A lesson in physics: Why will LOIS actually work?

Artist rendering by members of Brown and Caldwell shows an underwater view of interceptor system.

The new Lake Oswego Interceptor Sewer (LOIS) combines state-of-the-art design and technology - because the city deserves nothing less. So it might be logical to ask: Will it work? The answer is yes. Using rock-solid laws of physics, courtesy of such science heavyweights as Newton and Archimedes, the city's design engineering team, including consulting engineers from Brown and Caldwell, can assure you the LOIS system being designed will perform as intended for many decades to come.

What are those laws of physics behind this nearly two-mile length of buoyant, tethered, gravity sewer line? Some of them include:

Gravity: Gravity sewers have been in place since before the apple fell on Newton's head. The Romans installed the first sewer 2,500 BC. Even earlier, Roman aqueducts depended on gravity to deliver water to citizens. Without an opposing force in the way, if something is heavy, it will fall, or, in the case of sewage, flow downhill. Following this logic, the new interceptor will be kept at a slope that will allow gravity flow along the length of the pipe, all the way to the treatment plant. How can we be sure the pipe will maintain this constant downward slope? Read on.

Buoyancy: This is the lifting power of an object in a fluid and, like gravity, doesn't disappoint. Archimedes described the principles of buoyancy over 2,000 years ago, and they are as valid today as they were then. Accord-ing to Archimedes, the buoyancy of an object reflects the ability of an object to sink or float in water. If it is heavier than an equal volume of water it will sink; if it is less, it will float. Hence that redundant-looking 'buoyancy pipe,' filled with air and attached to the bottom of the sewer pipe. This smaller pipe will help keep upward pressure on the larger sewage pipe floating under the surface of the lake to maintain the proper slope for gravity flow. At the same time the buoyancy pipe is keeping the sewer pipe above the lakebed, the line is held firmly under the surface of the water (about 15 feet) by stainless steel tethers anchored into the bedrock below the lake. These opposing forces help maintain the proper pipe slope.

Thermal Expansion: Another factor to overcome in the design of the system is thermal expansion. Since the lake is a small, fairly shallow body of water, its temperature changes about 40 degrees through the seasons. With that kind of temperature change, the pipe will shrink and grow about 14 feet over the 2-mile long pipe. In the LOIS design, the tethers restrain the pipe from bowing up, the buoyancy pipe restrains the pipe from bowing down and the s-curve limits the side to side movement. These elements acting in unison ensure the right grade all the way to the treatment plant.

The interceptor has been designed with these physics lessons in mind, to provide decades of essential sewer service to Lake Oswego residents and businesses. Want to learn more? Go to http://www.loisnews.com , click on 'Reports' and Oswego Lake Intercep-tor Sewer Upgrade Project Predesign, Phase 1 - Executive Summary.

Stay tuned to this column for future in-depth looks at other questions we've received like: Why will some of the pipe be on piles and not buoyant, why not put it in the street like most sewers, what happens to sewer service when the lake is drawn down, and what if a boat anchor breaks the pipe?

Note: The in-lake portion of the LOIS Project is scheduled to begin in June 2009 and will continue for the next two years. The line replaces the existing interceptor pipeline in the lake that is undersized, overflows during rain events,and is seismically vulnerable.

Joel Komarek is the project director for the city of Lake Oswego's LOIS project. Brown and Caldwell provides engineering design services for the interceptor sewer project.