Volume and frequency of ice avalanches from Taconnaz hanging glacier, French Alps

Very large volumes of ice break off regularly from Taconnaz hanging glacier, French Alps. During winter, when the snow mantle is unstable, these collapses can trigger very large avalanches that represent a serious threat to inhabited areas below. Photogrammetric measurements have been performed over 1 year to assess the volume and frequency of the largest collapses. Major collapses occur when the glacier reaches a critical geometry. After a major ice collapse, the glacier is in a minimal position and subsequently recharges over 6 months to reach the maximum position again. This critical geometry is a necessary but not sufficient condition for further large collapses. Large collapses do not systematically occur in the maximum position, as ice is often removed by disintegration into small ice blocks. For two major collapses, the volume of ice breaking off has been assessed at,similar to 275 000 m(3). Photogrammetric measurements were used to determine an ice flux of 820 000 m(3) a(-1) through the studied ice stream, in agreement with an assessment based on ice-flow modeling. This ice flux estimation was used to determine the average ice volumes breaking off over surveyed periods.