May 13, 2002
Brightwater conveyance options studied
by Jeanette Knutson
Decisions, decisions. Not only must King County decide where to locate its next regional sewage treatment plant, Brightwater, it also has to settle upon a conveyance system that will bring untreated water into the plant and treated water away from the plant to an outfall zone on Puget Sound.
Plant locations under consideration are two: the Route 9 site north of Woodinville and the former Edmonds Unocal tank farm near Puget Sound.
Brightwater planners have already chosen the pipeline route that would bring the untreated water into a Route 9 treatment facility, if it were to be the final site selection. The path would run through Lake Forest Park along Ballinger Way Northeast, skirt along the northern tip of Lake Washington, through Kenmore and onward, northeast to the Highway 9 site.
Planners have not decided upon the pipeline route that would take treated water from a Route 9 plant site to Puget Sound. They are considering four options. Simply put, the first would be a straight shot across 228th Street Southeast. The second would run across the King-Snohomish border. The third would run from Route 9 down to Northeast 195th Street in King County, then up Ballinger Way Northeast and westward along the King-Snohomish border. The final option under consideration with respect to a Route 9 treatment plant follows the identical route the untreated water would take into the plant, only in reverse - from the treatment plant, back down to Kenmore, to Lake Forest Park and up to Edmonds.
According to Gunar Sreibers, conveyance manager for the Brightwater project, "Each corridor has attached to it an already assessed construction method."
Preliminary engineering indicates, said Sreibers, the pipeline could vary in size from 36 inches to 96 inches finished diameter. This means a combination of construction methods will be used regardless of the conveyance route selected, cut-and-cover (a near-surface construction technique), deep tunneling and microtunneling.
In addition to the pipeline itself, portals will be constructed every 10,000 feet or so. Portals will be points of entry and exit for tunnel machinery.
Dirt will be brought out at the portals. They will be 1 to 2 acres in size and will have room to store building supplies such as tunnel liners. Many of the portals will be intermediate in nature - used only for the duration of the construction, said Sreibers. Upon completion of the pipeline, the portal area may be restored to the condition found, it may slightly altered in that a manhole may be constructed there, or it may be retained in toto as a maintenance facility.
There will also be pumping stations along the conveyance route. These will be permanent structures built on 1- to 2-acre sites.
The outfall portion of the system is an engineered structure through which the treated water is discharged. A pipeline will carry the water between 5,000 and 8,000 feet offshore.
The final 500 or 600 feet of the pipeline has a diffuser section to spread the treated water over a wide area. It also mixes the effluent to get a high degree of dilution at a rapid rate, said Sreibers. The exact outfall location has not been determined.
Sreibers said costs, environmental impacts, traffic impacts, wetland/stream crossings and unique characteristics of particular areas along the conveyance routes will be considered before a final pipeline route and outfall zone are selected.
"Right now we are not at a point anywhere near making a final decision," he said.
In fact, before any decision is made, Brightwater planners would like to hear from the public.
On May 28, King County will be sending some 60,000 Scoping Meeting Notices to residents and businesses around the proposed plant sites and along the conveyance corridors. There are a number of scoping meetings scheduled for early June. These meetings are designed for the public to air specific concerns about the plant and the pipeline.
County officials encourage the public to read the scoping notice and attend the meetings held throughout the community.