[1] Despite their clear danger to humans, snow avalanches are hard to document. They occur in inaccessible and dangerous locations, often at times of bad weather. Observation instruments frequently malfunction in the harsh conditions or are destroyed. Measurements of powder snow avalanches are particularly difficult, as these occur less frequently and are usually very large. To understand the air flow in front of and inside powder snow avalanches, we have designed an air pressure sensor to survive the harsh conditions. It consists of a differential pressure transducer, with a high-frequency response, built into a specially designed housing unit. We mounted this 10 m from the ground on a measurement mast in the Vallée de la Sionne avalanche test site in Switzerland. Data from five powder snow avalanches over the winter of 2004 were recorded. Three of these were natural releases, and two were artificially triggered with explosives. We present an analysis of the sensor response and an interpretation of the signals in terms of simple flow fields. We show how these data can be used to deduce information about the speed, size, and location of the avalanches using a dipole approximation. Our sensor has two major limitations: The length of the internal tubing produces low-frequency resonances, and there is only one transducer, so a complete flow model is needed to deduce the three velocity components and pressure. We discuss these limitations and give a design for a new sensor to overcome them.

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A super-hydrophobic surface has been obtained from nanocomposite materials based on silica nanoparticles and self-assembled monolayers of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) using spin coating and chemical vapor deposition methods. Scanning electron microscope images reveal the porous structure of the silica nanoparticles, which can trap small-scale air pockets. An average water contact angle of 163° and bouncing off of incoming water droplets suggest that a super-hydrophobic surface has been obtained based on the silica nanoparticles and POTS coating. The monitored water droplet icing test results show that icing is significantly delayed by silica-based nano-coatings compared with bare substrates and commercial icephobic products. Ice adhesion test results show that the ice adhesion strength is reduced remarkably by silica-based nano-coatings. The bouncing phenomenon of water droplets, the icing delay performance and the lower ice adhesion strength suggest that the super-hydrophobic coatings based on a combination of silica and POTS also show icephobicity. An erosion test rig based on pressurized pneumatic water impinging impact was used to evaluate the durability of the super-hydrophobic/icephobic coatings. The results show that durable coatings have been obtained, although improvement will be needed in future work aiming for applications in aerospace.

[1] We present a model that underscores the role played by the porous snow cover in sustaining large, rapid, dilute powder avalanches over weakly cohesive snow. The model attributes massive localized material injection into the avalanche head to synergistic pressure gradients established within the porous cover by the very static pressure field that this influx induces along the pack surface. Treating massive frontal snow entrainment as a source of fluid, we show that static pressure time-histories recorded at the Vallée de la Sionne (Switzerland) conform to the classical two-dimensional Rankine half-body flow field. We calculate pore pressure within the snow cover and, from the resulting pressure gradient, find stresses on a vertical failure plane. After inferring an upper bound for snow cohesion from pressure records, we derive a sufficient condition for steady failure that sets the depth through which the cover changes from porous solid to fluidized suspension. Fluidization of the top surface imposes another relation among maximum density, internal friction and cohesion of the pack, maximum cloud size and minimum avalanche speed. Altogether, these conditions dictate which snow covers can produce powder snow avalanches. We suggest how similar "eruption currents" sustained by massive frontal entrainment may be relevant to other fluid-particle suspensions.Citation: Louge, M. Y., C. S. Carroll, and B. Turnbull (2011), Role of pore pressure gradients in sustaining frontal particle entrainment in eruption currents: The case of powder snow avalanches,

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