Engineer: Lake Dam Unstable
Aug. 3–Strengthening the dam at Lake Perris may focus on shoring up natural sand deposits that weaken its foundation, state officials said Tuesday, adding that its stability could be compromised after an earthquake less powerful than a magnitude 7.5.
“Anything above a magnitude 6, we certainly would be concerned,” about the potential for damage, said Rich Sanchez, a chief engineer for the California Department of Water Resources, which owns the dam and lake.
Computer modeling shows that under the worst-case scenario, a rare, magnitude 7.5 quake on the San Jacinto fault five miles away, the sandy pockets in the dam’s foundation would lose strength, causing severe cracking on the southernmost edge of the two-mile-long structure. In turn, the top of the dam would slump, allowing water to tumble over it.
The lake sits in a region laced with faults, including the Elsinore and San Andreas.
The state will begin draining the popular boating lake in western Riverside County by 25 feet beginning Aug. 15, Sanchez said. The move will allow the remaining water to stay in the lake and prevent downstream flooding toward Lake Elsinore should a powerful temblor occur.
Sanchez spoke at the monthly breakfast meeting of the Perris Valley Chamber of Commerce. It was the first public meeting held by state officials in the Inland region since they announced a week ago that the 31-year-old dam has seismic weaknesses.
The state will plan public workshops in the coming weeks, said Sue Sims, a spokeswoman for the Department of Water Resources.
Riverside County Supervisor Bob Buster asked if there was any need to delay construction projects in the fast-growing area.
Sanchez said that’s a question to be answered by local officials, but he stressed it would be rare for a disaster to occur.
Mark Yarbrough, Perris city councilman, said city officials haven’t yet discussed if they should hesitate to approve new developments because the dam issue is so new.
“It’s been dumped on us in a hurry,” he said.
Sanchez did say the state opposes the northernmost route proposed for the Mid-County Parkway, a 32-mile road linking San Jacinto to Corona, because it will need the land just southwest of the lake to work on the dam. Plus, Sanchez said, existing facilities that help operate the lake as a drinking-water reservoir could be in the way of that route.
The lake’s water, piped in from Northern California, will be drained into existing pipelines that serve drinking water to much of Southern California. Devices that pump oxygen in the lake are being installed this week, Sanchez said, to cut down on pollution.
Sanchez said the lake will be drained by 260 million gallons per day until Oct. 15. If problems emerge, the process will be slower, draining half that amount each day, until Nov. 26.
The number of boats allowed on the lake at any one time will be reduced from 450 to 300 as the lake slowly shrinks by 410 acres, or 18 percent, said Ron Krueper, superintendent of Lake Perris State Recreation Area.
Sanchez said the dam’s weakness lies in a stretch of the foundation that has natural deposits of sand that are five to eight feet thick. The earthen dam was built between 1970 and 1974.
“It’s not that the engineers did a bad job,” he said. “We’ve learned a lot since then.”
It could be a year before repair options are looked at and several more after that to repair it, Sanchez said.
SHRINKING LAKE
Popular Lake Perris will be partially drained to prevent flooding in the event a large earthquake occurs. The draw-down:
–Begins: Aug. 15
–Ends: Oct. 15, or later if problems occur
–Duration: unknown
–More information: www.perrisdam.water.ca.gov
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