The Army Corps of Engineers already knows water can work its way under the Martis Creek Dam, with the potential to destabilize the structure, letting a flood loose downstream in a worst-case scenario.
Now the engineers are investigating a series of earthquake faults in the Martis Valley, one of which could jut under the dam.
“A U.S. Geological Survey study showed the Martis Valley and Martis Peak are some of the youngest and most geologically active in the Tahoe area,” said Sarah Taddo with the Truckee Donner Land Trust.
The land trust owns Waddle Ranch in the Martis Valley near the dam, where the corps are cutting two trenches at likely fault locations.
“Through aerial photography we found two features we strongly believe are faults,” said Ron Rose with the Dam Safety Assurance Program of the corps.
In digging trenches, the engineers will be looking for displaced soil from an earthquake, and hope for material that can date the last earthquake activity, Rose said.
“We want to date the movement ” is it within the last 35,000 years? If not we consider it a non-capable fault, but if it is we have to take that into account with the dam,” Rose said. “And we think it is.”
Next year the corps will cut a third trench on the downstream side of the dam, and if there is evidence of movement from the same date, that means the fault runs under the dam, Rose said.
After all the research is complete, Congress will have to decide ultimately to rebuild the dam, repair it just for flood control (eliminating water storage from its functions) or remove the dam, Rose said.
The agency is keeping the water level low, currently below the level of the dam, to reduce any risk, Rose said, but a worst-case scenario could do serious damage downstream in Reno.
“If it failed rapidly tremendous amounts of water would go downstream,” Rose said. “It would take out most of Reno.”
The dam has a 1-in-350 chance of filling and failing, significantly riskier than the federal agency is willing to allow, Rose said.
This puts it in the top six riskiest dams in the country, he said.
The Truckee Donner Land Trust decided to allow the corps of engineers onto the recently preserved land, and Taddo said she has been working with them to make sure the work doesn’t damage sensitive resources.
“The sage brush and bitter brush soil is a lot hardier than the grassland and wet meadow areas,” she said.
Rose said study, environmental review, and planning could take four years before construction in a best-case scenario, and he said he thinks a flood control-only solution is most likely at this point.
“If we removed it would lessen flood protection for Reno,” Rose said.
20,400 acre-feet: Maximum flood control capacity
5,000 acre-feet: Water storage level
800 feet: Current level
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I-80 eastbound chain controls have been dropped according to Caltrans. I-80 westbound is still R-2 from Truckee to Rainbow Road.