Dam no longer at risk of failure
The National Weather Service on Thursday issued a flash flood warning for part of the Sierra foothills near Yosemite National Park, saying that Moccasin Damin Tuolumne County was at risk of “imminent failure.”
The dam was later determined to be sound and in no danger of failing, according to Caltrans.
The 60-foot high dam, built of compacted rock on a small reservoir near the town of Groveland west of Yosemite National Park, is part of the Hetch Hetchy water system that serves the Bay Area. If the dam fails, the water will flow into Don Pedro Reservoir onemile downstream, the Tuolumne County Sheriff’s office said. That reservoir, which is nearly 4,000 times the size of Moccasin Reservoir, was 84 percent full Thursday, with enough room to hold the extra water.
“There is no threat to the Bay Area’s water supply at this time, and all people potentially in harm’s way have been evacuated out of an abundance of caution,” the San Francisco
Public Utilities Commission said in a statement at 4:30 p.m. Thursday. “The dam has not overtopped and is still intact.”
The Tuolumne County Sheriff’s office said on its Facebook page: “Facilities downstream of the dam along the creek have all been evacuated. Precautionary evacuations are being conducted at Moccasin Point Campground. The town of Moccasin is above the damand will not be impacted.”
The reservoir holds a relatively tiny 554 acre feet of water. By comparison, Hetch Hetchy Reservoir holds 360,000 acre feet, and Lake Don Pedro holds roughly 2 million acre feet.
The dam, which sits near the junction of Highway 49 and Highway 120, was built in 1930. Its hydroelectric project, located near Priest Grade, is part of the Hetchy Hetchywater system, which supplieswater tomore than 2 million people in the San Francisco Bay Area.
In a conference call Thursday evening, SFPUC spokesman Todd Elmer vouched for the water system’s safety. Elmer said the reservoir was in the process of being emptied byworkers and should be completely empty Friday, and that the 30 to 40 individuals evacuated out of an abundance of caution would stay out overnight.
“The Moccasin reservoir is one of the smallest in our system,” said Steve Ritchie, assistant general manager for water enterprise at SFPUC. “We really did get a huge amount of water coming in today, and we even used a unique mechanism to run it through a tunnel to get the water out of the system.”
Ritchie said workers were surveying signs of minor damage, such as slightly loosened rocks and eroded earth, around the reservoir’s spillways. He also offered a memorable description of the reservoir’s water in the recent rains’ wake.
“Normally it ’ s very clear, and you can see the bottom of the reservoir,” Ritchie said. “When there’s a storm, we see erosion of soils from a natural creek that leads down to the water. It ends up looking very muddy, very murky. It literally looks like a chocolate milkshake.”