U. S. DEPARTMENT OF AGRICULTURE
WASATCH NATIONAL FOREST
ALTA AVALANCHE STUDY CENTER
Progress Report No. 1
TYPICAL ALTA FRACTURE LINE PROFILES
Avalanche Hazard Forecaster
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 occurred 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.
See Chapter 6 of "Snow Avalanches" (FSH 2332.81) for explanation of observation methods and symbols used in plotting the profiles.
Figures 1 and 2 illustrate a typical soft slab avalanche. Both profiles were taken on the same day on different avalanche paths to depict the uniformity of snow conditions which led to a cycle of artificial slide releases throughout the Alta ski area. In each case the layer of new snow from a single storm slid on a sun crust to which it was poorly attached due to the low January temperatures. In one case (Figure 2) a thin layer of old snow was located between the new snow and crust, but this apparently did not play an essential role in the avalanche release. Although the base consisted of unstable depth hoar which became involved in the slide as it built up momentum, this did not influence the origin of the slide, which was released as a direct-action soft slab running on the thin sun crust. Note that the ram profile shows very little stratigraphy In such a situation, the only detectable layer being the crust.
Figures 3 and 4 were also taken on the same day to illustrate a climax hard slab condition which extended throughout the Alta-area. In this case the sliding layer involved snow from several distinct storms. The sliding surface was a crust underlain by old snow which had undergone partial constructive metamorphism-, but which was stable enough not to become involved in the avalanche. The immediate cause of this avalanche cycle was a very weak, thin layer at the bottom of the slab which offered low shear strength and served as the lubricating layer for the slab release. It consisted of very fragile depth hoar crystals just above the crust. Weather records do not show clearly the origin of this layer, but it is presumed to be a shallow deposit of fluffy snow in which temperature gradient and constructive metamorphism were subsequently enhanced in relation to the surrounding snow. The roles of sliding surface (crust) and lubricating layer (depth hoar) are clearly illustrated in this case.
Figure 5 shows a climax hard slab running on a stable base of old snow. The origin of this slide represents a condition peculiar to the Alta area, the occasional deposition of graupel (pellet snow) in large quantities. Layers of graupel 20 to 30 cm thick are not uncommon, and these are an excellent source of slab avalanches by virtue of their high density and stiffness (high viscosity). A 32-cm layer of graupel at the bottom of the Figure 5 slab was probably a principal source of the instability. More interesting, though, if the formation of the lubricating layer by a shallow deposit of very poorly consolidated graupel, literally a case of natural "ball bearings." Stratigraphic weaknesses, other than a general low strength of the slab layer, are not suggested by the ram profile.
Figure 6 represents a climax slab condition which clearly originated from poor support by depth hoar in the lower snow cover. The sliding layer was built up from several snowfalls, the earlier of which suffered loss of tensile strength through constructive metamorphism. Even here, where the weakness in the snow cover is obviously generated by extensive- depth hoar development, the actual fracture has run back to a distinct lubricating layer, a shallow layer of depth hoar which was noticeably more fragile than the rest below it. In this case the very weak lower part of the snow cover is obvious from the ram profile, although the latter does not indicate the depth at which a slab would be expected to break away.
Figure 7 illustrates an earlier stage in the hazard development shown in Figure 6. The basic depth hoar development in December provided insufficient support for the snowfalls of early January, resulting in a soft slab release two and one-half weeks earlier than the hard slab of Figure 6. Note that both slides released in the same way, by sliding on the thin, very fragile depth boar layer. This cause of instability was recognized at the time of the Figure 7 slide, so that extensive control measures were undertaken following subsequent snowfalls in anticipation of more instability. This anticipation was fully justified on 24-25 January, when extensive avalanching occurred throughout the Wasatch Mountains following a 130-cm snowfall. The entire snow cover of Figure 7 is so weak that the ram profile offers no stratigraphic information.
Figure 8 is an uncomplicated case of wind slab resting on fragile depth hoar. Sliding surface at the fracture line was the upper part of the depth hoar coinciding with tops;of the rocks scattered across the slope. Once this slide picked up momentum, it removed all the snow cover back to the ground. Here the unstable configuration of a slab layer on depth hoar can be discerned from the ram profile.
In summary, the following is concluded from this brief study of fracture line profiles from the Middle Alpine zone:
1. Structural weaknesses in the snow cover leading to climax avalanching can readily be recognized by visual inspection of a pit wall.
2. The ram penetrometer alone is not a satisfactory instrument for detecting these weaknesses under the snow and climate conditions found at Alta. The snow is often too soft or weak to reveal much stratigraphy to the penetrometer, and when it does, the clues to instability may be obscure. (Compare this in contrast to the Berthoud Pass records, for instance Figures 32-35 in "Snow Avalanches.")
3. Avalanche release often originates with a thin layer of very low shear strength--the lubricating layer. This observation is based on general experience as well as the present collection of fracture line profiles.