City of Eureka Elk River Wastewater Treatment Plant Outfall Investigation and Armoring: Engineering, Survey, Geology, and Construction Observation

The City of Eureka’s wastewater outfall was installed in 1982 as part of the Elk River Wastewater Treatment Plant Project in Humboldt County, CA. Much of the outfall is installed under Elk River Spit. In 2008, City staff noticed significant erosion occurring on the western shore of the Spit that exposed the top of the outfall pipe in an area that was not protected by armor rock. This segment of the outfall was entirely covered within the Elk River Spit until the winter of 2008. Concerns over the potential causes and effects of the erosion, along with the critical status of the facility, prompted the City to initiate an investigation.

The City retained LACO to evaluate the 2008 erosion situation and our team quickly recommended rock armoring of the exposed segment of the pipeline to minimize further erosion and prevent undermining of the critical facility. We assisted the City in developing the armor design and specifications, ready for soliciting construction bids, and then provided construction staking and observation services. Construction photos and daily field reports were provided to the City. Our survey team prepared Record Drawings.

Following the armoring project,  the City retained our services to evaluate the overall stability of Elk River Spit and assess the levels of risk to the on-shore and off-shore segments of the outfall. Our project team analyzed historic aerial photos along with bathymetry and topography changes over time to determine erosional and depositional trends. These trends and observations became the basis for making recommendations for long-range actions pertaining to the facility. We accomplished the following study goals:

  • Identification of short and long term trends in the natural environment surrounding the outfall facility that could negatively affect its structural stability 
  • Assessment of the levels and types of threats that the outfall structure may be subjected to based on natural conditions and trend indications 
  • Identification of potential remediation measures based on the level and type of threat indicated
  • Recommended actions aimed at reducing risk and addressing potential threats

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