The Avalanche of Les Fonts d’Arinsal (Andorra): An Example of a Pure Powder, Dry Snow Avalanche

On 8th February 1996, in the north-western part of Andorra in the Pyrenees, the Les Fonts d’Arinsal (LFd’A) pure powder avalanche was triggered, descending some 1200 m to the bottom of the Arinsal valley and continuing up the opposite slope for about 200 m. This size 4–5 avalanche reached velocities of up to 80 ms−1, devastated 18 ha of forest, involved a minimum volume of up to 1.8 × 106 m−3 and caused major damage to eight buildings. Fortunately, no one was injured thanks to an evacuation, but 322 people lost their properties. This study describes the physical characteristics of the LFd’A avalanche path and provides data on earlier avalanches, the meteorological synoptic situation and snowpack conditions that generated the avalanche episode, the warning and preventive actions carried out, the effects and evidence of the large avalanche, and the defence system implemented afterwards. A discussion of the avalanche dynamics based on observations and damage, including the role of snow entrainment, the total lack of characteristic dense flow deposits, as well as the evidence of a two-phase flow (fluidisation and suspension), is presented. This case study is an example of a paradigmatic large, pure powder, dry-snow avalanche, which will be useful for model calibration.

Dynamic magnification factors for tree blow-down by powder snow avalanche air blasts

We study how short duration powder avalanche blasts can break and overturn tall trees. Tree blow-down is often used to back-calculate avalanche pressure and therefore constrain avalanche flow velocity and motion. We find that tall trees are susceptible to avalanche air blasts because the duration of the air blast is near to the period of vibration of tall trees, both in bending and root-plate overturning. Dynamic magnification factors for bending and overturning failures should therefore be considered when back-calculating avalanche impact pressures.

Brief Communication: Dynamic magnification factors for tree blow-down by powder snow avalanche air blasts

We study how short duration powder avalanche blasts can break and overturn tall trees. Tree blow-down is often used to back-calculate avalanche pressure and therefore constrain avalanche flow velocity and motion. We find that tall trees are susceptible to avalanche air blasts because the duration of the air blast is near to the period of vibration of tall trees, both in bending and root-plate overturning. Dynamic magnification factors for bending and overturning failures should therefore be considered when back-calculating avalanche impact pressures.

Configurational energy and the formation of mixed flowing/powder snow and ice avalanches

A long-standing problem in avalanche dynamics is to model the flow of a mixed flowing/powder avalanche. Here we use the thermodynamic concept of configurational energy to describe the blow-out of air from the avalanche core. Configurational energy is the mean potential energy associated with the location of snow and ice particles in the avalanche core. As such, configurational energy determines the avalanche flow density. Expansion of the particle ensemble reduces the flow density and leads to the intake of air. Compression of the particle ensemble causes the blow-out of the intaken air, now laden with ice dust. Once formed, the cloud moves independently of the flowing avalanche with the initial momentum acquired in the core. Configurational energy changes in the avalanche core are therefore intimately related to the formation of the powder suspension cloud. In this paper we use the concept of configurational energy to predict the mass of air taken into and blown out of the core. This requires calculating the dispersive pressure arising from random particle movements and configuration changes related to the expansion and collapse of the flowing particle ensemble. The ice avalanche that struck the Everest base camp on 25 April 2015 is simulated using the proposed concept.

Investigation of Avalanche Time and Carr’s Index of Poultry Litter Powder as Flowability Indices

The world’s dependence on chemical fertilizer as the primary source for enriching agricultural fields is continually increasing that cause nature pollution. This has led researchers to aggressively investigate renewable fertilizer resources, biomass, to produce organic crops and reduced wastage. Poultry litter is a bulk solid and biomass feed stocks. Flow behavior of bulk solid is a critical factor in designing and developing suitable equipments (e.g. pelletizing machine). The bulk density, tap density, Carr’s index and powder avalanche time technique were applied to evaluate the flow properties of poultry litter. The experiments were carried out at moisture content (10, 20 and 30% w.b.), particle size (0.3, 0.6 and 1.18 mm) for the bulk and tap densities as well as Carr’s index. In addition to the moisture content (10, 20 and 30 %w.b.) and particle size (0.3, 0.6 and 1.18 mm) the rotational speed of drum (0.5, 1 and1.5 rpm) were also investigated for the avalanche time. The results showed that with increasing moisture content Carr’s index increased significantly (P<0.01) in the ranges of 16.2% to 18.5% and with increasing particle size the Carr’s index decreased from 20.35% to 14.78%. The litter powder avalanche time (AT) increased significantly (P<0.01) with increasing moisture content and decreasing rotational speed and particle size. The bulk and tap densities of the litter powder was decreased with increasing moisture content and increasing the particle size. The bulk and tap densities of the driest and finest poultry litter sample were higher than other ones.

Return period calculation and passive structure design at the Taconnaz avalanche path, France

This paper aims to show how recent knowledge developed in the field of avalanche research can be used for a real case study, the Taconnaz avalanche path, where passive structures already existed but had to be improved. First a morphological analysis of the site is done and historical data are analysed. Second, each recorded event is back-calculated using a numerical model of dense-flow avalanches. For each surveyed avalanche, parameters at the entry of the runout zone upstream of the defence structures are defined. Third, a statistical analysis of these parameters allows characterization of 100 year return period events. Fourth, physical and numerical models of dense avalanches interacting with defence structures are combined in order to design the most effective passive structure able to contain the reference scenarios. Finally, physical and numerical modelling of the interaction between the powder avalanche and the designed defence structure is performed, to show that the proposed improvements do not increase the residual risk due to the powder part in areas downstream of the defence structures.

Avalanche of 18 January 1997 on Brenva glacier, Mont Blanc Group, Western Italian Alps: an unusual process of formation

At 1455 h on 18 January 1997, an airborne powder avalanche with a volume of about 3 × 106 m3 flowed along the tongue of Brenva glacier, high Aosta valley, on the southern flank of Val Veny, Italy It descended 2000 m and covered a distance of 5.5 km, with a rate of movement on the intermediate stretch of > 70 m s–1. It killed two skiers following a valley-bottom piste, damaged a hotel, destroyed a wide belt of woodland on the opposite slope and caused other minor damage. The avalanche occurred immediately after a large rockfall on the southern slope of the Sperone della Brenva (3567 m a.s.L). However, the relationship between the two events can only be considered indirect, since the main mass of the rockfall stopped at the base of the scar, on a large plateau on the glacier surface. The rockfall and subsequent shock caused the contemporaneous detachment of masses of ice and/or firn from hanging glaciers at the head of a nearby cirque, leading to the formation of the avalanche. The complex mechanism of detachment hypothesized was induced from available documentation and from in situ investigations. Its phases also coincide with seismograms recorded at a station 45 km from the avalanche area. Finally, a mathematical model of the avalanche was used to reconstruct its dynamics and path.