OCR Text |
Show The following points are noteworthy: 1. The first avalanche occurred when the P. I. Factor was 1.30 with an average P. I. of .16 inch. 2. The general avalanche cycle was touched off by the additional weight of 7 inches of snow and .50 inch of water, half of which fell at low accumulation rates. It is legitimate to speculate on when the cycle would have occurred if P. I. had remained high. Apparently about 3 hours earlier, 1800. 3. The general avalanche cycle took place immediately after a sudden increase in settlement. We have charted one other similar occurrence. We might theorize that this jump in settlement indicated the collapse of an unstable snow structure. As a hazard forecasting technique, it comes too close to the event to be of much value. 4. The now familiar combination of high P. I.- S. I. occurs again. But compared to No. 3, P. I. is higher and S. I. considerably lower. For the explanation we must look to the average densities: .17 for No. 5 against .09 fo; No. 3. 5. It is interesting to compare these two storms. No. 5 had one- fifth the duration, one- third the snowfall, less than one- half the water of No. 3. But it had 100 percent more dangerous avalanche action. The comparison suggests that the gross dimensions of a storm are not nearly so important as a variation in precipitation intensity of a few .01 inch per hour. Penetrometer Studies ( Penetrometer studies have been conducted in the United States during the winters of 1950- 51 and 1951- 52 by Snow Ranger Richard Stillman at the Berthoud Pass Winter Recreation Area, Arapaho National Forest, Colorado, in collaboration with Whitney Borland, Bureau of Reclamation. Berthoud Pass is in the high alpine zone.) The penetrometer- ram sonde- is an instrument developed by the Swiss to investigate the nature of the snowpack. It is a sectional rod, graduated in centimeters, which is driven into the snow by the blows of a falling weight. The objective is to determine the resistance to penetration of the various layers and from this data, estimate the cohesion and stratification of the snowpack. The technique is adapted particularly to the recognition of delayed action slab avalanche conditions. The procedure is as follows: The graduated rod is set upright on the snow and driven in by one or a succession of blows from the hammer. The weight of the rod, weight of the hammer, drop distance, number of blows and depth of penetration are recorded for each succession of blows. From this data a resistance profile of the snowpack can be calculated by a simple mathematical formula. An experienced observer can then interpret this profile in terms of avalanche hazard. - 107 - |