OCR Text |
Show - 1- The regular examination of stratigraphy in pits dug periodically in a snow study plot is a standard part of our hazard forecasting technique. In order to apply the information so gained to hazard evaluation on avalanche paths, it is necessary to know the characteristic snow profile patterns which lead to avalanche formation. These patterns differ widely from climatic zone to another. The most extensive study of relations between snow structure and avalanche formation in this country have been made at Berthoud Pass, Colorado, where high winds and low temperatures commonly produce a widespread hazard from hard slabs. Characteristic snow profiles at the fracture lines in this area, together with the applicability of the ram penetrometer in hazard forecasting, have been discussed at length in the Berthoud Pass annual reports. We have many fewer fracture line data from the other climatic zones. Information is especially lacking from the Coastal Alpine zone of Washington and California. In order to furnish the necessary information on avalanche origins in the Middle Alpine, numerous fracture line profiles have been measured in the Alta area in recent years. The Middle Alpine zone is characterized by frequent soft slabs falling during or immediately after a snowfall, plus numerous climax avalanches caused largely by depth hoar development. The latter usually fall in the category of hard slabs, although slab ram resistance averages much lower than in the High Alpine zone of the Colorado Rockies. During the period 1961- 1964 over which the present fracture line data were collected, depth hoar formation in the Wasatch Mountains of Utah was more extensive than usual. Consequently climax avalanches during this period were rather common while the larger soft slabs o^ urred infrequently. The fracture line profiles discussed in this report reflect this distribution. The avalanches chosen for investigation were all of medium or large size and were all artificially released during the course of routine avalanche control in the Alta area. |