surface water funds to match the EPA grant; $200,000 in city funds for street signs; and $175,000 from the city’s real estate excise tax fund.
In addition to rain gardens, the project includes changes to traffic lanes to accommodate bikes and more buses, which will move down from Commerce Street. Some parking spaces have been shifted to side streets, where parking will become diagonal and back-in in some places. Sidewalks will be repaired, and about 80 new trees will be planted in special material under the sidewalks, not in planters.
They will include varieties of oaks, elms, sweetgums that don’t produce spiny ball-like fruit, sour gum, sycamore, honey locust and ginkgo (only the male trees, because the female trees create smelly seed shells).
The trees will be quite large, said AHBL landscape architect Meghan Montgomery, with the goal being little-to-no obstruction of ground-floor businesses. And as they grow, the lower limbs can be removed, shifting the canopy higher.
DESIGNED TO FIT SPACE
The gardens range from 200-500 square feet and are designed to be easily maintained and to fit the space they’re in.
“We’re looking at this in terms of what will look interesting in all seasons and have texture, and also will look great in a minimal maintenance situation. So it will be big blocks of planting. It doesn’t need to be staked. It doesn’t need to be pruned,” Montgomery said.
The plants will include a dwarf variety of red-twigged dogwood, which is red in the winter, has a bloom in the spring and lots of leaves in the summer. Other plantings will include native plants such as the Quartz Creek rush grass and a native iris, which has many different varieties.
The gardens also are designed to use the stormwater as a feature, and they incorporate about 100 granite pieces from the city’s old curbs, which have been stored at the landfill.
“They’re designed to be attractive whether there’s water or not,” Gavin said. “It will look like a planted garden when there’s no water there.”
Gavin said key issues will be trash, pets and weeds. The city and the downtown Business Improvement Area, which represents downtown property owners, still are working on dividing maintenance duties, she said, but it’s logical that the city would maintain parts of the gardens consistent with their stormwater system upkeep, and the BIA would handle the immediate issues like trash.
The gardens will have a 3- to 4-inch guard meant to help visually impaired people navigate around them. That guard also should stop inadvertent entry by full-bladdered pets if they’re not turned off by the dense and rugged landscape.
“This isn’t something that visually you’ll want to step foot on,” Montgomery said.
The rain gardens planned for downtown Tacoma are one of the larger settings for urban rain gardens, Montgomery said. But when you look at Portland, “we’re definitely not breaking new ground.”
Kathleen Cooper: 253-597-8546
kathleen.cooper@thenewstribune.com
blog.thenewstribune.com/business
Twitter: @KCooperTNT
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