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International audience; On forested paths, dendrogeomorphology has been demonstrated to represent a powerful Vincent Jomelli* tool to reconstruct past activity of avalanches, an indispensable step in avalanche hazard Delphine Grancher* assessment. Several quantitative and qualitative approaches have been shown to yield Markus Stoffel † ‡ reasonable event chronologies but the question of the completeness of tree-ring records remains debatable. Here, we present an alternative semi-quantitative approach for the Christophe Corona † determination of past snow avalanche events. The approach relies on the assessment of Daniel Brunstein* the number and position of disturbed trees within avalanche paths as well as on the Nicolas Eckert § and intensity of reactions in trees. In order to demonstrate that no bias was induced by the dendrogeomorphic expert, we carry out a statistical evaluation (Classification and Regres-Michael Deschatres § sion Trees, or CART) of the approach. Results point to the consistency and replicability *Laboratoire de Géographie Physique, of the procedure and to the fact that the approach is not restricted to the identification of Université Panthéon-Sorbonne, UMR high-magnitude avalanches. Evaluation of the semi-quantitative approach is illustrated on 8591 CNRS, 1 place Aristide Briand, Fa well-documented path in Chamonix, French Alps. For the period 1905-2010, comparison 92195 Meudon cedex, France †Dendrolab.ch, Institute of Geological between the avalanche years recorded in a substantial database (Enquête Permanente sur Sciences, University of Berne, les Avalanches, or EPA) and those defined with dendrogeomorphic techniques shows that Baltzerstrasse 1 ‫ם‬ 3, CH-3012 Berne, the avalanche record reconstructed from tree-ring series contains 38% of the observed Switzerland events.

Each year in the Carpathian Mountains and the Sudety Mountains snow avalanches cause a great number of accidents. Avalanches also threaten buildings and affect the environment. The latest studies in Poland aim to implement advanced snow avalanche hazard mapping procedures, which would allow the creation of complex cartographic products for the location of avalanche hazard areas. These preliminary studies showed that results of these procedures strongly depend on the quality of the input digital surface data. The main goal of this study is to investigate this problem in detail through comparison of different types of Digital Elevation Models (DEMs), putting stress on high resolution DEMs generated from airborne and terrestrial laser scanning, in the context of estimating potential avalanche release areas and making run-out calculations. Test sites in the Tatra Mountains in the Carpathians and in the Karkonosze Mountains in the Sudety Mountains were selected for this study. The analysis was performed using Swiss Rapid Mass Movements (RAMMS) model and modified script on delineation automated release area. The study recognized that not only quality but also resolution of a digital surface models influence the accuracy of release area and volume estimation, calculated topography parameters, location of avalanche track and other parameters calculated by dynamic models.

The avalanche hazard in the United States is most severe in the continental zone of Colorado, where property damage and deaths exceed those in any other state. The continental zone is normally characterized by a shallow snowpack, faceted crystal growth, and relatively fewer avalanches as compared to the coastal and intermountain zones farther west. This study illustrates that variations in the avalanche character in the continental zone may, at times, resemble some less continental characteristics that are found farther west as a result of anomalous atmospheric circulation patterns. Results from cluster analyses show that some sites in the southern portion of the continental zone generally represent a less continental character. Anomalies of 500-mb heights explain the variability of avalanche climates for selected sites, particularly for Berthoud Pass, within the continental zone. Negative heights over the southwestern United States during early winter correspond with less continental conditions, but the ...

Since avalanche hazard mapping is a very time-consuming affair, it is our objective to develop an appropriate method to map large remote areas. This is based on satellite imagery and on a digital elevation model (DEM) tested at two sites in the Swiss Alps. To simulate the avalanche hazard, the existing Salm-Voellmy model was modified for the computer environment and extended to include the characteristics of avalanches within forested terrain. The forests were classified with Landsat Thematic Mapper (TM) data. So far, only a single forest class has been established. While the separation of forest, shrub and non-forested areas near the timberline poses problems, the classification of small openings and avalanche tracks inside the forest was successful. Comparison with the existing avalanche cadastral map revealed that 85 per cent of the risk areas were correctly classified. Separation into the defined ‘red’ and ‘blue’ danger zones, on the other hand, was not satisfactory. For an operational application, further improvements are necessary. However, the general approach is very promising and should lead to more reliable hazard maps for planning purposes, as well as to new and better insights into the mutual interactions between snow and forest.

