High-elevation glaciers are undergoing substantial warming in response to current climate change. This warming is affecting the ice/rock interface temperature, the primary control of glacier stability on steep slopes. Instability may give rise to the collapse of large parts of hanging glaciers. The rheology of ice-avalanches is still poorly known but expected to deviate from snow-avalanches rheology. Here we retrieve the avalanche basal friction law parameters (Voellmy model) from a back-analysis of the largest historical ice-avalanche which occurred in the Alps at Altels glacier in 1895 (Bernese Alps, Switzerland). Prior simulations with a (1-D) sliding block model are performed to initiate 2-D simulations with a shallow-flow model. The parameters found to fit of the avalanche runout are associated to very low basal friction processes compared to snow avalanches. These friction parameters are applied to possible ice avalanches released from a glacier in the French Alps (Taconnaz) that may become temperate and potentially unstable by the end of the 21 st century. Flow height and velocity are analyzed as a function of friction coefficients and release conditions (glacier thickness) revealing strong contrasts with snow-avalanches dynamics.