Napa, California

Trenchless Technology Leveraged to Rehabilitate Trunk Sewer

The Napa Sanitation District (NapaSan) oversees a sewer collection system in Napa, California consisting of 270 miles of mains, including a 66-inch diameter trunk sewer that parallels the eastern bank of Napa River for about three miles through wetlands and threatened species habitat. This pipeline connects the City to NapaSan’s Soscol Water Recycling Facility (SWRF), is the backbone of the collection system, and conveys up to 58 MGD peak wet weather flow. That is more than 90 percent of the wastewater flow generated within NapaSan’s service area. Installed in the 1960s, the trunk sewer is constructed entirely of reinforced concrete pipeline (RCP) without a protective lining or coating and the system has no redundancy for this asset.

NapaSan partnered with Woodard & Curran to conduct an inspection and condition assessment of the three-mile stretch of 66-inch diameter trunk sewer. The project team performed CCTV and sonar inspections, assessed the data, and developed remaining useful life projections, project prioritization and recommendations for repair. The team identified more than one mile of the inspected trunk sewer had reached the end of its useful life and required structural rehabilitation. With pertinent data on hand, the project team identified rehabilitation alternatives and developed a preliminary and final design to rehabilitate the compromised section, including the design challenge of routing more than 20 MGD through a bypass pumping system along the Napa River. The design opted for use of trenchless technology in the form of cured-in-place pipe (CIPP) lining.


Environmental Agencies Involved in Permitting


Special-Status Plant Species


Special-Status Wildlife Species

Navigating an environmentally sensitive project site     

The inspection, assessment, and resulting rehabilitation of this sewer trunk was inherently challenging due to its location along the Napa River, in an area which is critical habitat for 15 special-status plant species and 14 special-status wildlife species. This required Woodard & Curran’s experts to collaborate with eight different environmental agencies to ensure all permitting was procured for the project to move forward.  In addition, the Woodard & Curran team acquired the state’s environmental clearance under the California Environmental Quality Act (CEQA) to perform the work.

The design plans detailed distinct areas at each manhole site where contractors could safely work without impacting sensitive environmental areas along the existing alignment. This helped the contractor identify where best to deploy their crews for the CIPP work while still meeting stringent permit requirements. Certain manholes located within highly sensitive environmental areas were designated as lining reception locations to minimized or totally avoid impacting environmentally sensitive habitats.

Another critical environmental consideration the design team had to account for was the large volume of cure water necessary for the rehabilitation project. Given the drought status in California, this was a significant demand on water resources. However, the team worked with the client to leverage recycled water resources produced by NapaSan to help mitigate exacerbating drought conditions. A mobile filtration system was used for the cure water prior to discharging to the sewer system to comply with the maximum allowed styrene concentration levels at NapaSan’s treatment plant.

Minimizing impact to a heavily trafficked area

In addition to sensitive environmental habitat, the trunk sewer’s alignment also follows the Napa River Trail, a paved walkway providing area residents with recreation access, such as hiking, birdwatching, and fishing. The project team collaborated with a myriad of stakeholders and the construction crew to reduce impact to the trail for both public access to the resource and public safety.

The bypass required to ensure sewer service was not disrupted for ratepayers met an additional hurdle with an active railroad crossing. The bypass piping had to be manifolded and truncated to one 22-inch diameter HDPE bypass pipe. This allowed the bypass to fit inside an existing 24-inch diameter storm drain to cross through a wetland area and travel under active railroad tracks so as not to disrupt rail service. Similar measures were taken to reduce impacts to area vehicular traffic.

Receiving industry accolades

The complexity of this $5.6 million project was featured by Trenchless Technology Magazine in October 2022. It will also be recognized during the 2023 NASTT No Dig Conference in Portland, Oregon, as the 2022 Trenchless Rehabilitation Project of the Year – Runner Up.

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