This paper explores applications of digital photogrammetric modeling of mountainous terrain in forecasting snow depth and the meltwater avalanche hazard in a mining region of northwestern Russia. A series of initial maps (initial terrain points, relief of snow-flow centers, slope angles and orientations) provides successively more specific information, culminating in the compilation of a map of maximum snow cover depths at a center of water-saturated snow flows (representative reference site) on the northern slope of Mt. Aykuayvenchorr. Translated by Edward Torrey, Alexandria, VA 22308 from: Geomorfologiya, 1993, No. 3, pp. 75-83.

The present status of development work on avalanche hazard maps and their compilation is reviewed and the future prospects of such research are defined. Particular attention is devoted to methodolo...

The authors describe a procedure for the compilation of maps of the avalanche hazard in high-mountain regions of Afghanistan. The basic sources of information include space imagery (photographs and scanner imagery), weather station data, and other geographic information on relief, elevation, location of moisture sources, etc. The methodology features the compilation of series of increasingly more specific and informative maps and graphs regarding the avalanche hazard: terrain types, snow cover depth and seasonal extent, duration of snow cover and its elevational zonation, snow as a factor in avalanche formation, and summary map of avalanche hazard. Translated by Elliott B. Urdang, Providence, RI 02906 from: Materialy glyatsiologicheskikh issledovaniy, 1991, No. 71, pp. 86–93.

To protect rural areas and roads from disasters caused by snow avalanches, avalanche hazard maps are being developed. In Japan, when determining snow avalanche runout distances, empirical values are normally used. These assume that the line-of-sight angle at the starting point of an avalanche, viewed from its runout point, is less than 18° for surface-layer avalanches and less than 24° for full-depth layer avalanches. This rule of thumb ignores topographic features of the slope carrying the large mass of snow. We have made analyses to find a method that can obtain the avalanche runout distance, taking into consideration the longitudinal profiles of the sites, using regression analyses on data from 66 avalanches. The line-of-sight angle can be expressed sufficiently by functions, such as line-of-sight angle between the starting point of an avalanche and a point on its course of flow 10° in gradient, an angle at the starting point of an avalanche, and the curvature of an approximate quadratic curve of topography. Based on statistics for the above relationship, we have established four different ranking categories to classify the degree of hazard. If the starting points of avalanches are known, hazardous areas can automatically be ranked on a basis of topographic data.

Avalanche hazard forecasters must evaluate a number of different important parameters which often vary markedly over time and distance. Computer applications have been developed to help the avalanche forecaster manage the complex data. These include programs which simply graph and tabulate data into easily ingested displays, database and statistical software, deterministic models of the snowpack evolution and stability, and finally, networks of avalanche information. This paper is an overview of computer software currently available in English and in use in the United States.

Snow avalanches pose a considerable risk in the Patagonian Andes. Despite this, avalanche hazard maps are rare for this region. One way to reduce this risk uncertainty is to apply numerical models that predict avalanche runout. These models require that the avalanche release areas must be precisely determined (Maggioni and Gruber 2003) and the friction parameters used by the models must be adjusted to local terrain and snow conditions (Furdada et al. 2002). These parameters can be estimated by simulating avalanche events for which the extent of runout is known. Dendrochronological methods become a valuable tool for precisely defining the spatial extent of past events in the Andes where directly observed evidence is not available. Tree-ring studies are also important for reconstructing years of avalanche occurrences in such cases (Casteller et al. 2008).

