Submerged sewer line idea discussed
A Hawaiian consulting firm that specializes in deep-water pipeline systems may hold the key to Lake Oswego's sewer pipeline project.
The city is considering a submerged, buoyant sewer main that would run the length of Oswego Lake.
Mayor Judie Hammerstad said the buoyant main could be 'the least complex and most dependable' solution.
As designed by Makai Ocean Engineering, the approximately 21,000-foot-long pipe would be made of high-density polyethylene, which would be tethered by wire rope to a steel bar grouted up to 150 feet into the bedrock.
The city council will hear city staff's recommendation for an in-lake pipeline system, or an around-the-lake system, at a meeting at 6 p.m. Tuesday at city hall.
At a meeting Monday, the mayor, council members and staff indicated their preference for the buoyant pipeline design.
'That option would be technically and financially preferable,' said Hammerstad, noting that she wants to see the staff recommendation before she makes up her mind.
'I'm encouraged by the progress I've seen,' said Councilor Frank Groznik. 'As the design gets more refined, my comfort level gets much better.'
'It's such a simple and elegant system, and it works,' said Councilor Donna Jordan.
City Engineer Joel Komarek said the buoyant system is superior to having an underwater pipe connected to piles, because it would not be adversely affected by an earthquake.
'Instead of being held up, it's being held down,' Komarek said. He said if the lake dam failed, the buoyant system would be less advantageous than a pipe supported on piles, because the buoyant pipe would sag on the lake's floor. He also said maintenance on the pipe would be more difficult than a ground-laid pipe, because it would require divers.
But, overall, he favors the option.
'For me, all of the unknowns or concerns I had a year ago have been eliminated or assuaged,' he said.
Makai, which is a sub-consultant to the city's principal sewer pipeline design consultant Brown and Caldwell, is amply qualified to help design the project, according to a report prepared by the company and given to the council this week. Installation would be low risk and comparatively easy to deploy, the report said.
Makai has installed deep water pipes in Bora Bora, Everett, Wash., and Hawaii.
The cost of the buoyant pipeline would be about $100 million, compared with the cost of $120 million for an around-the-lake option. Those figures are adjusted for inflation, assuming completion of the project by 2010.
The around-the-lake option would cost about the same to build, but its operation and maintenance costs would be higher because it would require six pump stations. In contrast, the buoyant in-lake system would rely on gravity.
The buoyant system would be about 10 feet beneath the surface at the west end of the lake and about 18 feet at the east end.
The pipe carrying sewage would be supported by another polyethylene pipe that would be filled with air or foam.
The project is mandated by the state Department of Environmental Quality, because the current 1963 pipeline would be vulnerable in an earthquake.
Design would be finished by June of 2009 and construction would begin that fall.
The council must also decide whether to do a 12-month draw-down of the lake or two eight-month draw-downs, to allow for construction. Council members said they were inclined to go with the 12-month option.
The city will consider financing the project through revenue bonds or a general obligation bond. Hammerstad said she would like to put the issue to a public vote.
The council will consider the staff's recommendation on Tuesday, then issue its formal preferred alternative by early August.