The Intermittency Regions of Powder Snow Avalanches

Abstract

Powder snow avalanches are typically composed of several regions characterized by different flow regimes. These include a turbulent suspension cloud of fine particles, a dense basal flow, and an intermittency frontal region, which is characterized by large fluctuations in impact pressure, air pressure, velocity, and density, but whose origin remains unknown. In order to describe the physical processes governing the intermittency region, we present data from four large powder snow avalanches measured at the Vallée de la Sionne test site in Switzerland, which show that the intermittency is caused by mesoscale coherent structures. These structures have a length of 3–14 m and a height of 10 m or more. The structures can have velocities as much as 60% larger than the avalanche front speed and are characterized by an air/particle mixture whose average density can be as high as 20 kg/m3. This average density increases the drag on large granules by a factor of up to 20 compared to pure air, so that each structure can maintain denser snow clusters and single snow granules in suspension for several seconds. The intermittency region has importance for the dynamics of an avalanche, as it provides an efficient mechanism for moving snow from the dense layer to the powder cloud, but also for risk assessment, as it can cause large forces at large heights above the basal dense layer.

Bayesian Inference in Snow Avalanche Simulation with r.avaflow

“…A significant change in velocities may be achieved by modifying the process model by, e.g., including entrainment [11] or considering a different friction relation [16,51]. It is also important to note that depth averaged model approaches do not provide any vertical velocity information and are therefore unable of reproducing turbulent structures resulting in large local velocity variations, that are particularly interesting in the frontal region of the avalanche [52]. The challenge of properly considering velocities in avalanche simulation evaluations has been previously debated yielding similar results [29,30] and new, promising measurement [53] and evaluation approaches [54] are currently discussed.…”

Constraints on Entrainment and Deposition Models in Avalanche Simulations from High-Resolution Radar Data

“…In contrast, powder snow avalanches keep their speed when transitioning from a steep to more gently inclined slope. We attribute this mismatch to the missing intermittent [22] or fluidized [15] flow regime in our dense flow model. Avalanche #0019 shows an abrupt increase in the approach velocity at range 1300 m. This knick is clearly visible in the radar data, as well as in both simulation setups and shows the ability of the OpenFOAM solver to accurately capture complex natural terrain.…”

“…At about 1800 m asl, the avalanche joined the track of a previous avalanche and encountered deposited snow rather than undisturbed snow cover. Avalanche #0017 was a typical powder snow avalanche that showed multiple minor surges as the characteristics for the intermittent frontal region, and thus was able to progress over the shallow runout zone with high velocity [22]. Avalanche #0019 was released between Crêta Besse 1 and Crêta Besse 2 and merged into the flow path of avalanche #0017 at 2100 m asl after descending 400 m. The initial release with a volume of 2200 m 3 and the total volume of 29,500 m 3 were smaller by a factor of 2 to 3 than avalanche #0017.…”

“…Beside the computational improvements, modern high-resolution measurement equipment reveals a high variety of flow features and structures [19]. Experimental avalanche research recently put increased focus on thermodynamic processes within avalanches [20,21] as well as the transition to a fluidized flow regime [19,22] and powder snow avalanches [23]. A unified modelling of such flow regimes is not yet possible in depth-integrated flow models, although they surely influence the mobility of the avalanche.…”

Snow Avalanche Impact Measurements at the Seehore Test Site in Aosta Valley (NW Italian Alps)

“…The evolution of the pressures for such event suggests that an intermittency frontal zone was present in the front of the avalanche before the dense flow component impacted the obstacle. In fact, the recorded forces at the beginning of the impact showed large variations (as typical of the intermittency zone-see [18]): In particular, the upper transducers (numbers 7, 8, and 9) measured values oscillating from 0 to 4000 N during the first 2 s of flow (see Figures S11-S13 in the Supplementary Materials). Even for this event, as in 2010, the lowest measuring plate (II) was placed at 110 cm above ground, therefore we are not able to describe the vertical profile of the pressures in a complete way.…”