OBJECTIVE: We investigated whether frequency of avalanche accidents corresponds with the danger assessment given in avalanche hazard tables and with topographic factors of the avalanche origin. METHODS: A retrospective review of official avalanche surveillance data and of medical reports from helicopter-based emergency medical systems was conducted. RESULTS: Most victims involved in avalanche accidents are ski tourers and off-piste skiers, about 90% are male, and age ranges from 10 to more than 70 years. Sixty-seven percent of accidents occurred when moderate and considerable danger was forecast. In 94.0% of accidents avalanches were triggered by victims themselves or by nearby recreationists; 32.6% of avalanche slides occurred on extremely steep slopes at gradients exceeding 40 degrees . Overall incidence of accidents before noon was 25.0%. Professional stand-by avalanche parties significantly contributed to rescuing avalanche victims, but the chance of survival is diminished when arrival was delayed. All patients who were able to free themselves from avalanche burial survived. Of those rescued by companions, 78.0% survived compared with only 10.0% of those rescued by organized rescue teams (P

Medium-resolution scanner images from the “Meteor” satellite for different seasons were used in tracing the seasonal position of the snow cover boundary in Afghanistan. Supplemented by data from field measurements, these images were used to compile 13 snow cover maps at 1:5,000,000 showing seasonal snow distribution and depth, dynamics of the snow boundary, and dates of formation, melting, and duration of the snow cover. The influence of the Indian monsoon on snow cover distribution is discussed. These materials are important for evaluating the avalanche hazard. Translated by Edward Torrey, Alexandria, VA 22308 from: Materialy glyatsiologicheskikh issledovaniy, 1989, No. 67, pp. 44–49.

Canadians claim that the recently constructed Coquihalla Highway over the Rocky Mountains is one of the most spectacular and scenic superhighways in the world (Fig. 1). Unfortunately in winter snow avalanche activity is widespread and about 150 avalanche sites affect almost 17 km of the route. In one critical area a roof structure called the Great Bear Snow Shed has been constructed over the highway to protect traffic throughout the winter. This particular area of extreme avalanche hazard has a history of high avalanche frequency. Here heavy snowfall, steep terrain and adverse geological factors combine to create potentially unsafe ground above the road (Fig. 2). The Coquihalla Highway is a four-lane dual carriage-way and the only tollway in British Columbia. It starts approximately 200 km east of Vancouver near the township of Hope on the Trans-Canada Highway and runs northeastwards for 195 km through Merritt to rejoin the Trans-Canada Highway at Kamloops. The new highway follows a series of mountain corridors through the rugged Cascade Ranges and thereby reduces the Hope-Kamloops travel distance by 80 km. The main corridors are primarily glacial in origin and contain extensive deposits of moraine and fluvioglacial drift in the valley bottoms. Along the route elevations greater than 1900 m are commonplace and relief may be as much as 1300 m. The annual snowfall in this part of the Cascade Range is usually less than 8 m, but in particularly cold years the snowpack may reach depths of more than 16 m. Extensive drifting during

The risk resulting from natural hazards can be derived from the combination of parameters of physical processes and the damage potential. Even though the damage potential has been taken into account more frequently, quantifying statements are still missing. This study presents a detailed recording of the damage potential in the study area and describes the development of the damage potential since 1950 in decades. In the community, the increase of the number of buildings and their values is above average of the region. 37% of the existing buildings are located in legally declared avalanche hazard zones. In these areas, the probability of presence of persons increased considerably due to tourism activities and shows substantial seasonal fluctuations. If the analysis of the damage potential and the hazard estimation are performed with the same degree of precision, risk analysis and risk management will be improved significantly.

The EU programme CADZIE (Catastrophic Avalanches: Defence Structures and Zoning in Europe), was established after the catastrophic 1999 avalanche winter in Europe. The overall objective of the programme is improved snow avalanche risk management by: (1) improved avalanche hazard zoning by computational models; and (2) improved understanding of the interaction between defence structures and avalanches. One contribution to meet the objectives is a database of well-documented extreme or deflected avalanche events in the six countries of the partners of the programme. The database contains observational, topographical and meteorological snow avalanche data with reliability, as well as references, copyrights, etc., all in a convenient framework based on common formats. The structure, contents, and potential use of the database are described. Example calculations of extreme and deflected events made by the NGI user interface "SKRED", for practical use of avalanche computational models, present applications of the database. Finally, further development of the database and of the computational models to meet the future needs in avalanche hazard zoning is proposed.

