Overview
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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 did the City assess the problem?
Pre-design Work
View of the exposed interceptor during 1980 drawdown.
In 2002, the City retained Brown and Caldwell, Inc. a national environmental engineering firm with a local office, and commissioned a pre-design study to determine how to best correct the LOIS deficiencies. This Phase I study built upon earlier engineering evaluations of possible alternatives to replace the LOIS system. Those earlier studies suggested an in-lake gravity system would likely be the most cost efficient solution to the interceptor problems.
Updated construction cost estimates prepared as part of Phase I were substantially higher than preliminary estimates prepared in earlier studies. With the findings of the Phase I study in hand, and acknowledging the significant complexity and costs associated with any alternative to replace the current LOIS system, the City Council authorized Brown and Caldwell to begin Phase II of this study effort.
- To read more about the Phase I and Phase II findings or to read the predesign reports, click here.
- The City established feasibility and reliability criteria to guide their decisions. To learn more click here.
In the Lake Versus Around the Lake Options
The City Council directed project staff to pursue the in lake option in July 2007 after several months of analysis and public comment. The decision was based on several factors, but the primary differences between the in lake and around the lake options were in long-term operations and maintenance costs and impacts to the community.
Unlike the gravity system proposed in the lake, the around the lake option would require the construction of six large pump stations throughout the community. Maintenance and operations for those pump stations would add $20 million to the cost of the project over the life of the system. The pump stations would have to occupy private property-many with above ground structures.
These six pump stations would be disruptive to the private property owners who would be asked to accommodate them. In addition, the around lake option would require the construction of an additional four miles of upland sewer pipes below the streets of Lake Oswego. Hundreds of Lake Oswego residents would be impacted if pipelines were constructed under busy Lake Oswego thoroughfares. Lastly, the around-the lake option would not preclude a draw down of Oswego lake. A draw down would be required to collect the flows from three trunk lines at the west end of the lake impacting the 700+ household with lake access.
- Read Article: Sewer interceptor: Right decision then and now by Alex McIntyre, City Manager of Lake Oswego for the Lake Oswego Review (10/2/2008)
Project Finance
The total current estimate is approximately $110 million for the in-lake buoyant pipeline gravity system. This estimate may change somewhat as designs are refined and the lake down contractor is selected. To read more about the total cost of the project and what the project team has done to keep costs down, click here.
The City Council voted in July 2008 to finance the LOIS project through a 25 year revenue bond. Revenue bonds are repaid through utility bills.
A rate increase was approved by City Council that took effect July 1, 2009. 30% rate increases are anticipated in the next two budget cycles. It is possible that the increase could be reduced to 14% in the 2012-2013 fiscal year. Starting in July, 2010, a ratepayer, who currently pays about $77 per month, could see wastewater rates rise to $100 in the 2010-2011 fiscal year, $130 in 2011-2012, and $148 in 2012-2013. Increases after 2013 should mirror inflation.
Service Area
Homes and businesses connected to public sewer within the City will pay for the interceptor replacement through increases to their sewer bills. Some homes or businesses at the western edge of the City are served by Clean Water Services and will not see their bills increase because of the LOIS Project. If you are unsure if you are inside the City's service area, call Utility Billing at (503) 635-0265.
How Will the System Work?
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.
- Read Article: Physics and Chemistry and Math, Oh My! (It's What's Keeping LOIS in Line) (9/16/2008)
System Components
The Lake Oswego Interceptor Sewer is no electron supercollider in terms of complexity, but its design has received serious scrutiny by a team of engineers to ensure it provides reliable, long-term service. Last week's column addressed how the principles of gravity, thermal expansion, and buoyancy are guiding the LOIS pipeline design. This week, the focus is on interceptor component design and how the LOIS replacement will be stronger, more corrosion resistant, more flexible, and more seismically resistant than the existing pipeline.
The sewer pipe: The design team evaluated a wide range of materials to find the most suitable for the environment (the lake) and for the contents it will carry (sewage). These materials included steel, ductile iron, polyvinyl chloride (PVC), carbon fiber reinforced resin, fiberglass, high density polyethylene (HDPE), and concrete.
