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Mountain Tunnel Improvements Project

July 18, 2019 Update: The environmental review document concerning this proposed project is now available at the San Francisco Department of Planning’s Website here:

About Mountain Tunnel
The San Francisco Public Utilities Commission (SFPUC) is proposing the Mountain Tunnel Improvements Project (project) to improve the condition of the existing Mountain Tunnel (tunnel), and to ensure the tunnel’s continued ability to provide quality drinking water reliably to its 2.7 million customers in the Sierra Foothills and San Francisco Bay Area. The project is in the central Sierra Nevada Mountain Range near the town of Groveland in Tuolumne County. 

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. 

construction of mountain tunnel

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.

Map of Mountain Tunnel Project

The Proposed Project

To address the deficiencies, the proposed repairs and improvements include:
  •  Repair with wired mesh and shotcrete the defects in the 11 miles of existing tunnel lining to fix deterioration;
  • 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 proposed project would take place over seven years, including five winter shutdowns (when water demand is lowest).

Project Status:
Current environmental review process: anticipated completion at end of 2019
Current project design: anticipated completion Fall 2019
Forecast construction bidding: anticipated at end of 2019
Anticipated Construction: 2020-2026
Current Estimated Construction Contract Cost: $130M to $140 M

Lumsden Road Closures 
Forest Service Road 1N10/Lumsden Road is public road used by recreationists and commercial rafting companies for activities on the Tuolumne River. Portions of Lumsden Road would be widened and resurfaced to improve access and roadway safety. If the project is approved, construction crews would need to limit access to the Meral’s Pool and Lumsden Campground areas in year 2020 and potentially 2021. 

To minimize impacts on public use of this road during project construction, the SFPUC, in coordination with the U.S. Forest Service, has identified the following schedule: 

May 1, 2020 through Labor Day, September 7, 2020
• Monday through Thursday: Lumsden Road open daily 7 a.m. to 7 p.m.
• Sunday through Thursday: Lumsden Road closed evenings, 7 p.m. to 7 a.m.
• Weekends, Friday 7 a.m. through Sunday 7 p.m.: Lumsden Road open.
• Holiday weekends that include a Monday: Lumsden Road open through Monday 7 p.m. 

September 8, 2020 thru April 30, 2021
• Full Lumsden Road Closure 

September 7, 2021 thru November 30, 2021
• Potential full Lumsden Road closure


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Last updated: 7/18/2019 4:27:25 PM