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Colorado Geological Survey Bulletin 49

In 1992, the Colo. Geological Survey published Snow-Avalanche Hazard Analysis for Land-Use Planning and Engineering by A.I. Mears. This document has served as the standard reference in the U.S. for almost 20 years, and is still relevant. However, time, technology and knowledge advance. Some of the more important changes since 1992 are described below. The Internet and related abilities to collect, store, analyze and distribute data have expanded significantly.

Significant Changes since Bulletin 49

  1. Google Earth
  2. Computer modeling
  3. Major Avalanches in Europe
  4. LiDAR Mapping
  5. GIS - Spatial Databases
  6. Climate Change

Google Earth

We've had stereo and ortho aerial images and 1:24,000 topos since the mid-20th century and, as Bull. 49 describes, these images and maps are still useful. Now, Google Earth gives us instant access to recent images, terrain and map data - even to our hand held devices while we are in the field! And we can send site info, including photos and text, instantly.

Computer Modeling of Avalanche Dynamics

Snow is one of the more difficult natural materials to describe mathematically. Its dynamic motion varies tremendously within a single avalanche event. Dynamic behavior also varies with avalanche type, size, channelization, release volume, snow density and free-water content. For a simplified approximation, a two-parameter fluid friction model developed in the by Voellmy in Switzerland in the 1950s provided the foundation for later dynamic models. Basically, the model correlates avalanche speed to a basal (Coulomb) sliding friction and a velocity-squared dependent inertial friction.

Subsequent models and advances in computing power have provided improved tools for research, mapping and load calculations. The PCM model developed in 1980 by Perla, Cheng and McClung uses two friction parameters to calculate dynamic properties of the center of mass of the flow for segments throughout the avalanche path. The PLK model developed in 1984 by Perla, Lied and Kristensen uses discreet non-interacting particles to allow entrainment and deposition in the avalanche path. The NIS model created by Norem, Irgens and Schieldrop in 1987 treats avalanche flow as a 2D granular process with frictional and viscosity input properties.

Since 1992 (post-CGS Bulletin 49), the following avalanche dynamics models have been developed:

  • Aval-1D - A Voellmy-Salm based model using depth-averaged flow; Includes a separate fluids-based powder avalanche module. Developed by the SLF and commercially available.
  • Samos-AT - An Austrian 2D and 3D model developed by the (BMLFUW) Forest Technical Service for Avalanche and Torrent Control and a private company (AVL List). It includes a granular flow model for the dense flow and turbulent mixture model for the powder flow. Originally released in 1999, it was revised and improved in 2007. It has been applied in Austria and Iceland, but is not sold commercially.
  • RAMMS - a 2D Swiss avalanche and debris flow simulation program for 3D terrain. Released commercially in 2010.

Models have advanced and are useful tools. However, judgment by experienced avalanche control engineers is still essential. There are important limitations, including:

  1. friction input values must be estimated but cannot be measured;
  2. avalanche-dynamics models must simplify the complex and variable physical processes;
  3. difficulties accounting for entrainment and deposition of snow and debris.

More information on avalanche dynamic models, along with their applications and limitations, is described in Jamieson, Margreth and Jones, 2008

Major Avalanche Cycles in Europe

Iceland - 1995

Two catastrophic avalanches hit two small towns in the northwest Iceland in 1995 causing 34 deaths. Iceland also suffered 12 fatalities in a small community in 1974. Total population in Iceland is about 300,000 people. The Icelandic Government responded with public funding to help local communities address avalanche hazards.

The Icelandic Meteorological Office (IMO) provided a national technical resource and worked with international colleagues to improve avalanche hazard mapping and analysis needed to design and construct protective measures.

Iceland shared its experience in 2008 at the International Symposium on Mitigative Measures against Snow Avalanches in at EgilsstaĆ°ir in Eastern Iceland . (proceedings)

Avalanche Winter 1999 in Central Europe

Not since the 1950-51, "Winter of Terror" when more than 265 people died, had the European Alps experienced such major avalanche impacts. The winter of 1999 might have been more hazardous, but advances in avalanche knowledge and protection limited total fatalities to 75 people. The SLF provided good documentation: (in German) or (French)

Among the hardest hit was Galtur, Austria where 34 died. In 2009, ten years later, this small community hosted an international conference to describe and discuss avalanche work completed in the decade. Both Austria and Galtur went to extraordinary lengths to protect people and facilities from avalanches. Galtur Alpinarium


LIDAR (Light Detection And Ranging) is an optical remote sensing technology that uses reflected pulsed signals to measure the distances. Terrain and vegetation height can be measured from aircraft to create 3D surfaces of the ground and forest canopy. This technology allows creation of detailed topographic maps and a mapping of tree canopy height. Lidars's ability to differentiate tree heights provides a new tool for vegetative analysis in avalanche mapping. This tool can be used to efficiently focus field observations and dendrochronology efforts.

Geographic Information Systems

GIS has grown into a large and important field of cartography. Essentially, GIS allows maps to be separated into layers and to connect map features with databases. This allows users to view maps showing the features of interest and to search and analyze data based on location or feature properties. In avalanche mapping, features of interest include slope angle, aspect, vegetation and zoning or land use. By creating Avalanche Hazard Maps on a known horizontal and vertical coordinate system, the hazard zones can be viewed by GIS users with other selected features or layers.

Climate Change

The Intergovernmental Panel on Climate Change (IPCC) has concluded that temperatures are rising worldwide. The rates and implications of warming are uncertain, but this trend deserves recognition for avalanche risk analyses.

Warming vs. Precipitation

General warming would suggest that the avalanche season will become shorter, and thus the overall hazard might become less. If precipitation patterns do not change, this reasoning might have some validity. However, most major avalanche periods periods of importance in land-use planning and engineering result from periods of prolonged heavy precipitation often accompanied by high winds. Thus, it is possible that climate change could result in an increase in avalanche hazards if storms become longer and/or more intense. The probability of these changes is unknown.

Forest Conditions

Large areas of forests in North America are experiencing high mortality rates due to drought and insect infestations. This phenomenon might be expected to continue if warming persists. When forest mortality occurs on steep slopes additional avalanche terrain could be created. Similarly, burned areas can become new or expanded avalanche terrain due to loss of vegetation in potential starting zones.

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Copyright 2011, Arthur I. Mears, P.E., Inc. & Wilbur Engineering, Inc.