AVALANCHE
Avalanches are a rapid down-slope movement of snow, ice and debris that can be triggered by
ground shaking, sound, extreme weather, human, or animal movement. Avalanches consist of a
starting zone where the ice or snow breaks loose, a track, which is the grade or channel the
debris slides down and a run-out zone where the snow is deposited.
Since the 2019 plan, Utah has had numerous avalanches. Avalanches are one of the deadliest
types of natural disasters in Utah. According to the National Oceanic and Atmospheric
Administration’s (NOAA) National Centers for Environmental Information (NCEI) database,
between 1996 and 2022, avalanches have caused 128 deaths in the state accounting for 70% of
severe weather-related deaths.
An avalanche is a mass of snow, ice, and debris sliding rapidly down steep slopes. Snow
avalanches occur in the mountains of Utah during the winter and spring seasons as a result of
snow accumulation and unstable snowpack conditions. January through April avalanche risks
are the highest in Utah where they pose a significant mountain hazard. Annually, avalanches
account for more deaths than earthquakes in the US. Determining the probability of an
avalanche occurring can help save money and protect human life.
The two primary factors impacting avalanche activity are weather and terrain. Large, frequent
storms deposit snow on steep slopes to create avalanche hazards. Additional factors that
contribute to slope stability are the amount of snow, rate of accumulation, moisture content,
wind speed and direction and type of snow crystals.
Topography also plays a vital role in avalanche dynamics. As shown in Figure 4-11 and as
discussed by Utah Avalanche Center, slope angles between 30 to 45 degrees are optimal for
avalanches. The risk of avalanches decreases on slope angles below 30 degrees. At 50 or more
degrees they tend to produce sluff or loose snow avalanches that account for only a small
percentage of avalanche deaths and property damage annually.
Dry or slab avalanches occur when a cohesive layer of snow fractures or shatters like a pane of
glass and slides as a unit down the mountainside breaking apart as it slides, typically producing
a snow dust cloud. Slab avalanches occur when additional weight is added rapidly to the
snowpack, overloading the buried, weaker layers of snow. Dry snow avalanches usually travel
between 60- 80 miles per hour, reaching this speed within five seconds of the fracture and
resulting in the deadliest form of snow avalanche.
CASE STUDY
Park City deflection dam project
The Utah Avalanche Forecast Center (UAFC) has made progress forecasting and identifying avalanche risks. By utilizing new technology UAFC is able to provide up-to-date information on their website and hotline on weather conditions, snow stability and avalanche danger ratings for areas across the State of Utah. UAFC and Utah ski resorts have also increased their efforts to educate outdoor enthusiasts and city and county emergency managers about avalanche hazards so the public can safely recreate in Utah during the winter months.
Permanent structures like the Park City deflection dam pictured above are built on mountains above inhabited areas with limited precautionary evacuation measures to limit the size of slides by stopping small avalanches before they gain momentum and impede fracture propagation. & Channelizing is most effective on 12 to 20 degree slopes and is commonly used when multiple structures need to be protected.
In 2002 the State of Utah created an ordinance with avalanche hazard zoning for Salt Lake County ski areas and Big Cottonwood Canyon, Little Cottonwood Canyon, and Provo Canyon. This created a red zone where development was prohibited because avalanches were too frequent or too large to mitigate and a blue zone where residential development is allowed with engineered mitigation strategies in place.
The Town of Alta has also adopted a regulation requiring that an avalanche expert certify that any structure proposed for construction will withstand the impact of an avalanche.