The Napa Sanitation District (NapaSan) located in Napa, California owns and operates a 66-in. diameter trunk sewer that parallels the east side of the Napa River for approximately three miles, connecting the City of Napa to NapaSan’s Soscol Water Recycling Facility (SWRF).
The pipeline serves as NapaSan’s collection system backbone and conveys up to 58 MGD peak wet weather flow which is more than 90 percent of the wastewater flow generated within their service area. The alignment travels through wetlands and threatened species habitat with no redundancy. The pipeline was constructed in the 1960s and consists entirely of reinforced concrete pipeline (RCP) without a protective lining or coating.
Woodard & Curran’s team completed the inspection and condition assessment of the three miles of 66-in. reinforced concrete pipe (RCP) sewer interceptor for NapaSan. As part of the work, they performed CCTV and sonar inspections, assessed CCTV footage and sonar data, and developed remaining useful life projections, project prioritization, and recommendations for repair.
Findings indicated that approximately 7,000 ft of the inspected pipeline was at the end of its useful life and required a structural rehabilitation solution. The team evaluated existing large-diameter sewer rehabilitation lining systems available in the marketplace and prepared a recommendation to rehabilitate the 66-in. trunk sewer.
Based on recommendations from the alternatives evaluation, the team completed the preliminary and final design for rehabilitation using cured-in-place pipe (CIPP) lining. In addition to the design of the trunk sewer rehabilitation, final design also included rehabilitation and/or replacement of 10 manhole structures, as well as design of a 15 MGD bypass pumping system through an environmentally challenging location along the Napa River including habitat that was home to 29 special status plant and wildlife species.
As part of this project, Woodard & Curran’s team also prepared the California Environmental Quality Act (CEQA) documentation, acquired all permits, and provided engineering services during construction including biological monitoring. The project was constructed in the summer of 2021 by SAK Construction.
This project is outstanding because of the number and diversity of issues that the team had to navigate during planning, design, and construction:
Complex Environmental Project Landscape: 15 special-status plant species and 14 special-status wildlife species had the potential to occur in the project area. The project team had to acquire permits from eight environmental agencies to complete the work.
Environmental restrictions associated with bypass and over-the-hole CIPP wet-out set-ups: To allow for contractor means and methods, the design team showed allowable work areas at each manhole location so that SAK could choose which manholes to launch lining from. The design plans also indicated manholes designated as lining reception locations only because they were in an environmentally sensitive habitat where im-pacts could not be avoided.
Access restrictions associated with public walking trail: Much of the trunk sewer alignment is located along the Napa River Trail which is a paved walkway that meanders along the Napa River with access for fishing, birdwatching, and hiking. SAK’s work area had to be truncated to avoid impacts to the trail as much as possible and had to be secured for public safety.
Coordination with multiple stakeholders: For a rehabilitation alignment that is just over one mile, there were 16 different stakeholders to coordinate with.
Large diameter bypass under active railroad tracks: Bypass piping had to be manifolded and truncated down to one 22-in. diameter HDPE bypass pipe to fit inside an existing 24-in. diameter storm drain crossing in a wetland area and under the railroad tracks near the treatment plant.
Cure water procurement during drought: The total volume of cure water required for rehabilitation of the entire alignment was approximately 3.7 million gallons including water used for circulation during cool-down. This is a big ask in an area of the country caught in the middle of a severe drought. The use of recycled water as produced by NapaSan was promoted by the District as a better use of existing resources during an on-going drought and was the ultimate choice of SAK.
Cure water disposal close to treatment plant: To come under the maximum allowed styrene concentration levels at the treatment plant, SAK used on-site filtration of the cure water prior to discharge into the sewer.