The team determined that HDPE pipe is the ideal material:
- It's tough. A 50-year track record in severe marine environments demonstrates its impact resistance. Unlike more brittle materials, HDPE will not be damaged by the anchors from recreational boats on Oswego Lake.
- It's strong. Fifty-foot pipe lengths joined into 1,500-foot segments by heat fusion welding results in a continuous, jointless pipe, unlike the existing interceptor, which has a gasketed, push-on joint every 32 feet. The walls of the new pipe will be over 3-inch thick to resist more than 2.5 times the maximum water pressure for 100 years and to ensure that any nicks, cuts, or scrapes are structurally insignificant.
- It's flexible, making it the ideal material for seismic events and handling during installation.
- It can't corrode, unlike concrete, steel, and ductile iron.
Anchors: Anchors are needed to hold the pipe solidly into the bedrock under the lake. A wide range of anchor types were considered. Rock anchors were selected because they are assured to stay in place in the lake's unique geologic conditions. They have a long track record of performance in critical facilities such as dams, buildings, bridges and tunnels. Upon installation, every installed anchor will be tested to 150 percent of the extreme load condition to ensure proper installation. By keeping the anchors close together, any upward movement of the pipe is controlled to ¼ inch over 100 years.
Seismic: The entire system is designed to survive without any repairs an earthquake with a 200-year average recurrence interval and with only minor repairs an earthquake with a 1,000-year average recurrence. This high level of protection comes at no additional cost as the buoyant approach provides an inherently flexible, seismically resistant system.
- Read Article: New Design Requires Less Ongoing Maintenance (10/14/2008)
Kelok Road Pump-Around
Although initial designs called for the replacement of the pile-supported sewer line inside the main canal, project designers figured out that pumping-around the main canal and building a new, underground pipeline in Kelok Road might reduce costs and cause less disruption to Main Canal property owners. Those initial designs required a drawdown of the main canal for at least a year. Unlike pumping around the entire lake, pumping around just the Main Canal provides a solution to a small, focused area that has benefits that appear to exceed costs for this feeder line to the interceptor. The in-lake gravity solution chosen for the bulk of the interceptor project still provides the most economical and efficient solution over the life time of the interceptor.
Staff initiated discussions with the Lake Corporation and with neighbors from the area. The result was almost universal support for the Kelok Pump-Around option. The City Council supported the replacement option in June. Some of the reasons for this recommendation include cost, preserving lake summer access, impacts to canal docks and seawalls and a slight improvement in dredging opportunities.
Mussels
In 1988 Zebra mussels were discovered in Lake St. Clair, Michigan. This invasive species, native to Russia and Eastern Europe, has since been carried to lakes in the western U.S. Since this species and its relative, the Quagga mussel, both cling to any available hard surface, LOIS team members knew it was important to investigate the impact they could have on the Interceptor sewer if they ever found their way to Oswego Lake.
The City invited six nationwide experts to a "Mussel Summit" on May 19, 2008 along with project consulting engineers and Lake Corporation representatives, to learn about the life cycle of these species, whether they could adapt to conditions in the lake and what types of prevention or mitigation could reduce their harmful effects. What the team learned was:
- Zebra and Quagga mussels will likely reach Oswego lake during the 75 to 100-year life of the LOIS line,
- The calcium, pH, alkalinity, dissolved oxygen and temperatures in the lake will not encourage the proliferation of these mussel species, and
- Even a dense buildup of mussels could be accommodated by slight increases in the length of the buoyancy pipe used in the project in order to maintain the proper grade.
The project design team now has a solution to what could have been a surprising and potentially untimely arrival of these mollusks. The team also learned from the experts that these shellfish are not even good to eat!
Click here for more information about environmental work related to the interceptor project.
Where can I go for current information?
This website will be updated as new information is available. Questions not answered here? Contact the Lake Interceptor Hotline: (503) 699-7466
If your community or neighborhood organization would like a presentation on the status of this project, please let us know by contacting the LOIS communications manager: Jane Heisler, Communication Director (503) 697-6573 (direct).