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Dynamic modelling of extreme speed profiles of dry flowing avalanches Borstad, Christopher Paul

Abstract

A brief review of different approaches to avalanche dynamics modelling is considered, drawing on observations and knowledge of avalanche behaviour and analogies with other physical phenomena. The numerical model used for this analysis, DAN, is described with respect to the formulation of the equations of momentum and continuity. Avalanche path geometry is approximated as a rectangular channel with the possibility of variable width. The flow mass is discretized longitudinally into a variable number of constant-volume blocks. The uncertainty surrounding the dynamics of frontal ploughing of undisturbed snow, basal erosion, and mass entrainment is discussed. In order to avoid this uncertainty, which is implicit when modelling a slab avalanche beginning from rest in the starting zone, the calculations in DAN begin near the middle of the slope where the flow is expected to achieve maximum speed. Knowledge of the maximum probable speed for an extreme avalanche is used to supply an initial, non-zero speed to the flow near the middle of the path, and entrainment is assumed negligible as the flow decelerates. A sensitivity analysis explores theoretical considerations surrounding the modelling of a flow mass in multiple dimensions, given this new procedure as well as traditional from-rest approaches. The model is shown to be more sensitive to changes in flow width than flow length or depth. DAN calculations are compared against examples of recorded extreme avalanche data, with generally good agreement. The new modelling procedure was able to reproduce the observed sharp deceleration of avalanche frontal speed in runout zones. This is worthy of remark, because most decisions about land use planning and hazard assessment are made in the lower reaches of avalanche paths.

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