Abstract. Avalance activity in Iceland for the last 860 years is briefly outlined and the present avalanche problems in Iceland are explained. Statistics are given for the number of deaths from avalanches by years, months and avalanche categories (for both residents and travellers).

Climatic conditions and terrain features are described for the main avalanche-hazard areas in Iceland. Data from meteorological stations for precipitation, wind, and air temperatures have been analysed for all the major avalanche cycles in the present century. Synoptic weather patterns that trigger avalanches are explained. A distinction is made between conditions that cause wet and dry avalanches. The results form the basis of an area-wide avalanche forecast in Iceland.

Abstract. This paper summarizes the state of research in China on snow cover, snow-drift control, and avalanche defences, and also reports the results of observations of avalanching above Batura Glacier, Pakistan, during two expeditions.

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.

A 70-Year record has been compiled for avalanches affecting the Canadian Pacific Railway at Rogers Pass, British Columbia. Time series are presented for avalanche frequency, avalanche mass, and length of avalanche debris on the rail line for 26 avalanche paths as well as for winter snow-fall. Winters with the heaviest avalanche activity were 1971-72, 1934-35, 1919-20, 1932-33, and 1953-54. Time-series analysis indicates that the size of avalanches has decreased in recent decades. Spectral analysis shows avalanche activity to the similar to white noise but with a weak periodicity of about 18 years. An examination of the climatology of big avalanche winters reveals two distinct circulation patterns: a strong zonal flow with frequent Pacific storms and heavy snow-fall; or a pronounced meridional flow, Arctic air outbreaks, and catastrophic avalanching released by rapid advection of warm moist Pacific air. Major avalanche winters need not be big snow-fall winters.

A simulation process model is developed for rating avalanche danger for twelve east-facing avalanche paths loaded by westerly winds. The model simulates layer age and densification, snow depth, snow transport and deposition, formation of melt crusts, snow temperatures, temperature gradient metamorphism, and avalanche danger on a 6 h basis. Conditioned on avalanches alone, the model predicted avalanche potential on 86% of the 175 avalanche days during an eight-year period. It indicated avalanche potential 50% of the time on non-avalanche days. A sensitivity analysis is under way to improve model performance, and simulation of danger from additional avalanche samples is planned.

Stability indices at different slope aspects and characteristic parameters describing the snow cover and the weather conditions are recorded simultaneously and automatically in potential fracture zones of avalanches during the whole winter season. A remote, solar-powered system including measurements of snow depth, mass flux of wind-blown snow and of the snow temperature profile as well as on-line and off-line data processing has been developed and tested. The results show that this type of data-recording and processing system makes quantitative case studies of natural stability variations possible.

Conventional avalanche forecasting is practiced as a mix of deterministic treatment for snow and weather parameters and inductive logic to reach actual forecast decisions. Inductive logic of the scientific method dominates, making frequent use of iteration and redundancy to minimize decision uncertainties. The mental processes involved are holistic rather than analytical. Elementary information theory can be used rationally to sort data categories for minimum entropy and optimize inductive reasoning. Recognizing these principles affords a chance to improve the practice and teaching of conventional forecasting techniques.

A study of 30 slab avalanches at Whistler Mountain ski area, British Columbia, indicates that slab instability initiates with highest frequency on 40° slopes, that the temperature at the slab bed surface is about –5°C averaged for the 30 cases, and that on the average the slabs have a snow density of about 220 kg/m3. These results agree well with results from earlier studies. A rich variety of crystals appear in photomicrographs of samples extracted from the bed surfaces of the 30 slabs. In many cases, crystals extracted from the plane of critical weakness do not differ markedly from crystals extracted from adjacent strata.

Avalanche warnings can provide a valuable public service. To be effective, the warning program must inspire public confidence. Experience gained from the Colorado Avalanche Warning Program is used to develop guidelines for establishing and maintaining credibility. The topics discussed arc the requirements for a good forecaster, the working relationship between forecaster and field observers, relations with the news media and the public, and forecast accuracy.

The principles of snow dynamics, including practical field work, constitute a feasible university-level course where seasonal snowfall and terrane of varying steepness are accessible. Two to three lectures or discussions per week are combined with one full afternoon in the field to provide a workable course format. We have successfully used the U.S. Department of Agriculture Forest Service Agriculture Handbook 489 (Avalanche handbook) as a text but also assign readings in diverse world literature.

Field work has centered on standard techniques for snow study in a realistic alpine setting but could easily be adapted to more simple “roadside” conditions if necessary. Student interest during and following the course usually leads to spontaneous and practical research that tends to develop life-long skill and application in the subject.

Our experience indicates that one instructor can manage a maximum of 25 students in the field, considering proper logistics, safety, and necessary adaptibility of field procedures to changing weather conditions.

The numbers of people rambling in the mountains is increasing every day. Consequently, the number of avalanche accidents is very significant. In France, the main mountain users organizations, headed by ANENA, work for a better education of those in the mountains. Various aspects of training courses and public information services are discussed.

