Predesign Study: Phases I and II

The Phase I study refined earlier engineering studies through an extensive analysis of the challenges and costs associated with designing and building an in-lake gravity pipeline to replace the existing LOIS system. This analysis resulted in the following findings:

• Deep, soft, lake bottom sediments, limited access to the lake for construction and staging. Construction work over multiple winter seasons added significantly to the challenge and costs of replacing the current LOIS system;
• Two viable systems to support a new gravity pipeline are available - a conventional pile supported system and a buoyant pipeline held below the lake surface with anchors and tethers;
• An around-the-lake pumping and piping system, appears competitive with the cost of the gravity alternatives and merits evaluation to the same level of detail as the gravity system alternatives;
• Capacity in trunk lines connected to the interceptor at the west and east ends of the lake can be restored by removing accumulated sediment and debris;
• Deteriorated segments of the system downstream from the lake along Foothills Road should be replaced and upsized.

Phase II was comprised of additional engineering analysis and the implementation of certain recommendations from the Phase I study effort. Specifically, Phase II:

• Analyzed both in-lake and around-the-lake pumping alternatives. These alternatives also satisfy the City's 'Criteria for Success';
• Evaluated two temporary capacity-relief options that would allow temporary deferral of LOIS system replacement;
• Restored capacity in certain segments of the existing LOIS system through cleaning;
• Recommended repair alternatives for portions of the LOIS system in the Foothills area;
• Completed a comprehensive analysis of interim and long term alternatives for replacing the current LOIS system resulting in a definitive body of work upon which the residents of Lake Oswego and the City Council can deliberate to determine the appropriate next steps for a solution.

Visit the documents and reports page to download a copy of the Phase I or Phase II predesign report.