Hetch Hetchy is the name of a valley, a reservoir and a water system in California. The glacial Hetch Hetchy Valley lies in the northwestern part of Yosemite National Park and is drained by the Tuolumne River. It now serves as the primary water source for the City and County of San Francisco and several surrounding municipalities in the greater San Francisco Bay Area.
From Hetch Hetchy Reservoir, the water flows through the Canyon and Mountain Tunnels to Kirkwood and Moccasin Powerhouses, which have capacities of 124 and 110 megawatts, respectively. The Mountain Tunnel, specifically, runs 19 miles and connects the Kirkwood Powerhouse to the Priest Reservoir, just north of the town of Moccasin.
San Francisco Chronicle reports:
The San Francisco Public Utilities Commission will shut down the tunnel for an unprecedented 90 days beginning early next year, completely cutting off the Bay Area from its main water supply in the craggy heights of Yosemite National Park. Crews will inspect the tunnel and make patch repairs. The improvements will be followed by seven to eight years of construction to stabilize the water system’s main artery.
Jacobs Associates is part of the Black & Veatch winning team that was selected by the San Francisco Public Utilities Commission (SFPUC) to provide engineering services for two projects in the Hetch Hetchy Regional Water System: the Coast Range Tunnel Inspection and the San Joaquin Pipeline Number 3 (SJPL#3) Joint Repair Project, and the Mountain Tunnel Access and Adit Improvement Project.
Built in 1934, the Coast Range Tunnel is a 25.1-mile-long (40.4 km) tunnel through the Coast Range Mountains, part of a network delivering drinking water from the Hetch Hetchy Reservoir to 2.6 million residents in four Bay Area counties. A scheduled system shutdown in 2015 provided a five-day window for inspection activities, which included assessing the tunnel’s condition, identifying areas in need of repair, and taking water quality samples. The Coast Range Tunnel is a gassy tunnel requiring pre-inspection planning, ventilation and special precautions. The inspection also included inspection of the Tesla Portal manifold and its transition to SJPL#3. The manifold consists of a 126-inch-diameter (3,200 mm) welded steel pipe with 79.5-inch-diameter (2,020 mm) branch outlets.
Based on the data gathered from the inspection, the Black and Veatch / Jacobs Associates project team provided a condition assessment report for the main tunnel and basis of design report for the pipeline manifold, which included design criteria for repair of identified problems.
Approximately 85% of SFPUC water deliveries to the Bay Area come from Hetch Hetchy and the Tuolumne River watershed. The conveyance path from the river to the Bay Area includes the Mountain Tunnel, which is almost 100 years old. There is no redundancy for the tunnel, so keeping the tunnel functioning is a priority for the SFPUC.
Therefore, SFPUC needs to improve its ability to monitor conditions within the tunnel, and reduce the time required to return the Mountain Tunnel back into service. To help achieve these two goals, the project team will developed an emergency restoration plan for the existing facility and access points, designed a monitoring program to assess any changed conditions within the tunnel, and is improving the existing access roads and tunnel adits to minimize the length of time it takes to return the tunnel to service.