A review of circumstances surrounding 19 backcountry deaths in Alberta, Canada, between 1980 and 1991 suggests several factors that increase the risk of injury or death. This study provides a descriptive profile of a fatally injured backcountry skier and the circumstances surrounding his or her death. The individual most likely to suffer a fatal injury while participating in a backcountry ski activity is a 36-year-old man. He is typically an experienced backcountry skier who chooses to ski in areas where the avalanche hazard is known to be moderate to extreme. Delineating the personal characteristics of those at risk for backcountry injury and identifying situations that put them at risk will enable better design of education programs. Targeting high-risk groups may also reduce the incidence of death from this activity. Language: en

Predicting heavy mountain snowfall, which is critical for avalanche hazard forecasting, is difficult due to complex interactions between rugged topography and atmospheric circulation. In this study the relationship between atmospheric circulation patterns and extreme snowfall events is examined at Bridger Bowl, Montana, U.S.A. which has a 26-year winter record of daily snowfall. Five hundred millibar composite and anomaly maps were constructed for days of heavy snowfall (greater than 32.8 cm). These maps show that during and prior to heavy snowfall, Bridger Bowl is located beneath the back side of a upper-level trough, with predominant winds and storms coming from the northwest. This atmospheric circulation pattern differs from those for other high-elevation sites in the North American interior due to the surrounding regional topography. High mountain ranges to the southwest and west often block incoming moisture, while relatively lower topography to the northwest allows Pacific moisture to reach Bridger Bowl. The results of this study can be used to complement operational forecasting models for predicting heavy snowfall at Bridger Bowl, thereby facilitating snow avalanche forecasting in the region.

This article examines the likely impacts of the United States' “new federalism” on natural hazard land use management in Colorado. Flood and avalanche hazard programs are the focus of the study Both short- and long-term effects of changes in funding are discussed, and the conclusions summarize probable positive and negative program impacts for the state. These results may help researchers and state authorities mitigate similar effects in their states prior to the enactment of new federalism.

Principal problems concerning the raw data and methodological limitations of statistical and conventional avalanche forecasting methods are summarized. The concepts of four statistical models based on multivariate data analysis, are outlined in a few words. In order to give an idea of the potential and quality of the different methods, test runs over two winters are discussed and a tentative store is established. Statistical models I and IV, together with the conventional forecast, attain a score of 70-80%, whereas statistical models II and III show a slightly poorer performance.

The avalanche-hazard index is a numerical expression of damage and loss as the result of an interaction between snow avalanches and vehicles on a road. The index is determined by multiplying the frequencies of moving and waiting vehicles being hit by various types of avalanches by a weighting which indicates the severity of the consequences. The avalanche-hazard index has proved to be a useful decision-making tool for identification of those avalanche paths where control would be most beneficial, for evaluation of options for control measures, and for comparison of the risks for different roads.

General principles governing a structured, multiscalar approach to avalanche-hazard mapping are outlined. At an initial [regional] stage, intermediate-scale mapping (1:300,000) reveals spatial patterns of relevant avalanche parameters and possible lines of research into avalanche forecasting and control. Mapping at the second [local-planning] and third [site-planning] stages, at scales of 1:100,000 and 1:10,000 respectively, is oriented toward formulation of specific site selection and protection measures, and the development of monitoring and forecasting procedures. Excerpts from representative maps at each level are provided, along with detailed legends. Translated from: Materialy glyatsiologicheskikh issledovaniy, USSR Geophysical Committee Publication No. 51. Moscow: Mezhduvedomstvenniy geofizicheskiy komitet, 1984, pp. 216-223.

