MAXIMUM AVALANCHE RUNOUT MAPPING; A CASE STUDY FROM THE CENTRAL SIERRA NEVADA

In 1983 an avalanche hazard map (AHM) had been prepared for Galena Basin in the Central Sierra Nevada. That area was to be deve-loped as a ski resort and the map was later incorporated in the Con-cept Plan for the resort. Fortynine avalanche paths were identified in that eight square mile area and for each path the limits of a 20-years and a 100-years avalanche were drawn on a large scale topo-graphic map. Two years after the map had been prepared a very severe snow-storm hit the Sierras. Numerous avalanches released and felled trees up to 300 years old. The storm is considered to be the "storm of the century".In Galena Basin 16 avalanches ran and their outer edges were mapped and compared to the predictions of the AHM. The observed runout distances of those February 1986 avalanches were an excellent opportunity for a calibration of the two friction coefficients ~. and 5 used in the Voellmy equations for the computa-tion of avalanche velocity and runout distance. With a slab depth of 2.0 meters, a sliding friction coefficient ~ = 0.17 and a turbu-lent friction coefficient 5 = 1200 m/s2 the results of the Voellmy model fitted fairly well the observed runout distances