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D. Sanders, A. Preh, J.T. Sausgruber:
"Long-Runout Rock/snow Flows: An Underrated Type Of Mountain Hazard";
Poster: GeoTirol 2016, Innsbruck; 25.09.2016 - 28.09.2016; in: "ABSTRACT VOLUME GeoTirol 2016", (2016), 1 S.

In the Alps, LIDAR topography and field mapping reveal ever more mass-flow deposits that most probably descended on and with snow. Specifically, an hitherto neglected flow type is represented by two-layer rock/snow avalanches (TLA) of: (a) a lower layer of pure snow entrained along the avalanche flow path, and (b) an upper layer of unsorted rock debris. TLA are triggered by impact of rockfalls or rock avalanches onto slopes covered with granular snow; after impact, within a short distance downslope, the twofold layering develops by snow entrainment along the avalanche front while the rock debris rides passively piggyback. Observations of several TLAs in the past few years and numerical modeling suggest that the rock debris carried atop the moving mass markedly prolongs avalanche runout. Notwithstanding potential physical explanations for this type of avalanche, another question is how to recognize their 'fossil' deposits not available for direct observation.
A difficulty in identifying fossil rock/snow mass flows is that a major determinant of flow behaviour - the snow - is
vanished by melting; its former presence thus must be deduced from the distribution of rock debris only. In
addition, rock/snow flows not necessarily all were TLAs. The presented critera for identification of fossil rock/snow
flows are neither claimed to be complete nor to be strictly diagnostic for any case: (1) superlong runout relative to
rock debris volume, (2) overall low thickness (one clast to a few clasts) of unsorted rock debris not explained by
debris-flow transport, and/or (3) laterally highly variable thickness to patchy distribution of rock debris, (4) lateral
spread of thin veneers to 'litters' of rockfall cobbles to boulders near the snout of a presumed rock/snow flow, and
(5) evidence for highly mobile flow, as recorded by local interruptions of rock-debris veneers fitting with
topographic obstacles. With time and progressive overprint by other geomorphic processes, however,
reconstruction of rock/snow flows and in particular of TLAs will get ambiguous. Large parts of the present Alps are covered with snow over three to five months of the year, i.e., the probability for rockfalls and rock avalanches to hit snow-covered slopes is high.

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