On its way westward from the granite-lined basin at the Hetch Hetchy Reservoir, the pristine water that flows to 2.7 million Bay Area residents makes a remarkable journey through pipes and tunnels that span more than 150 miles. 

A key part of this vital network is the 19-mile-long Mountain Tunnel, completed in 1925. The tunnel collects water that has passed through the Kirkwood Powerhouse and then takes it underground through the rugged Sierra mountains until releasing the water to power the next electrical generation plant at Moccasin.

After nearly a century in constant use, the San Francisco Public Utilities Commission (SFPUC) is now undertaking improvements that will ensure the Mountain Tunnel will continue to provide fresh water to future generations. Chief among these is to improve the ability to control the flow of water and its level in the tunnel. The SFPUC is also upgrading access to several points along the tunnel, called adits, where crews can enter, evaluate, and make repairs and improvements. 

Another key aspect of this project is inspecting the existing tunnel lining and making repairs and changes that will ensure its smooth future operation. Where Mountain Tunnel terminates at Priest Reservoir, two vital pieces of infrastructure are also under construction. The Priest Adit, which is an access tunnel that allows people and machinery to enter Mountain Tunnel (when it’s empty), is being solidly reinforced. Next to that is the all-new Flow Control Facility (FCF), which will operate as an enormous valve, allowing operators to manage the consistency and volume of water flowing through the tunnel.

“We’re pleased with the progress so far on this complex set of individual projects within the overall large project," said City Project Manager, Randy Anderson. “A huge milestone this past year has been finishing the FCF shaft in time for the winter shut down. Our crew is also working non-stop to ensure a significant amount of rehabilitation work inside the tunnel gets completed by the time we re-open the flow of water from Hetch Hetchy in March," he explained.

This work inside the tunnel will need to continue with future winter shutdowns until the inside of the tunnel is fully repaired. After the winter shutdown, work will then transition to finishing up the large building that will be constructed over the top of the FCF shaft and completing the Priest Adit. "This rehabilitation work is mainly comprised of repairing portions of the concrete lining that has deteriorated over the years and filling in the voids between the concrete lining and the native rock to ensure another 100 years of service," Anderson said. 

Of course, to do this work, the tunnel must be emptied of water, which means the SFPUC has to rely on other reservoirs to temporarily fill the gap. Since Hetch Hetchy typically provides 80% of all the water in our system, a shut-down of Mountain Tunnel needs to be limited in duration. For that reason, this project is taking place over six years, including five winter shutdowns (when water demand is lowest).