The Calavares Reservoir dam replacement project is the largest among 82 in the Hetch Hetchy Water System Improvement Program and now, this summer, it is about to get bigger by tens of millions of dollars.
The Calavares Reservoir has been filled only to about 40 percent of capacity since 2001 because the 88-year-old dam holding the water back is subject to liquefaction. If the dam does fail, the lower water level is intended to limit damage downstream.
"There's some loose material in the base of this dam that could liquefy in a major earthquake. We are about 500 yards from the Calavares fault which has a maximum credible earthquake of 7.25," said Dan Wade, the Regional Project Manager.
Heavy equipment called "scrapers" now do laps digging and hauling material out of the canyon that will be filled with water when the new dam is done. It will rise just downstream of the old one and was supposed to be done in 2015, but an ancient landslide was just found during excavation one of the slopes. That unstable geology means the project had to be re-designed, at a cost of two more years and $163 million more dollars.
However, in the event of a drought or if Hetch-Hecthy is unavailable, we need the Calavaras. It represents 50 percent of our storage capacity.
"It actually is being used during construction and its continued to supply water to the customers, but we won't be able to have full use of the reservoir until the project in complete and so it is extremely important for us to continue moving forward expeditiously," said Wade.
Along with a re-design and a delay, the ancient landslide yielded fossils from when the area was the beach.
"This is a shark's tooth. We think it's probably related to a carcharodon, which is the same general family that the Great Whites are in," said Jim Walker, the project paleontologist.
Walker says the fossils probably date from 20 million years ago. He said there is a fossil much like a whale. The whale is still in the hillside where it is not expected to add to the delay.
Most of the additional cost and contingency will come out of a voter approved 2002 system-improvement bond, but another $55 million will have to come out of a separate San Francisco Public Utilities Commission maintenance fund and needs to be approved by the supervisors.