On snow entrainment in avalanche dynamics calculations

The analytical and numerical models that are used in practical snow avalanche dynamics calculations contain a questionable simplification: they assume that avalanche mass is constant — meaning that no snow cover entrainment or mass deposition takes place along the avalanche path. This assumption is in clear contradiction to post-event observations which show that much of the snow cover has been eroded and that deposits are left along the avalanche track. The aim of this work is to investigate the influence of snow entrainment on avalanche dynamics calculations. A simple Grigorian–Ostroumov type entrainment model, which assumes entrainment localized in an area close to the avalanche front, is introduced into a depth-averaged numerical model containing different constitutive flow laws. The snow cover distribution along the avalanche path must be specified. The results of the numerical simulation with entrainment are compared to existing Swiss calculation procedures. Six well-documented extreme avalanche events are used for this purpose. We find that inclusion of a simple entrainment model 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; that is, the parameter range needed to calculate small and large-sized avalanche events narrows. However, one-dimensional models, as presented here, are limited because the flow width and therefore the potential erosion area, must be specified in practical situations by the user. This can introduce additional uncertainties into practical calculations.