The work will replace an unused tunnel with two 2-million-gallon storage tanks for treated water. The estimated cost is $26 million.
The Marin Municipal Water District has launched a project nearly 14 years in the works to bolster water storage in the Ross Valley and decommission a century-old tunnel used as a water storage tank.
The five- to six-year project will close off the 9,000-foot-long Pine Mountain Tunnel that was built in 1919 to transport raw water from the Alpine Lake reservoir to Cascade Canyon for drinking water. After new water treatment standards were implemented in the late 1960s, the tunnel was disconnected from Alpine Lake and repurposed in 1971 to store up to 3 million gallons of treated water.
“You can think of it as a 9,000-foot-long tank,” said Crystal Yezman, chief engineer at the district.
The project, for which planning began around 2009, will replace the tunnel with two 2-million-gallon storage tanks for treated water. The tanks will be on the western side of Concrete Pipe Road near Bolinas and Sky Oaks roads.
A risk assessment performed by the district in 2019 found that the tunnel had one of the highest risks of failure from disasters such as earthquakes, which could threaten water supply and fire response for Ross Valley communities. The tunnel was also found to have cracks, resulting in water loss.
“The project is going to be really beneficial to the community because it will increase the storage and reduce water loss,” Yezman said.
The project launched this month and will be completed over two phases.
The first phase will prepare the site for the new storage tanks through grading work and building a 400-foot-long soil nail wall to stabilize the nearby hillside.
The project will require the district to remove 45,000 cubic yards of soil, or enough to fill more than 2,800 dump trucks. About 5,000 cubic yards of topsoil will be hauled to a landfill to prevent the potential spread of a pathogen that causes sudden oak death disease, Yezman said.
The loaded trucks will be tarped and will take a one-way route through the watershed and exit out of Natalie Coffin Greene Park. The work will result in the temporary closure of some parking spaces at the park.
The remaining soil will be deposited at Bullfrog Quarry, with some being used to create the soil nail wall. The first phase is set for completion in early 2025.
The larger second phase set to begin in the summer of 2025 will build the new water tanks and cap off Pine Mountain Tunnel.
The first phase is estimated to cost $6 million. While an engineer’s estimate is not finalized, Yezman said the second phase is expected to cost about $20 million. The project will be funded by district funds, including revenues from its capital maintenance fee.
The project will result in traffic control measures and parking restrictions in Ross and nearby areas during the first phase of the project. Some trails and fire roads in the watershed will be closed through October.
The Ross Town Council heard a presentation on the project on Thursday. Mayor Elizabeth Brekhus said she was reassured by the presentation and that the town will work with the district to monitor and address any impacts.
“And while no one likes construction, the presentation persuaded me the work is necessary and other alternatives were explored to avoid the impacts,” Brekhus wrote in an email. “If the work is performed as described, it sounds like it will be a short-term inconvenience and will result in important benefits to the watershed capacity.”