Avalanche accidents are increasing in Scotland and fatalities now average two each year. The victims are either mountaineers or cross-country skiers as the problem is confined to the steeper and remoter slopes. Studies in the Cairngorm Mountains have shown that Scottish snow differs from alpine snow in three respects: depth hoar does not occur in Scotland, higher values are found for density and rammsonde penetration, and ice layers are more abundant. Slab avalanches are found on slopes of about 35° and sluffs on slopes of 30° and steeper. Although most avalanches are small, crown fracture lines reach a maximum length of 500 m, and slabs may be up to eight metres thick. Preliminary correlations between avalanche release and weather conditions have shown the importance of cold periods following heavy snow falls and the significance of thaws. Much more needs to be done before an avalanche-forecasting service could be provided, not the least of which is persuading likely beneficiaries that such a service would be worthwhile.

Many developed areas in British Columbia are exposed to snow-avalanche hazards. Avalanche-hazard zoning has been undertaken by the British Columbia Ministry of Transportation, Communications, and Highways during the past five years. Recommendations from these zoning studies are forwarded to those agencies responsible for land-use zoning and development approval. Existing and possible legislation are described, as well as problems associated with implementation of the legislation. Technical considerations are outlined; interpretation of vegetation is a very important factor in evaluating each avalanche site. Calculation of run-out distances and consideration of other factors serve as a check on the vegetation interpretation. A special safety factor has been developed. Socio-political considerations with respect to British Columbia are described. Existing developments have the most wide-ranging implications.

On the evening of 5–6 September 1981, an avalanche on the east slopes of Mount Egmont buried the walking track to the Stratford Mountain Club skifield under several metres of debris. The weather conditions preceding the event were typical of those producing direct-action slab failures. However, the volume of debris and the presence of shearing layers above the thick ice bands in the snowpack suggest that old snow contributed to the magnitude of the event. Whether the initial failure occurred in the old snow or in the new snow layers cannot be determined from the available information. A systematic programme of snowpack and meteorological observations is required in Mount Egmont and Tongariro National Parks to provide information necessary for avalanche hazard assessment.

Mapping of areas exposed to avalanche hazard for the Norwegian Army was started by the Norwegian Geotechnical Institute in 1986. The background to this mapping is that large-scale military exercises are held annually in northern Norway, in terrain where there is a high danger of avalanche activity. Avalanche areas are divided into two zones: potential starting zones, and potential run-out zones. All potential avalanche areas are indicated on maps, and mapping is carried out by computer using a terrain model and digital maps. An interactive graphic work station is used to outline danger areas. Starting and run-out zones are identified by using terrain parameters which may be extracted from digital maps. The usual scale of the avalanche maps is 1 : 50 000, with 20 m contour intervals.

Using 20 years of storm and ramsonde profile data, the probability of avalanche hazard to the Alta Highway and Village is plotted as a function of ten contributory factors. Results indicate that the hazard probability is related strongly to precipitation and wind direction. Certain factors previously thought to be important (wind speed, settlement, and ramsonde profiles) do not seem important for the south facing slopes and large avalanches tested in this study.

This is a report on the avalanche hazard area of Rebun-island, northern Hokkaido, researched on March and August in 1966.1. There are slopes of marine terraces along the coast, on which snow-cornice and -drift are usually formed by heavy wind in winter. They often developed to snow avalanches and caused large damages.2. From late February to early March in 1966, air temperature became very high and many ground avalanches broke out. The layer of depth hoar or of wet granular appeared to play an important part in these avalanches formation and release. Potential Magnitude (P.M.) of these avalanches were generally 3.0-4.0 and maximum 5.1.3. The earth wind-baffle on the terrace is successful. On the longer slope, however, it is necessary to make both, the wind-baffle on the terrace and the defence structure in the sliding zone.

