EVALUATION AND IMPROVEMENT OF SNOW STABILITY TESTS BASED ON MEASUREMENTS ASSOCIATED WITH REAL AVALANCHES
Résumé
A number of snow stability tests (Rutschblock, CT, ECT, PST) have been in use for long. However, some dramatic cases have shown their limitations as soon as they started being popularized (Munter). This is why we initially ascribed them (since 2006) little predictive value, as for classic stratigraphic profiles. Nevertheless, by systematically practicing them during avalanche accidents judicial investigations since 2018, A. Duclos observed constants that seemed empirically robust, among the hundreds of tests (more or less formal before 2020) practiced elsewhere in the context of operational missions for roads avalanche safety. This is why we have developed a methodology for conducting, recording, and sharing snow stability tests, gathered in a unique online database, now rich with over 700 tests, all conducted according to the same protocol. Among them, more than 50 are associated with real avalanches, dated, geolocated, and described. The tests have sometimes been conducted in the immediate vicinity of the avalanche starting zone (e.g., crown rupture) immediately after its occurrence (e.g., for accidental avalanche triggered by observers). More often, they could only be carried out one day or two after the avalanche, but always as close as possible to the starting zone. Sometimes they were also conducted before the feared avalanche release. These observations and measurements are analyzed taking into account this diversity of situations. They are compared to the exhaustive population of tests not associated with an avalanche. The initial results have allowed determining PST initiation length thresholds, and crack and column behaviors, typical of situations where an avalanche is very likely to occur, provided that other parameters as topography, snowpack continuity. . . etc., are also favorable. In some well-identified situations, they also show that ECTs deliver more significant results than PSTs. However, CTs are mainly used to identify and locate the presence of weak layer(s), without reliable predictive value. Continued measurements by the same operators led to protocol evolution, notably through a relevant ECTs integration. The objective remains to best combine terrain constraints (feasible in alpine conditions by two experienced mountaineers within a reasonable time lapse) and a result as reliable as possible derived from the combination of several tests.
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