City of Los Angeles - Hyperion Treatment Plant 1- and 5-Mile Outfalls 50-Year Checkup

The City of Los Angeles with Parsons undertook an extensive program to assess the condition of the Hyperion Treatment Plant 1- and 5-Mile Outfalls. The program incorporated the latest modeling and surveying technologies and included: man-entry into the outfalls for visual inspection; precision bathymetric studies; coring and testing of the reinforced concrete pipe walls; peizometric studies to establish the hydraulic performance; offsite wave tank modeling; and geotechnical investigations.Field activities involved shutdown of the 5-mile outfall and diversion to the backup 1-mile outfall. Meetings with stakeholders and an extensive testing program to ensure water quality standards during the bypass were conducted.The 50-year check-up determined that the outfalls are in good condition, requiring mainly routine maintenance and minor rehabilitation work. This paper describes the methodology developed for the assessment program. The detailed information and conclusions developed regarding the condition of the outfalls are summarized, and lessons learned are discussed.