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An accurate prediction of avalanche runout distances, flow velocities and impact pressures in natural three-dimensional terrain is the driving motivation behind the development of improved snow avalanche dynamics models. These numerical models are typically applied in avalanche hazard mapping studies, both to back-calculate documented avalanche events at a particular site as well as to forward calculate the consequences of possible hazard scenarios. The quality of the results depends highly on the experience and know-how of the avalanche expert, especially when defining the initial conditions of the hazard scenario, but also on the ability of the numerical model to accurately simulate flowing snow in natural three-dimensional terrain. The computer model must allow the expert to easily and realistically define terrain, release conditions, track roughness and vegetation; the model output must be easily visualized and interpreted so that the expert can confirm or discard simulation results.
Because RAMMS is used for hazard mapping in Switzerland, it must be used with regard to existing calculation guidelines in Switzerland (Salm et al., 1990). For this reason, the Voellmy-Salm (VS) model (Salm, 1993), which is used in the Swiss Guidelines for avalanche runout calculation as well as in the one-dimensional AVAL-1D code is implemented in RAMMS. In practical applications, the VS model has proven to be simple (it contains only two flow parameters) and numerically accurate.