Scientists call for safety evaluation
New evidence about the area’s past seismic activity is prompting some scientists to call for a safety evaluation of the Lake Tahoe dam, plus more studies, monitoring stations and geologic mapping.
A team from the University of Nevada in Reno stressed the need for disaster planning in a March 24 report to the Tahoe Regional Planning Agency.
“We think the dam is a serious concern,” said Rich Schweickert, Department of Geological Sciences chairman. He said the dam, which is in Tahoe City, is old and “filled with cracks.”
With two major fault zones at the lake, the dam should be evaluated for safety, an evacuation route should be mapped, an emergency response plan should be put in place and building codes and utility routes should be planned with earthquakes in mind, the team said.
If an earthquake breached the dam, it could flood Reno and Sparks, said geophysicist Robert Karlin.
A strong earthquake also could wipe out buildings, beaches and roads around the lake, Schweickert said.
“In (magnitude) 6.5 or 7 earthquakes, flooding, tsunamis and landslides could take out shoreline properties…and cause major disruption of utilities,” he said. If an earthquake occurred in winter, “major avalanches could take out utilities and roads” and, especially in summer, fires could result.
The effects would be devastating in the short term on tourism, gaming and recreation, he said, and in the long term on water quality, rebuilding costs and retrofits.
While the TRPA plans no action on the report, it will consider its findings in future planning decisions, said the bistate agency’s spokesperson Pam Drum.
Learning of the report the following week, a representative of the Lake Tahoe dam’s owner, the U.S. Bureau of Reclamation, responded angrily to the scientists’ allegations about the dam.
“That’s totally and completely false and unfounded,” said Mike Larson, a civil engineer in the bureau’s Carson City office, which operates and maintains the structure.
The dam, completed in 1915, is “extremely sound and completely safe,” Larson said. “It was structurally modified in 1988 and has no cracks except for superficial ones in the grout. No structural deficiency exists for any hazard, including seismic. It would take something much more extreme than a 7 (magnitude quake) to breach it.”
Furthermore, if the dam was completely breached, “it would absolutely not flood Reno/Sparks,” Larson said.
“The entire dam could fill the Truckee River only bank-full to Truckee. It would be within flood stage in the river between Tahoe City and Truckee and they’d see a slight increase in Reno, but that’s about it.”
The bureau is concerned with protecting lives as well as with recreation, Larson said, and it has “one of the most advanced dam safety programs in the world.”
In addition to almost daily visual checks by a dam keeper, the structural integrity of the dam is given an annual review and, every three years, a comprehensive mechanical, structural and seismic analysis is done, he said.
The dam, in fact, poses so little threat to downstream users, his department considered leaving it out of an emergency response plan it is in the process of putting together with fire departments and other agencies in five California and Nevada counties, he said. But it will be included because of the number of people living downstream.
The plan, required in 1995 for all federal dams with significant downstream populations, is focusing on those at Boca, Prosser and Stampede reservoirs, as well as Tahoe. The reservoir dams pose far more flooding threat to Reno/Sparks than Tahoe’s dam, Larson said, hastening to add that they also have no structural deficiencies and receive regular checks and maintenance.
The response plan has been completed for Reno/Sparks and is now focusing on Tahoe, he said.
The Tahoe Basin also has a coordinated emergency response plan for disasters, including earthquakes, said Mike Boyle, assistant director of emergency services for Placer County, where the Lake Tahoe dam is located.
“Our disaster plan does take earthquakes into consideration,” Boyle said, adding that “disaster planning is nothing new to Tahoe.”
“We’ve seen floods, avalanches, even some fires,” he said. “Almost every year in winter, any of the roads into the basin can be closed by weather or accidents. There are a number of natural hazards in the mountains and earthquakes are certainly one of them.”
The area’s limited access is recognized, he said, but it would be difficult to develop further evacuation routes.
“Evacuation routes are limited to whatever’s open,” he said. “The roads are all evacuation routes. When (Highway) 89’s closed, we hope 267’s open.”
The UNR team said disaster planning could be aided if scientists could learn more about earthquake trends in the area and thus be better able to anticipate future occurrences.
Just in the last year, U.S. Geological Survey imaging has allowed them to clearly view the lake bed for the first time and to track the results of past earthquakes, they said, but more studies are needed because not enough is yet known to make predictions.
Faults, earthquakes, landslides
The area’s most recent quake, about 5 miles north of Incline Village, was a 4.9 on October 30, 1998. It was strongly felt throughout North Tahoe and Truckee, but bottles and cans knocked off shelves were the only reported damage.
Because there was no surface rupture, scientists can’t be certain which fault it was on, said structural geologist Mary Lahren.
The only 6-magnitude quakes this century were closer to the Verdi-Reno area than the basin, Schweickert said.
“The last fairly large one (near the basin) was a 5.9 north of Truckee in 1966,” he said. “There were two 5s about five miles south of South Lake Tahoe in 1953 and there were a number of 4s this century, but they weren’t well documented.”
The Sierra Nevada range is creeping northwest at 12 millimeters a year, Schweickert said, placing strain on two major faults along the east, north and west sides of the lake.
“There have been many 4s, 5s and 6s (earthquake magnitudes) on these faults, which we’re actively monitoring,” said seismologist Ken Smith. But, he said, “we don’t understand the behavior or know when to expect them.”
While “we’ve discovered a lot of new active faults” which are still being mapped, said Lahren, there are two known main fault zones, “north and northeast trending.”
One, dubbed the North Tahoe-Incline Village fault zone, can be seen in the lake bed from Stateline Point at North Tahoe. It extends into the lake bed, goes under Incline Village and over the Mount Rose summit toward Galena Forest, she said.
The other, the West Tahoe-Dollar Point fault zone, runs northward in the lake bed along the West Shore from Emerald Bay. It cuts across McKinney Bay, goes on land close to Dollar Point and continues north toward Truckee.
Although these faults have been monitored since the 1970s, funding is needed “to allow us to predict into the future,” Lahren said. Currently there is “no way to know the rates of movement and the recurrence interval – how far apart the earthquakes can be.”
Funding for marine coring – boring holes in the lake bed – would also help determine such information, she said.
Evidence of massive earthquakes – at least three of magnitude 7 or above within the last 10,000 years – “much more recent than thought before,” has shown up in the last few years with new equipment, Schweickert said.
It is now thought that the basin’s largest seismically induced landslide, in West Shore’s McKinney Bay, occurred within the last 10,000 years.
The quake and resulting landslide carried glacial moraines from Ward Creek and Blackwood Canyon and deposited the huge boulders – “some the size of city blocks” — into the lake, Schweickert said, where they “occupy a circular portion of the middle of the lake of about 2 to 3 cubic miles volume.”
“This mega-landslide was produced by failure of the West Tahoe fault,” he said.
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