Sierra dam can handle more storms after $22 million fix
A leaking dam that prompted evacuations in the Sierra foothills during an intense rainstorm last year has been repaired and is again storing drinking water for 2.7 million Bay Area residents, San Francisco water officials said Monday.
The San Francisco Public Utilities Commission spent almost $22 million over the past year repairing and reinforcing Moccasin Dam in Tuolumne County. A storm in March sent a torrent of water and debris into the reservoir, raising fears the earthen barrier would collapse.
The 60-foot-tall structure was overwhelmed by floodwaters from Moccasin Creek west of Yosemite National Park, prompting utility workers to evacuate the area. It was the closest the dam had ever come to failure, and the neardisaster spurred a major investigation.
“We pursued a top-to-bottom safety review of the reservoir,” said Harlan Kelly, general manager of the SFPUC, who got permission to reopen from the California Department of Water
Resources Division of Safety of Dams. “We have made interim repairs and improvements to enhance the safety and security of this important facility while we continue to develop plans for longer term improvements.”
The diminutive Moccasin Reservoir was built in 1930 and is primarily used to regulate stream flows between larger reservoirs in the Hetch Hetchy system, which is managed by the SFPUC. Moccasin is one of the agency’s smallest reservoirs, with a capacity of about 554 acre-feet of water.
The trouble began on March 22, 2018, when 5 inches of rain fell in 24 hours. During one 4hour stretch, 3.1 inches of rain pounded the region and filled creeks and tributaries. The downpour came at the end of what utility officials said was the seventh wettest March in the history of the watershed.
Moccasin Creek overtopped a concrete diversion barrier upstream, sending floodwater, mud, trees and logs pouring into the reservoir.
Dan Wade, director of water capital programs for the SFPUC, said the surge overwhelmed the main concrete spillway and the rock-lined backup spillway as water rose to within 1.2 feet of overtopping the dam.
Water flowing over the top of a dam is considered among the worst possible failures, putting pressure on the structure and increasing the odds of a complete collapse.
In this case, it would have been less of a problem because Don Pedro Reservoir, a mile downstream, would have captured the overflow. Still, Wade said, the floodwater could cause damage to a fish hatchery, campgrounds, a marina, SFPUC offices and a handful of homes along the lake.
As if things weren’t already bad enough, utility workers also noticed that water was seeping out of the face of the dam.
“When you have seepage through an earthfilled dam, you know you have major concerns,” Wade said.
About 25 workers and residents downstream were evacuated. Although water levels eventually receded, the utility decided to drain the reservoir and divert water around the dam so inspectors could figure out what went wrong.
The inspectors found extensive damage to the spillways, diversion dam, valves and pipelines, in addition to severe erosion below the dam. They determined that the leaking had occurred because the water had risen above the interior concrete core of the dam and was sloshing against compacted dirt.
Workers have since extended the core wall to within about a foot of the top of the dam. The concrete had previously been 4 feet below the top, Wade said.
They also repaired erosion at the bottom of the spillways, repaired outlet valves and built a bridge that will give dam workers the ability during emergencies to manually operate an automated valve that opens the outlet gate.
The utility also built a berm to protect the Moccasin fish hatchery from flooding and repaired the badly damaged diversion dam, which had been so clogged by debris that it caused the creek to flood. Debris barriers and water pipes were replaced, and floating log booms were added to help catch trees and branches.
“This event was much bigger than any event we’ve had in the past,” Wade said. “There was a lot of damage we had to repair before we could restart it, but it looks good. The reservoir is full, and we’re able to serve water again through the reservoir.”
The advanced age of California’s water infrastructure is one reason two spillways at Oroville Dam partially collapsed in 2017. Federal inspectors concluded that outdated construction and maintenance problems led to problems at the 50-year-old dam. Nearly 200,000 people were evacuated during the scare.
State inspection reports on the Moccasin Reservoir over the past five years did not turn up any major problems.
The SFPUC has completed 97 percent of a $4.8 billion overhaul of its water distribution system, but rehabilitation of Moccasin Dam was not part of the work.
Wade said the utility will conduct studies to reassess the system to determine what additional work is needed, especially considering that more intense storms are predicted as a result of climate change.
“We do expect more events like this going forward,” Wade said, “so we will be assessing the hydraulics of this system and modifying the system over the next five to 10 years to be able to handle those very large events.”