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Colorado Geological Survey Bulletin 49In 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 over 30 years, and is still largely relevant. Some of the more important changes since 1992 are described below. |
Significant Changes since Bulletin 49
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 size, channelization, release volume, snow density and free-water content. For a simplified approximation, a two-parameter fluid friction model developed by Voellmy in Switzerland in the 1950s has become a practical tool used worldwide. Basically, the model correlates avalanche speed to a basal (Coulomb) sliding friction and a velocity-squared dependent inertial friction.
Since 1992, 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.
- RAMMS - a 2D Swiss avalanche and debris flow simulation program for 3D terrain. Released commercially in 2010.
- 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::Extended - The latest Swiss avalanche dynamics model for mixed flowing/powder avalanches in three-dimensional terrain allows for snow entrainment and temperature effects. It is particularly useful for quantifying impact pressures of tall objects where avalanche snow densities and pressures vary with height.
Models have advanced and are useful tools. However, judgment by experienced avalanche engineers is still essential. There are important limitations, including:
- friction input values must be estimated but cannot be measured;
- avalanche-dynamics models must simplify the complex and variable physical processes;
- difficulties accounting for entrainment and deposition of snow and debris.
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 Egilsstaðir in Eastern Iceland . (2008 proceedings)
A subsequent symposium was held in in 2019 at Siglufjörður in northern Iceland where international avalanche scientists and engineers shared and learned from a wide range of experiences. (2019 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: (der Lawinenwinter 1999, in German)
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
2019 Avalanches in Colorado
In 2019, a major statewide avalanche cycled occurred causing infrastructure damage, including interuption of utilities and transporation corridors. At least 40 structures were destroyed. and loss of lives. An article in the Avalanche Review described an engineer's perspective.
A video by CAIC Director Ethan Greene shows some amazing avalanche images.Lidar
LiDAR (Light Detection And Ranging) is an optical remote sensing technology that used 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.