Federal officials consider making Truckee dams safer | SierraSun.com

Federal officials consider making Truckee dams safer

The U.S. Bureau of Reclamation is studying how to reinforce Stampede and Prosser Creek dams northeast of Truckee to make them safer during a major earthquake or flood.

A preliminary review of the decades-old earthen dams showed that over time the structures could be vulnerable to collapse during a strong earthquake or erode during a flood.

“Overall, there is no issue with stability of these facilities and they’re not in danger,” said Jeffrey McCracken, a bureau spokesman in Sacramento.

The improvements being considered are more of a result of improved technology to detect potential problems, rather than any immediate threat, he said.

He compared the renovations to “taking a vintage ’60s car and putting new seat belts in it.”

“We do ongoing evaluations of all our facilities,” he said.

California and Nevada rank second and third in the nation Ð behind Alaska Ð in seismic activity.

Researchers at the University of Nevada, Reno have estimated there is a 34 percent to 98 percent likelihood that an earthquake of a 6 magnitude or higher will occur in the next 50 years in the Reno-Carson City area.

Craig dePolo, a research geologist for the Nevada Bureau of Mines and Geology, said one of the most recent significant earthquakes of magnitude 6 or larger occurred in the Sierra Nevada north of Truckee. It caused minor to moderate damage to dams, highways and railroads. Plaster fell in Reno and Carson City, dePolo said.

The most powerful temblors researched in the area Ð centered in the mountains southeast of Lake Tahoe Ð were magnitude 7.1 and 7.5 in 100 B.C. and 1400 A.D, he said. They caused the ground to offset as much as 16 feet vertically and created fractures in the ground to just south of Reno.

The dam on Prosser Creek is about one mile upstream of where the creek meets the Truckee River just north of Lake Tahoe. Prosser Reservoir was completed in 1962. It has storage capacity of 30,000 acre-feet of water and is about one-third full.

Stampede Dam, completed in 1970, is on the Little Truckee River. Stampede Reservoir holds about 136,000-acre feet of water out of a capacity of 226,000 acre-feet. An acre-foot of water can supply a family of four for one year.

Water from both is used primarily to maintain flows in the Truckee River and to Pyramid Lake for fish and as reserves for municipal drinking water in Reno.

McCracken said engineers want to determine if liquefaction Ð when earthquake shaking causes sediment to lose its strength and act like a fluid Ð could cause the dams to slip.

“We’re still investigating whether it’s an issue here or not,” he said.

The other potential concern is overflowing.

“In the event of a huge, huge storm event, any dam can be over-topped by more water than it can hold,” McCracken said.

Because Prosser and Stampede are earthen dams, they would erode.

Possible improvements could include making the dams higher or enlarging the outlets, he said.

“These are potentialities Ð however remote Ð but they exist,” McCracken said. “This is a bunch of water sitting upstream from Reno and you don’t need it all at once.”

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