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Improve the condition of the existing Mountain Tunnel, and to ensure the its continued ability to provide quality drinking water reliably to its 2.7 million customers in the Sierra Foothills and San Francisco Bay Area.
Mountain Tunnel is an approximately 19-mile-long water tunnel that has been serving the Bay Area since 1925 as part of the Hetch Hetchy Regional Water System. It transmits drinking water originating from Hetch Hetchy Reservoir through Kirkwood Powerhouse, where it generates hydropower, to Priest Reservoir downstream. Water flows entirely by gravity through this tunnel, which is unlined upstream for over 7 miles and has an unreinforced concrete lining for 11 miles in its downstream section.
Constructed between 1917 and 1925, this tunnel has been in service for more than 90 years. Inspections of the tunnel showed deterioration of the tunnel lining and other deficiencies of the Mountain Tunnel. These deficiencies result in reduced flow rate, increased groundwater intrusion, and increased turbidity. The deficiencies also diminish the ability to provide drinking water reliably to customers, and increase the difficulty of performing maintenance in the tunnel during normal operation.
To address the deficiencies, the repairs and improvements include:
Construct a new 1075-foot Adit Tunnel at Priest Reservoir to improve maintenance access;
Construct a new Flow Control Facility with large flow control valves at the downstream end of the tunnel near Priest Reservoir to better control pressure in the tunnel, help protect the tunnel lining, and improve operational flexibility;
Lay concrete along the floor of approximately 5,000 feet of the unlined portions of the tunnel to improve maintenance access and improve hydraulic flow;
Construct a 750-foot bypass tunnel (siphon extension) at South Fork Siphon to reduce river water infiltration and associated adverse water quality effects;
Construct a larger entry portal access at Early Intake to facilitate maintenance inside the tunnel;
Make surface improvements at one of the shaft locations to reduce rainwater infiltration into the tunnel;
Remove debris that has settled on the floor of the tunnel to improve hydraulic flow;
Install slope protection, roadway stabilization and drainage improvements to provide safer access to tunnel entry points; and
Perform environmental mitigations and site restoration in compliance with permit requirements.
It is important to note that much of this work would take place underground, within the tunnel itself. This crucial water tunnel must be drained of water and taken out of service in order to work in it. Therefore, this project would take place over seven years, including five winter shutdowns (when water demand is lowest).