PLANNING AND OPERATIONAL PROCEDURES ARE PRESENTED, AND CRITERIA FOR AVALANCHE CONTROL AND THE EVALUATION OF AVALANCHE HAZARD ARE DISCUSSED BASED ON FIELD DATA. A METHOD FOR DETERMINING WHEN GUNFIRE SHOULD COMMENCE IS OUTLINED. BASIC INFORMATION NECESSARY FOR FORMULATING OPERATIONAL FORECASTS FOR HIGHWAY MAINTENANCE AND AVALANCHE CONTROL PERSONNEL IS GIVEN BASED ON: MOUNTAIN WEATHER FORECASTS, TRAFFIC DENSITY, AND CURRENT SNOW CONDITIONS. INSTRUMENT NEEDS ARE EXPLAINED IN DETAIL. /AUTHOR/

International audience; When determining risk related to natural hazard processes, many studies neglect the investigations of the damage potential or are limited to the assessment of immobile values like buildings. However, persons as well as mobile values form an essential part of the damage potential. Knowledge of the maximum number of exposed persons in an endangered area is of great importance for elaborating evacuation plans and immediate measures in case of catastrophes. In addition, motor vehicles can also be highly damaged, as was shown by the analysis of avalanche events. With the removal of mobile values in time as a preventive measure this kind of damage can be minimised. This study presents a method for recording the maximum number of exposed persons and monetarily assessing motor vehicles in the municipality of Galtür (Tyrol, Austria). Moreover, general developments of the damage potential due to significant socio-economic changes since the mid-twentieth century are pointed out in the study area. The present situation of the maximum number of persons and mobile values in the official avalanche hazard zones of the municipality is described in detail. Information on the number of persons is derived of census data, tourism and employment statistics. During the winter months, a significant increase overlaid by strong short-term fluctuation in the number of persons can be noted. These changes result from a higher demand of tourism related manpower as well as from varying occupancy rates. The number of motor vehicles in endangered areas is closely associated to the number of exposed persons. The potential number of motor vehicles is investigated by means of mapping, statistics on the stock of motor vehicles and the density distribution. Diurnal and seasonal fluctuations of the investigated damage potential are pointed out. The recording of the number of persons and mobile values in endangered areas is vital for any disaster management.

Avalanche hazard maps of high accuracy are difficult to produce. For land-use planning and management purposes, a good knowledge of extreme run-out zones and frequencies of avalanches is required. In the present work, vegetation recognition (especially focused on Pinus uncinata trees) and dendrochronological techniques are used to characterize avalanches that have occurred in historical times, helping to determine both the extent of large or extreme avalanches and their occurrence in time. Vegetation was studied at the Canal del Roc Roig (eastern Pyrenees, Spain) avalanche path. The avalanches descending this path affect the railway that reaches the Vall de Núria resort and the run-up to the opposite slope. During winter 1996, two important avalanches affecting this path were well documented. These are compared with the results of the vegetation study, consisting of an inventory of flora, the recording of vegetation damages along eight transverse profiles at different altitudes on the path and a dendrochronological sampling campaign. The data obtained contributed to a characterization of the predominant snow accumulation in the starting zone, the 1996 avalanches and the range of frequencies of large avalanches. Also, traces of avalanches that increase the path mapped in the avalanche paths map published by the Institut Cartogràfic de Catalunya in 2000 were identified, improving the initial existing information.

The road along the Óshlíð hillside in the West Fjords region of Iceland is one of the most hazardous roads in Iceland due to avalanches, rockfalls and debris flows. The road has little traffic, but nevertheless traffic accidents caused by the severe conditions at the site are common. A number of avalanche tracks are found on the hillside. In some of these tracks, avalanches occur more frequently than in others. When there is an avalanche threat, avalanches generally flow over many tracks in a short time. Monitoring vibrations in the tracks with the highest avalanche frequency can indicate when avalanches start flowing down the hillside in a snowstorm, and avalanche hazard can then be declared with the specific site indicated. The same methodology can be used for rockfalls and debris flows, which are strongly affected by weather conditions and typically occur in clusters. Based on this knowledge, a research project was initiated in February 1996 with the objective of developing an automatic system based on seismic measurements to detect and analyze avalanches on the Óshlíð hillside and to instantly send a warning to a control station.

