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RUNOFF - SEATTLE SEAS STREETS DIFFERENTLY
In a series of stormwater greening projects that began in 2000-each project increasing in scale-the city of Seattle is trying to mimic the permeable forest floor and pasture that once covered the land. Between 1972 and 1996, as the city grew and urbanized, its canopy cover shrank to 13 percent, while stormwater runoff increased by 7.5 million cubic feet. The increased stormwater load and the sediment, grease, oil, pesticides, and other urban pollutants it carries meant trouble for the Chinook, coho, chum, and sockeye salmon and cutthroat trout that live in Puget Sound and rear in its watersheds.
Seattle first tackled parts of the city that had old, "unimproved" gutter and ditch drainage systems, replacing them with vegetated swales and rain gardens in the public right-of-way along the street. In 2000, it installed its first "natural drainage systems" pilot project, known as SEA Street (for Street Edge Alternative Street) in an older neighborhood northwest of downtown Seattle. The city's transportation and public utilities departments collaborated to test the soils, meet with all neighbors (to make sure no one objected) and the fire department, and implement the project. After some initial hesitation and much discussion at kitchen tables with neighbors (all but one neighbor signed on) the residents of SEA Street helped decide on the species that would be planted in their swales and where to put them. Today the swales are lush and green, and bright with flowering currant and other natives, mixed in with a few drought-tolerant non-natives that grow well in the Pacific Northwest. "We're trying to make an urbanized environment think like it's still forested," says Bob Spencer, the city's Creek Steward. To allow fire trucks access, curbs are flat and "jumpable"; the street itself curves to echo the shape of the swales and to slow traffic. The narrower, curved street also meant that less pavement could be used, says the city's Jim Johnson. The project cost $850,000, and was funded through drainage fees.
Two years later, the city moved a few blocks south and ripped out four blocks of a ditch and culvert system, replacing it with a series of vegetated pools that stair-step down a fairly steep hill. This project, known as "110th Cascade, drains 21 acres of the Pipers Creek watershed. Here, there was less neighbor involvement because no homes faced onto the street; the project doubled the number of trees planted on the street.
The next project, Broadview Green Grid, was larger still, draining 32 acres of the Pipers Creek watershed, and encompassing 15 city blocks. Completed in 2004 at a cost of $5.1 million, funded by drainage fees, it built upon what was learned at SEA Street and 110th Cascade, and incorporates swales on the north-south oriented streets and cascade step pools at the east-west boundary streets. In October, another similarscale project-called Pinehurst Green Grid-was put in the ground.
Seattle is also applying natural drainage systems to more traditional neighborhoods with straight streets and conventional curbs. Phase one of the largest scale such project to date has just been completed-redevelopment of a 130- acre World War II-housing development called High Point, located south of downtown Seattle, with "mixed-income" homes. The project-a collaboration among the Seattle Housing Authority, Seattle Public Utilities, and other city agencies-has followed the city's new low impact development guidelines, and uses porous pavement, disconnected downspouts, rain gardens, and swales; there was also a Herculean effort to preserve existing large trees. Its most impressive feature is the series of vegetated swales- modeled after SEA Street but put in next to traditional straight streets- growing between the sidewalk and street. The idea here was to fit natural drainage systems into a new urbanist framework, landscape architect Peg Staeheli says.
Although some grassy swales were used as well, the general consensus, says Staeheli, is that the vegetated swales, with their bunchgrasses, redtwig dogwood, and vine maples, among other species, are more attractive and popular and erode less than the grassy swales. Popular with more than humans, too: songbirds flit through the canopy, while the swales and rain gardens give the site a charm rarely seen in large-scale new housing developments. Models predict that the natural drainage system here will detain and treat stormwater from the two-year storm event; at SEA Street, University of Washington monitoring has shown runoff from the two-year storm to be reduced by 99 percent-better than was expected. "The root mass and debris production [from the plants] has helped it function even better," says Shane DeWald, with the city's transportation department, who spent lots of one-on-one time with residents of SEA Street, addressing their concerns. DeWald says the city has found that by maximizing the benefits for every dollar spent on drainage-adding a sidewalk, disabled access, etc.-the projects have become that much more attractive to neighbors.
Even in downtown Seattle, the city is making an effort to call attention to stormwater-this time, using art. On Vine Street, a giant cistern with a beckoning hand takes water from a downspout and carries it into a series of planters along the sidewalk. On that same street, a series of terraced water gardens step down a steep slope, slowing runoff before it enters a small jade pool (and the stormwater system). While the projects obviously can't mitigate runoff from the entire downtown area, they do have a huge public educational effect. Seattle's one-percent art tax helped fund these projects.
The cost of the natural drainage projects is "comparable to traditional gutter and ditch," according to Spencer, who expects future projects to cost less as the city applies the lessons learned along the way. After maintaining the projects for three years- with help from the conservation corps-the city turns maintenance over to the neighbors (with occasional help from the city in the form of "mulch parties").
"But you have to maintain pipes too," says DeWald. "And pipes don't give you habitat, beauty, and livability and social benefits. We realized there was so much more to be gained by using a more natural mode." LOV
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