Numerical models that predict flow velocities and runout distances of extreme avalanche events are used to delimit hazard maps. These models contain many simplifications, one of which concerns snow entrainment. Most avalanche dynamics models assume that avalanche mass is constant along the track, meaning that no entrainment takes place. This assumption is in clear contradiction to post-event observations of avalanche paths which show that much of the snow cover has been entrained into the avalanche and that deposits are left along the avalanche path.
The primary aim of this work is to show the influence of mass variations on avalanche simulations used for hazard mapping. Suggestions are given to improve hazard mapping procedures.
To verify the influence of entrainment in practical calculations, extreme avalanches from the Winter 1998/99 were back-calculated using the Swiss Guidelines procedures and a new theory considering entrainment. It is shown that if entrainment is neglected, the mass and energy balance of the event is in error. Inclusion of entrainment leads to: (1) a better prediction of runout distances, (2) a more accurate determination of flow and deposition depths and (3) a better control over model parameters.
A simple rule for practical calculations that can help to define the correct avalanche mass is suggested.