This paper describes the various maps concerned with avalanche hazards in Switzerland and gives examples of parts of specimen maps. These include the Wald- und Lawinenkarte der Schweiz (1907) at 1 : 250 000 which marks actual avalanche paths, the Karte der lawinengefährdeten Gebiete (1972) at 1 : 30 0000 which gives a summary view of avalanche hazard, the Gefahrenkarte der Schweiz (1975) at 1 : 100 000 which is intended to give an over-all view of all natural hazards that will influence land use, the Lawinenatlas/Lawinenkarte Uri (1978) at 1 : 25 000 which contains all identifiable avalanche paths in the Canton, the Kombinierte geomorphologische Gefahrenkarte 1: 10 000 von Grindelwald (1977), a Pilot study of mapping natural hazards in a mountain area, and the Lawinengefahrenkarten des Eidg. Institutes für Schnee- und Lawinenforschung at 1 : 10 000 included in 26 advisory reports on 561 individual avalanche paths, and which can act as pilot studies to assist cantons in preparing their own maps. This paper is being published in full in German in Interpraevent 1980, Bd. 3, as part of the proceedings of the symposium held at Bad Ischl, Austria, 8-12 September 1980.

The inter-relationship between settlement pressures in the Town of Vail, Colorado and the hydrological and geophysical natural hazards of the upper Gore Creek Valley poses problems shared by other rapidly developing mountain towns in Colorado and other Rocky Mountain states. What is unique in Vail's case is that town government officials and some early developers sought expert scientific opinion on the identification and mapping of these natural hazards before such action was legally required. The scientific information was incorporated into hazard zoning ordinances. This paper examines with several case-studies how the munici- pality of Vail has used the expert scientific information in the implementation of its avalanche hazard zoning and how it has responded to land-use decisions in relation to the pressures of property development in those zones.

Forty-two rock avalanche deposits, with volumes ranging from a chosen lower threshold of 1 × 10 6 m 3 up to 500 × 10 6 m 3 , have been identified in a 10,000 km 2 area of the central Southern Alps, New Zealand. The observed frequency of one per 244 yr over 10,250 yr is biased by erosion. A nearly complete sample of the past 1,700 yr indicates a frequency of 1 per 94 yr. On the average, the largest rock avalanche in any century is 56 × 10 6 m 3 , and in any millennium, 103 × 10 6 m 3 . At about one per century, most large rock avalanches are probably triggered by large earthquakes and only a few by large storms. The risk of rock avalanche debris reaching a specified distance from a source of known elevation is given by a simple stochastic model, locally calibrated to assess rock avalanche hazard.

Tree-ring analysis can be a reliable method of determining past snow avalanche events when good historical records are lacking. Characteristic features in the tree-ring record indicative of disturbance include: (1) the occurrence of reaction wood, (2) abrupt changes in growth rate, (3) age of scars caused by avalanche impact, (4) age of trees in reforested tracks, and (5) examination of a “new leader.” This study was conducted at the town of Ophir, a small community in southwestern Colorado with a severe snow avalanche hazard. Historical records indicate that snow avalanches reached the town, or its vicinity, four times during this century (1918, 1951, 1958, and 1959). Tree-ring analysis substantiated this record in part and produced evidence of at least one additional event (middle to late 1880s). From this combined information, the recurrence interval for major snow avalanches capable of endangering the town is thought to be at least as frequent as once in 20 yr. The limitations of tree-ring analysis encountered in this study are also discussed.

To the Editor:— The article on snowmobile injuries ( 209 :1672, 1969) by Chism and Soule evokes another facet of the problem which needs emphasis. This is the need for training in winter survival, winter first aid techniques, and hazard recognition. In Montana during the past winter, a man and his son died after their snowmobile fell through the ice on Yellowtail Reservoir. Another serious problem is avalanche hazard. The National Ski Patrol System has developed a slide lecture on avalanche hazard which members are happy to present at meetings of snowmobile organizations. Those of us who are experienced in ski touring are aghast at the potential problems posed by large numbers of untrained individuals in poor physical condition, many miles from assistance.