Lake Eleanor Valve Repair

Yosemite National Park » Lake Eleanor Valve Repair » Document List

The City and County of San Francisco - Hetch Hetchy Water and Power (HHWP) - is requesting a waiver from the February 1981 Stipulation [Stipulations for change of location of right of way, for power line to proposed Cherry-Eleanor pumping station pursuant to map c-5393 filed February 2, 1981 in the United States Land Office at Sacramento, California] , item #6: "During years when Lake Eleanor spills the holder (the City and County of San Francisco) shall maintain the level of Lake Eleanor above elevation 4651.0 ft from the date the spill ends at Lake Eleanor through September 30". HHWP is requesting a waiver to drop below 4651.0 ft after September 2, 2013. This one time waiver is being requested to allow access and completion of maintenance and repair work on the lower release outlets. This work requires lowering the lake to an elevation of 4619 feet (HHWP datum). The repair work is estimated to take 7 to 8 weeks to complete.

HHWP must perform needed maintenance and repair work at Lake Eleanor Dam at the lower outlet works of the facility. To access these assets, the lake must be lowered to 4619 feet, or minimum pool. This is to remove all water from behind the valves to safely dismantle the valve. There are no other reasonable or safe alternative approaches to performing this work.

In the past to perform work at the lower elevations of Eleanor Dam a coffer dam was constructed, approximately 1 mile upstream of the dam. Subsequent to the work projects, the coffer dam has been breached in order to allow water to drain from behind it. An estimate of the volume impounded is less than 1,000 acre-feet. This storage allows for controlled water volumes and flow rates to be at the face of the dam while work is being performed. The existing portion of the coffer dam has an 18 inch culvert through it to allow some water to pass, which allows for maintaining minimum downstream releases.

HHWP intends to re-establish the coffer at the current location. The breached section is approximately 20 feet across and is located in the historical stream and remains the preferred pathway for water when the lake is drained. Material from previous construction and breaching of the coffer dam is stored as a mound in the bottom of the reservoir. However due to the small sizing of the material, higher turbidity in the effluent may occur when the material is put into place. In order to alleviate this issue and avoid future water quality problems when the coffer dam is needed again, an alternative construction of the breached section will be completed this fall. Clean material will be brought in to create the initial lower section of the coffer dam.

This material is locally available and will reduce the water quality impacts when it is placed. Also 3-36" culverts will be laid into place approximately 1 to 2 feet from the bottom of the coffer dam. The culverts will have seats for slide gates to be placed which allow for the water level to be regulated behind the coffer dam. This will allow for water to be drained from behind the coffer dam and allow it to remain in place following the work. When future work on the dam is necessary, the culverts will allow the reservoir to be drawn down and the culverts in the coffer dam can be operated to impound water.

The work entails six steps:
1. Lowering the lake to 4619 feet,
2. Re-establishing a coffer dam upstream of the dam,
3. Routing and pumping water out the upper 2 outlet valves to maintain minimum flow requirements,
4. Removing the valve stems and gates,
5. Completing repair work, and
6. Re-installing the valves and refilling the lake.