Snow Avalanches J
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, . ^^'- If A HANDBOOK OF FORECASTING AND CONTROL MEASURE! Agriculture Handbook No. 194 January 1961 FOREST SERVICE U.S. DEPARTMENT OF AGRICULTURE Ai^ JANUARY 1961 AGRICULTURE HANDBOOK NO. 194 SNOW AVALANCHES J A Handbook of Forecasting and Control Measures k FSH2 2332.81 SNOW AVALANCHES :}o U^»TED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE SNOW AVALANCHES FSH2 2332.81 Contents INTRODUCTION 6.1 Snow Study Chart 6.2 Storm Plot and Storm Report Records CHAPTER 1 6.3 Snow Pit Studies 6.4 Time Profile AVALANCHE HAZARD AND PAST STUDIES CHAPTER 7 CHAPTER 2 7 SNOW STABILIZATION 7.1 Test and Protective Skiing 2 PHYSICS OF THE SNOW COVER 7.2 Explosives 2.1 The Solid Phase of the Hydrologie Cycle 7.21 Hand-Placed Charges 2.2 Formation of Snow in the Atmosphere 7.22 Artillery 2.3 Formation and Character of the Snow Cover CHAPTER 8 2.4 Mechanical Properties of Snow 8 SAFETY 2.5 Thermal Properties of the Snow Cover 8.1 Safety Objectives 2.6 Examples of Weather Influence on the Snow Cover 8.2 Safety Principles 8.3 Safety Regulations CHAPTER 3 8.31 Personnel 8.32 Avalanche Test and Protective Skiing 3 AVALANCHE CHARACTERISTICS 8.33 Avalanche Blasting 3.1 Avalanche Classification 8.34 Exceptions to Safety Code 3.11 Loose Snow Avalanches 3.12 Slab Avalanches CHAPTER 9 3.2 Tyi)es 3.3 Size 9 AVALANCHE DEFENSES 3.4 Avalanche Triggers 9.1 Diversion Barriers 9.2 Stabilization Barriers CHAPTER 4 9.3 Barrier Design Factors 4 TERRAIN 9.4 Reforestation 4.1 Slope Angle CHAPTER 10 4.2 Slope Profile 4.3 Ground Cover and Vegetation 10 AVALANCHE RESCUE 4.4 Slope Orientation 10.1 Rescue Equipment 10.2 Immediate Action CHAPTER 5 10.3 FoUowup Action 10.4 Avalanche Accident Report o AVALANCHE HAZARD FORECASTING 10.5 Case Histories 5.1 The Contributory Factors 5.11 Factor Analysis CHAPTER 11 5.2 Delayed-Action and Climax Avalanches 5.3 The Ram Profile 11 AREA SAFETY PLANNING 5.4 Wet Snow Avalanches 11.1 Terrain Analysis CHAPTER 6 11.2 Climate Analysis 11.3 Hazard Classification STANDARD SNOW OBSERVATIONS AND 11.4 Operations Plan TERMINOLOGY BIBLIOGRAPHY For sale by the Superintendent of Documents, U.S. Government Printing Office, t , Washington, 25, D.C. — Price 60 cents 2332.81 SNOW AVALANCHES Introduction The first publication by the U.S. Forest Service in 1952, has now become a widely accepted and ef- dealing with snow avalanches, "Alta Avalanche fective means of avalanche control in the Western Studies," appeared in 1948. This booklet reported United States. On the other hand, the basic the results of experiences and studies at Alta, principles of snow mechanics and avalanche for- Utah, where the problem of snow safety for the mation, and the commonsense rules for safe con- skiing public had confronted forest officers since duct in avalanche terrain have not changed, the area had been established 10 years previously. though an understanding of snow mechanics in- The study program at Alta continvied, and ex- creases with passing time and longer experience. perience was rapidly gained in methods of This new handbook provides an opportunity to avalanche control which made possible increasing bring forest officers up-to-date instructions on the use of excellent alpine slopes without compromis- most modern methods of avalanche hazard fore- ing public safety. Study stations similar to that casting and control, and to recast the general text at Alta were also established at Stevens Pass in on avalaiiches in a more informative and concise Washington and at Berthoud Pass in Colorado, pattern. This handbook is intended as a textbook, where the investigations and tests of control as a training aid, and as a field manual for the use methods could be extended to encompass a wider of forest officers and others whose duties involve range of climatic conditions. In 1952 the expe- avalanche hazard. rience of forest officers working in these areas was Directly, or indirectly, many persons and or- compiled and published as the "Forest Service ganizations have contributed to this handbook. Avalanche Handbook." This was designed to be a We acknowledge a particular debt to our European manual of instraction for foresters confronted colleagues for material on the physics of snow and with snow safety problems following the rapid avalanche defense structures. postwar expansion of skiing and development of new ski areas on national forest lands. It also served as a general text on the nature, character- istics, and formation of snow avalanches. The h extensive appendix included a report on avalanche studies and research programs then in progress. The "Forest Service Avalanche Handbook" was ■à M ' the only text in English on this subject widely available in the United States, and it was soon accepted by