Wednesday, November 20, 2013

USGS releases debris-flow study

Sites vulnerable to post-fire mudslides identified


By GREG MOORE
Express Staff Writer

A backhoe removes debris from a driveway in Greenhorn Gulch after a post-fire landslide in September. Express file photo

    A study conducted by the U.S. Geological Survey in the Beaver Creek Fire burn area has confirmed Blaine County’s choices of sites to install an early-warning system for mudslides.
    According to the study, released Nov. 14, debris flows are most frequent within two to three years after wildfires, when vegetative cover is absent or reduced and abundant materials are available for erosion and transport.
    The USGS developed a model to estimate the probability of occurrence, volume and combined hazard ranking of debris flows in the burned area. The study determined debris-flow hazard based on slope gradient and height, burn severity, clay content of soil and liquid-bearing capacity of the soil.
    The models evaluated three storm scenarios:
 A two-year-recurrence, one-hour-duration rainfall of 13 millimeters, referred to as a two-year storm (a 50 percent chance of occurrence in any given year).
 A 10-year-recurrence, one-hour-duration rainfall of 19 mm, referred to as a 10-year storm (a 10-percent chance of occurrence in any given year).
 A 25-year-recurrence, one-hour-duration rainfall of 22 mm, referred to as a 25-year storm (a 4 percent chance of occurrence in any given year).
    According to the study, the probability of debris-flow occurrence is highest in the eastern part of the Willow Creek watershed, the southern part of the Greenhorn Creek watershed, most of the Deer Creek watershed, the Democrat Gulch sub-watershed and a couple of drainages in the Baker Creek watershed. Probabilities of debris-flow occurrence were low for most of the Warm Springs Creek watershed except for a basin in the Thompson Creek sub-watershed.
    Estimates of debris-flow volume were highest in two basins in the Willow Creek watershed, the headwaters basin in the Baker Creek watershed, and the Thompson Creek sub-watershed in the Warm Springs Creek watershed.


It’s helpful to have scientific confirmation.”
Angenie McCleary
Blaine County commissioner




    Because those basins are not coincident with the high-probability basins, debris-flow hazard did not reach a level of 5 on the agency’s 1-5 scale. The hazard ranking was 4 for one basin in the burn area—Willow Creek in Camas County—using the 10-year-recurrence storm model and for three basins—two in Willow Creek and one on the south side of Deer Creek in the vicinity of Clarendon Hot Springs—using the 25-year-recurrence storm model.
    The study stated that those areas face a significant possibility of debris-flow impact to homes, buildings, roads, bridges and culverts, both within and downstream of the basins.
    Hazard rankings were at a level of 3 in Alden Gulch in the Baker Creek watershed and in the Thompson Creek drainage.
    Blaine County Commissioner Angenie McCleary said the county had already used a draft version of the USGS study to choose the general location for six precipitation monitors—two in the Baker Creek drainage, two above the Deer and Willow Creek basins, and two in the Croy and Greenhorn creek basins. McCleary acknowledged that everyone involved knew where the hazardous areas are based on mudslides that already occurred in September.
    “There was nothing surprising to come out of the debris-flow model,” she said. “Still, it’s helpful to have scientific confirmation.”
    McCleary said such confirmation is needed for any grant applications for post-fire rehabilitation work.
    Tim Merrick, public information officer for the USGS’s Idaho Water Science Center, said agency employees will be in the Wood River Valley this week to select the exact sites for the precipitation monitors. The monitors will transmit precipitation levels in real time to the National Water Information System at USGS headquarters in Reston, Va. That information will then be transmitted to Blaine County’s emergency dispatch system so residents in danger can be alerted. McCleary said the details of how that will occur are yet to be worked out.
    County Disaster Services Coordinator Chuck Turner said the county’s plan is to have the devices installed in early April.
    Merrick said exact site selection will depend on debris-flow and flood hazard levels as well as on transmission capability.
    Total first-year project cost—including installation, operation and maintenance—is estimated at $48,100. The USGS has about $10,000 available for cost share, and McCleary said the county has applied for $11,000 from the Idaho Bureau of Homeland Security, based on the county’s designation as a disaster area following the fire.
    The USGS study did not evaluate flash-flood hazard in creeks, which it said can remain for many years following a fire. However, McCleary said the U.S. Army Corps of Engineers has informed the county that it could fund a flood study. She said the Army Corps is concerned about the possibility of a landslide into the Big Wood River near the mouth of Greenhorn Gulch, and would like to determine the potential likelihood and consequences of such an event.
    The USGS study can be found at http://id.water.usgs.gov/projects/BeaverCreekFireDebrisFlow/index.html.
Greg Moore: gmoore@mtexpress.com




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