Wednesday, January 15, 2014

County urged to address flood hazard

Army Corps of Engineers study indicates high risk

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By GREG MOORE
Express Staff Writer

    Blaine County should have plans in place to deal with a flood caused by a debris-dam collapse, since the risk of a damaging flood in the Big Wood River has about tripled since the Beaver Creek Fire, an Army Corps of Engineers study has determined.
    The fire burned about 614 square miles within the Big Wood drainage in August, leaving bare hillsides with charred surfaces less permeable to raindrops. An unusually heavy rainstorm in September caused flooding and mudslides in Greenhorn Gulch and Deer Creek.
    Darrell Eidson, the Corps’ regional technical specialist in sediment transport at its Walla Walla District, presented the study’s findings to the Blaine County commissioners in a phone conference Tuesday.
    Eidson said engineers’ modeling had determined that what had been a 3.5 percent chance of a flood of 4,000 cubic feet per second at Hailey in any one year had turned into a 10 percent chance. He said that increase will be in effect for the next three to five years.
    Eidson said the Corps’ biggest concern was the creation of a debris dam due to a flood in a tributary caused by heavy rain on denuded soils. He said that if such a dam ponds water behind it and then is breached, it can send the water down the Big Wood River in a flood.
    “You should be very vigilant in addressing this should it occur,” he said.
    Eidson said modeling of such an event was done for the river’s confluence with Timber Creek, north of Greenhorn Gulch, and for its confluence with Osborn Creek, just south of Deer Creek. He said that what had been a 10-year debris volume would create ponding 4 to 5.5 feet higher, resulting in a rise in the river of between a few inches and more than a foot at Hailey.

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“You should be very vigilant in addressing this should it occur.”
Darrell Eidson
Army Corps of Engineers

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    “Cleaning big debris flows in a tangled drainage sounds like a real challenge,” Commissioner Larry Schoen said.
    Eidson said the most hazardous scenario would be heavy rain on top of a spring snowpack.
    “We are hoping for more snow, though,” Schoen said with a laugh. “I do have to admit that.”
    Ryan McDaniel, with the Idaho Bureau of Homeland Security, said during the phone conference that 381 homes in Blaine County are within the reach of potential alluvial fans caused by mudslides.
    Ellen Berggren, Idaho Silver Jackets coordinator, said the Army Corps will release a more detailed report on flood hazard in the Big Wood Basin in February.
    Commissioner Jacob Greenberg said that as soon as the report comes out, the county should get input from all its relevant departments on how to address flood potential.
Greg Moore: gmoore@mtexpress.com

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