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Acceptable Avalanche Risk

Risk is the probability of loss, injury or peril to persons, property, the environment. The degree of risk depends on both the frequency and severity of the hazard, as well as the exposure of the resource. The exposure of mobile resources such as autos, trains, and winter recreationists can be actively managed to reduce risk. Permanent facilities such as buildings and transmission lines have continuous exposure, so their risk depends on frequency and severity of the hazard.

Factors to Determine Acceptable Risk

  • risk tolerance of individuals and society,
  • importance of facilities and structures exposed,
  • replacement costs and other economic factors,
  • political factors,
  • legal issues and their associated costs,
  • environmental considerations
  • other factors.

The level of risk that should be considered acceptable depends on the probability of encounter with an avalanche and the consequences of the encounter. For example the risk from a 30-year return period avalanche to a storage shed used primarily in summer is much less than the risk to an elementary school to the same avalanche. Similarly there would be a much smaller risk to traffic on a county road with small traffic volume to a 30-year return period avalanche than to traffic on a busy highway exposed to a 3-year return period avalanche. Risk from avalanches should be balanced with other risks we accept in life. The type and cost of mitigation should also be balanced with the risk.

The following table provides typical return periods for various activities. The acceptable risks in the table are taken from various sources in North America and Europe but they vary widely depending on location. The values in this table are combined from CGS Bulletin 49 and Guidelines for Snow Avalanche Risk Determination and Mapping in Canada published by CAA .

Design Period (years)
Highways and railroads*
< 10 active control
Highways and railroads*
10-100 temp. closures
Ski trails*
< 1
Ski Lifts
Ski Base Facilities
Transmission (electric power) lines
10 - 100
Industrial road (no active mitigation)
3 - 30
Residential Development
30-100 w/ mitigation
100-300+ w/o mit.
Critical facilities (hospitals, schools, fire stations)
* requires active management by trained avalanche technicians

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Copyright 2014, Arthur I. Mears, P.E., Inc. & Wilbur Engineering, Inc.