The friction coefficients needed to solve Voellmy’s avalanche-dynamics equations and as input to the numerical, finite-difference computer program AVALNCH are calculated from case studies. The following coefficients of internal friction v and of surface friction f worked well for program AVALNCH: for midwinter dry snow v = 0.5 to 0.55 m2/s and f = 0.5 to 0.55; for hard slab v = 0.7 to 0.8 m2/s and f = 0.7 to 0.8; for fresh, soft slab v = 0.4 to 0.5 m2/s and f = 0.4 to 0.5. The program predicted run-out distance well for a variety of conditions but performed less well in cases of sharp, adverse grade in the run-out zone. For the Voellmy approach, large design-size avalanches required turbulent friction coefficients ξ of 1200 to 1600 m/s2 and kinetic friction coefficients of 0.15. Two hard-slab avalanches, a slow-moving,wet-slab avalanche, and a soft-slab avalanche that ran through scattered mature timber required ξ of 700 to800 m/s2 and μ of 5/V when V is velocity in m/s. The coefficient of sliding friction for a hard-slab avalanchethat encountered damp snow in the run-out zone was computed directly from movies to be 0.35, 0.43, and 0.32 for three measured sections of the run-out zone.

To fix the limits of different hazards in the avalanche-hazard maps one uses criteria pertaining to avalanche dynamics. These criteria are at present the velocity and the run-out distance of a given avalanche for a given place. In 1955 A. Voellmy published his theory of avalanche dynamics which has widely been used in practical map preparation. Since 1962 his equations have also been used by the Eidg. Institut für Schnee- und Lawinenforschung (EISLF) to calculate avalanche pressures and run-out distances. Furthermore B. Salm (EISLF) developed another equation for the calculation of run-out distances in 1978. Both the equations of Voellmy and of Salm contain two friction coefficients, µ and ξ. Little is known about them and opinions, even among specialists, differ on what values should be given to them.

This paper presents field observations on very long run-out distances. These observations are used to calculate values for pairs of µ and ξ. For avalanche zoning, only extreme values are of interest, i.e. very low values for µ and very high values for ξ. For the calibration of those coefficients, ten avalanches from the winters 1915–16, 1967–68, 1974–75, and 1977–78 have been used. Those avalanches occurred during heavy and intense snowfalls. For those avalanches, the pair µ= 0.155, ξ = 1 120 m/s2 was found for the Voellmy equation and the pair µ = 0.157, ξ =1 067 m/s2 for the Salm equation. These values only partially agree with those used up to date by EISLF. It is recommended for example that for extreme flowing avalanches (newly fallen snow, soft slabs) the pair µ = 0.16,ξ = 1 360 m/s2 be used.

A numerical computer model, based on the finite differencing of the Navier-Stokes fluid equations, is used to simulate snow-avalanche flow. In order to verify and calibrate the numerical model, snow-flow tests 0.20 m deep with flow velocities between 0-18 m/s were conducted. Data concerning position, velocity, and flow depth versus time were collected and compared to model runs on the computer. The frictional force on moving snow is investigated and found to be modeled by a term that is proportional to the square of the flow velocity.

Observations and measurements were made of high-speed avalanches at Shiai-dani, the Kurobe canyon, to reveal their real features and properties, using strain-gauge-type load cells in measuring impact force and speed. A maximum impact pressure of 140 - 104 N m-2 was obtained, as well as data indicating the existence of a number of wave fronts, ranging in speed from 9 to 60 m s -1 inside an avalanche. Violent changes in atmospheric pressure ranging from -21 to +5 mbar were observed near its path when a high-speed avalanche passed by.

Field data on dry–slab avalanches obtained during the period 1975–79 from several snow climates suggest the following: (1) most of the mass of the typical avalanche studied consisted of fragments with lengths greater than 5 cm, (2) transverse and longitudinal shear planes formed during deceleration in the avalanche run–out zone, and (3) the flow height exceeded the slab height in most cases. No correlation was found between the run–out distance and the track or run–out zone slope, or between the run–out distance and the released slab height.

The field data suggest that avalanche motion is best described as a flow of fragments in which boundary shearing stresses are sensitive to fragment size distributions and volumetric solids concentrations. These factors may be more important than the roughness of the avalanche boundary in determining maximum velocity and run–out distance.

A method for calculation of “maximum” avalanche run–out distance based on topographic parameters only is described. 423 well–known avalanches have had their maximum extent registered. The average gradient of avalanche path (α–angle), measured between the highest point of rupture and outer end of avalanche deposit is used as description of avalanche run–out. The topographic parameters which determine α are described.

A regression analysis of 111 avalanche paths based on 8 terrain parameters is performed, applying 26 independent combinations of these parameters as variables. The four best combinations of variables are used. These variables are: second derivative y’’ of avalanche slope described by a second–degree function, average gradient of avalanche track β, total vertical displacement of the avalanche H, and gradient of rupture zone θ.

The equation has a correlation coefficient of 0.95 and standard deviation of 2.3°. This relationship makes possible a fairly accurate prediction of avalanche run–out distance.