the public for use far beyond that for which it was designed. The training program of the National Ski Patrol System alone generated a heavy demand for it. Requirements of other groups, such as ski instructor associations, plus mterest of the general skiing and mountaineering public, soon exhausted stocks at the Government Printing Office. Eight years have passed since the first "Forest Service Avalanche Handbook" was published. During this period much practical experience has been gained, both through study programs and through the ever-widening practical application of snow safety measures. Many techniques pro- posed or described as innovations in 1952 are now routine procedures used by Forest Service snow F-495188 FIGURE 1.—105 mm. recoilless rifle mounted on a Forest rangers. For example, the use of artillery (fig. 1 ), Service truck. Tliis is a highly mobile weapon for which was barely out of the experimental stage avalanche control. 2332.81 SNOW AVALANCHES Chapter 1 1 AVALANCHE HAZARD AND PAST location than lack of potential. In the first World STUDIES. The remote mountain areas of War, on the Austro-Italian front in December America are the scene of hundreds, perhaps thou- 1916, thousands of soldiers lost their lives in snow- sands, of avahxnches each winter. No man wit- slides during a single 24-hour period. Total cas- nesses them, and tliey have no measurable effect on ualties from avalanches during the war on that civilization. But in certain locations, which are front were greater than from military action. increasing in number, avalanches and mankind do This was the greatest avalanche disaster in rec- meet. This creates avalanche hazard—a threat to orded history; it was brought about by an ab- life and property from the cascading snow (tig. 2). The avalanche is a great natural force, not in- normal concentration of men stationed in the ferior in ix)wer to the tornado, the flood, and the mountains, abetted by an abnormal winter. It earthquake. It is not so well known as a destroyer demonstrates the difference between avalanches of life and property more because of timing and themselves and avalanche hazard. r-49B189 FIGURE 2.—A climax, hard slab avalanche ( HS-AE-5—meaning of abbreviations is given in figure 38). This slide nearly buried a lift terminal, but lost its momentum just in time to prevent any damage. Such large slides have not been observed on this slope since it has been stabilized by routine artillery fire during and after every storm During the Gold Bush period, from about 1860 Alta, with its grim history of death and de- to 1910, the mountains in tlie Western States were struction at the hands of the "white death" was miicJi more heavily populated than they have been reborn as a winter recreation area and the site of at any time since then. the first avalanche observation and research cen- From the Sierras and the Cascades to the Rock- ter in the Western Hemisphere. ies, miners swarmed into the high country and In 1960, avalanche hazard threatens more per- built their camps wherever they found silver and sons and more enterprises than ever before, and gold. Telluride and Aspen in the Colorado Rock- the number is increasing. The spectacular in- ies, Atlanta in the Sawtooths, Mineral King in the crease in winter sports activities throughout the Sierras, Alta and Brighton in the Wasatch—to United States in the decade ending 1960 has been name a few—were fainous in their day for fabu- responsible for the greatest increase in hazard. lous riches. They were known also for the sudden, The number of skiers in the national forests of the dramatic, and complete destruction by snowslides Intermountain Region has increased two and one- that often obliterated both miners and their half times since 1950. The number of persons ex- camps. posed to possible danger continues to increase The mining era was the first time that human yearly. beings in large numbers invaded the alpine zones Witli the skier have come businesses, hotels, pri- of the United States. The consequences were vate cabins, and heavier highway traffic, all con- often disastrous. Verifiable records of avalanche tributing to the economy and development of the accidents in the vicinity of Alta, for instance, go West. As new highways are planned and use back as far as 1874, when the mining camp was of the older ones increases, more and more non- practically obliterated by avalanclies; more than skiers are also exposed to possible snow hazards 60 lives were lost. During the next 35 years, ava- in the many mountain passes. The development lanches killed 67 more persons. Three slides in and increasing population in the Western States three diiferent years wiped out more than 10 per- is bringing more and more people in contact wúth sons each. snow avalanches.