|Both sides of the channel are lined with MSE walls.|
A construction project in the Northgate neighborhood of Seattle is creating a natural drainage system that will mimic the way nature works to clean the water entering Thornton Creek, slow it down and allow it to flow through an open-air channel year-round.
The Seattle office of Walsh Construction Co. is building the $6.3-million Thornton Creek Water Quality Channel for the Seattle Department of Public Utilities on a comma of open parkland between two substantial building projects that are under construction on 10 acres just south of Northgate Mall.
Thornton Creek historically flowed through the site but was diverted to a 5-foot-diameter pipe many years ago, when the land was filled to create a parking lot using materials excavated during the construction of Northgate and Interstate 5, said John Gilson, project manager for Walsh Construction. Neighbors have been telling the city they wanted the creek back since the 1960s, and now they are getting their wish, he added, thanks to an agreement between property owner/developer Bruce Lorig and the city.
Located at the headwaters of the South Branch of Thornton Creek, the 2.7-acre site offers the last available opportunity to improve the quality of stormwater runoff before it reaches the creek, according to SPU. The channel design will divert stormwater from the drainage pipe under the site to a series of surface swales (or small ponds) landscaped with special soils and native plants. These swales slow down the water, allow it to seep into the soil, and remove pollution before the water reaches the creek.
The channel will have water flowing in dry weather, as well as clean stormwater from the frequent storms. The existing storm drain pipe will stay in place to carry high storm flows when the channel cannot handle all the stormwater volume, Gilson said.
The design also includes native landscaping and pedestrian pathways that provide access throughout the site with connections to the Lorig mixed-use development, ERA Care Development, the King County Transit Center, the Northgate Mall, the new library and community center on 5th Avenue NE, NE 100th Street and 3rd Avenue NE.
"It will really be quite stunning," Gilson said, "a quiet, pleasant spot."
Walsh Construction started work on the channel April 1, with Gary Merlino Construction, Seattle, as the major subcontractor for earthwork.
The work involved 75,000 cubic yards of excavation to create the channel, Gilson said. The dirt, being old fill material, was of low quality but did not pose any problem while the channel was being excavated, he added. Merlino Construction also built mechanically stabilized earth walls that line both sides of the curving channel.
An interesting aspect of the channel is the use of granite blocks to build several waterfalls along the length of the channel. The granite is being recycled by the SPU, which has removed it from various places around the city where it originally was installed as curbing for streets.
Another key subcontractor on the project is Cerna Landscape Inc., of Renton, WA. Cerna's landscaping tasks include the installation of thousands of native and non-native plants on the property, plus installing the irrigation system and setting boulders and logs that will serve as weirs to affect the water flow.
The work also involves placing a substantial amount of mixed gravel and organic material that will sustain the plant life, said Larry Cerna, supervisor for Cerna Landscape.
The company bought a new Bobcat T300 compact track loader from Bobcat Northwest specifically to use it on the project, Cerna said. According to Bobcat, compact track loaders weight significantly more than skid-steer loaders of the same size, giving them higher pushing force and the ability to lift larger loads. At the same time, the weight of the machine is distributed over a larger area to provide increased flotation and minimal ground disturbance.
The T300 is Bobcat's largest compact track loader, with a bucket width of 80 inches and a rated operating capacity of 3,000 pounds. It is powered by an 81-horsepower, Tier II-compliant turbo diesel engine.
Cerna said the T300 has worked out well, able to do all the hauling tasks his crew has asked of it, from placing dirt to hauling logs and boulders.
"This job is kind of a challenge for us, but we're going to do it," Cerna said. "We wouldn't have been able to do it without this machine."
Gilson said Walsh Construction expects to finish construction of the channel by the end of October 2008. Water will continue to flow through the underground pipe for about a year after that to allow time for the plants to become established. After that, water will be diverted into the channel, and the neighborhood's longstanding dream for Thornton Creek will come true.