Project Overview

What is the Lake Oswego Interceptor Sewer?

Click play above for an overview about construction plans and the need to replace Lake Oswego's aging sewer line inside Oswego Lake.

Constructed in the early 1960's, the Lake Oswego Interceptor Sewer lies submerged some 14 to 21 feet below the surface of Lake Oswego. The LOIS system includes over 13,000 feet of 16 to 36-inch diameter pipe traversing Lake Oswego and over 5,000 feet of smaller diameter trunk sewers constructed through canals and bays. Today, the LOIS system collects flows from a 4,500 acre service area and transports these flows down to the City of Portland's Tryon Creek Wastewater Treatment Plant (TCWTP) by gravity, where it is then treated and discharged to the Willamette River. While some of the pipe is buried, nearly 9,000 feet is pile-supported above the lake bed to maintain a uniform slope to the treatment plant. About 40 manholes are scattered across the lake, canals and bays to provide access for maintenance and inspection. The LOIS system has two major deficiencies that must be addressed: capacity and structural soundness of the pile-supports.

What is the problem?

Capacity (insufficient pipe size to pass flows from the service area) - The interceptor was designed to handle sewer flows from about 3,500 developed acres. Prolonged periods of rain can cause the system to back up and flow out of manholes into the lake or onto streets near the lake. Such spills violate the Federal Clean Water Act (CWA) and are unacceptable to the City. In February the City entered into an agreement with the State Department of Environmental Quality (DEQ), to establish a strict compliance schedule and measures the City will take to reduce overflows until the interceptor upgrade is constructed.

Structural soundness (related to earthquake vulnerability) - More than 9,000 feet of the line in the main portion of the lake is supported on steel piles, designed and constructed at a time when seismic design standards were much less stringent. In addition, the steel pile system is corroding; corrosion weakens the steel supports further reducing their strength. The pipeline and its pile support system will not likely survive a moderate earthquake. Engineering studies predict that seismic ground movement sufficient to cause widespread failure of the pile support system has a 15% chance of occurring within 25 years. Severe breaks in the pipeline would cause millions of gallons of raw sewage to flow into Oswego Lake. Up to 15 million gallons per day of lake water could drain through the pipe, potentially overwhelming the Tryon Creek Wastewater Treatment Plant and spilling to the Willamette River.

How Will the System Work?

Buoyant pipe

Rendering of the buoyant, tethered pipe

Tethered pipeline system

The tethered pipeline system ties a floating pipeline directly to bedrock.

Much of the replacement pipe installed in the lake will be buoyant pipe attached to the lake bottom with anchors and tethers. The pipe will float 8-17 feet from the surface of the lake.

Like the existing interceptor sewer, the new system will be a gravity line meaning that changes in elevation from the west end of the lake to the east will keep material moving through the line without pumping.

Lake temperatures can vary up to 40 degrees through the year which will cause the pipeline to lengthen and shorten. In the LOIS design, tethers restrain the pipe from bowing up, the buoyancy pipe restrains the pipe from bowing down and installation in an s-curve limits the side to side movement. These elements acting in unison ensure the right grade all the way to the treatment plant.

More information
Return to the City of Lake Oswego website