AN EXAMPLE OF DAMAGE FROM A POWDER AVALANCHE
M. MARTINELLI, Jr. and K.D. DAVIDSON (1)
Principal Meteorologist and Avalanche Control Engineer
A powder avalanche moved a 3,200 kg truck 19.8 in horizontally and dropped it 15.2 in into a gully without serious damage. Data from the site are used to compute the velocity of snow-free air and of a snow-air mixture necessary to cause this event. These velocities are found to be within the velocity of the avalanche as estimated from the equations published by A. Voellmy.
An adequate theory to explain the dynamics of snow avalanches has never been developed. Until such a theory is available, it seems desirable to document as many relevant events as possible in the hope that these will eventually suggest an acceptable theory. In the mean time, the observations furnish a continual check on existing empirical expressions. The following incident is described with these thoughts in mind.
On December 28, 1964, an avalanche was intentionally released by gunfire on
Red Mountain near Berthoud Pass, Colorado (2).
After the avalanche, a 7,000-pound (3,200 kg) dump truck and two heavy attachments for a tractor (3) were found a considerable distance from where they had been, even though the avalanching snow apparently did not touch them. At the time, this was thought to be an example of the airblast from a fast-moving avalanche. It will be shown, however, that it could also have been the action of a lightly-loaded powder avalanche.
Prior to the avalanche, the truck was parked on a horizontal, packed-earth storage area which is bordered on one side by the deeply incised avalanche path and on the other by a deep stream channel (Fig. 1). The long axis of the truck was perpendicular to the avalanche path and parallel to the stream channel. A bucket (2,300 lbs. - 1,045 kg) and a fork lift attachment (1,200lbs-546 kg) for a tractor were on the ground on the left (downhill) side of the truck (Fig. 2).
After the avalanche, the truck and one of the tractor attachments were found in the streambed 50 feet (15.2m) below and 65 feet (19.8 m) horizontal distance from their former position (Fig. 3). The truck was upright, essentially undamaged, and aligned as it had been prior to the slide. The fork lift attachment was beside the truck but on the right side of it. The bucket attachment was part way down the bank (Fig. 3). The only damage to the truck was a dent where the fork lift attachment fell against the hood, and a slight jamming of the two doors. The glass, tires, and springs of the truck were intact, and two large rear-view mirrors mounted on the cab were undamaged. There was no glass in the window on the driver's (left) side of the truck. This had been broken prior to the avalanche and had not been replaced. After the avalanche there was a light dusting of pulverized snow over the entire area where the truck and equipment had been parked and in the streambed where they landed. There was no evidence that any of the sliding snow had passed either place. The undamaged mirrors and lack of dents or scratches on the cab of the truck indicate that the truck did not roll down the bank into the stream.
(1) Authors are Principal Meteorologist, Rocky Mountain Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, with central headquarters maintained at Fort Collins in cooperation with Colorado State University; and Avalanche Control Engineer for American Metal Climax, Inc., Climax, Colorado.
(2) 39 degrees 45' N and 105 degrees 50' W on the Berthoud Pass, Colorado, quadrangle map of the U.S. Geological Survey.
(3) Attachments were for a 966 b Caterpillar Traxcavator.
Fig.1-Artist's sketch of Red Mountain 4B avalanche near the Urad Mine
in Colorado. The dump truck and two tractor attachments moved parallel
to the avalanche track in spite of the impression given here that they
moved toward the avalanche track.
How the truck got on the other side of the heavy tractor attachments is somewhat of a mystery since there were no eye witnesses. It is hard to believe that the truck could have been lifted high enough to go over them, since a vertical lift of about five feet (1.5 m) would have been necessary.
Fig. 2 - Location of truck and attachments before
and after the avalanche. The sketch is not to scale,
and contours are approximate.
For the five days prior to the avalanche, new snowfall totaled 41 inches (104 cm), and the wind at the ridge crest (12,500 feet - 3,810 m) averaged more than 40 mph (18 m sec-1) for 9 six-hour periods, with peak gusts to 100 mph (45 m sec-1). Snow drifting was moderate to heavy during most of the 5-day period, and air temperatures were below freezing for the entire period.
The 60-inch (1.5 m) soft slab released by the explosives fractured to the ground and moved down the track very rapidly. A large cloud of snow was generated by the avalanche before it was one-third of the way down the track. At this time the observers took cover, which explains why there were no witnesses to the truck movement.
The avalanche had run for approximately 2,600 to 2,700 feet (790 to 825 m) and had lost 1,400-1,500 feet (425-460 m) of elevation when it passed the truck. Snow avalanched from only about one-third of the 4-1/2 acre (1.8 ha) catchment basin. The remainder of the catchment basin was either bare or had a stable, shallow snow cover. The avalanche track is funnel-shaped with a cliff band (average gradient 52 degrees) across the track about 800 feet (244 m) from where the truck was parked and 350 feet (107 m) above it. Below the cliff band, the track has been deeply channelized by excavation and by an earthen bank along one side. There is a rather sharp reduction of gradient (from 52 degrees to 15 degrees) in the track just above where the truck was parked. The avalanche debris was distributed in the lower part of the track, in front of where the truck had been parked and in the flat runout zone 80-100 feet (24.4-30.5 m) downslope from this point.
Fig. 3 - Dump truck and tractor attachments after the avalanche.
The snow in the truck was present before the avalanche.
It is interesting to see if the velocities and forces given by Voellmy's (1955) (1) empirical expressions are sufficient to account for the events described here.
The force necessary to move the truck can be estimated by means of the following greatly simplified assumptions:
(1) The truck was pushed sideways from A to B (Fig. 4) by either a blast of snow-free air preceding the avalanche or by the dust cloud of the powder avalanche.
(2) The truck dropped from B to C (Fig. 4) as a projectile with initial horizontal velocity, VT. This is a gross simplification since a drop of 50 feet (15.2 m) would cause considerable damage to the truck. One possible explanation is that the turbulence from the avalanche caused a strong enough updraft in the gully into which the truck fell to cushion its fall.
(3) The velocity of the truck at A was zero; at B it was VT, which can be computed (appendix II A(1)).
(4) The coefficient of sliding friction, Ks was 0.2, (1) the drag coefficient, CD for the truck was 1.2
(1) VOELLMY, A., 1955. Ober die Zerstorunskraft von Lawinen. Schweizerische Bauzetung (in English as: On the destructive force of avalanches. U.S. Dept. of Agriculture, Forest Service. Alta Avalanche Study Center, Wasatch National Forest. Transl. no. 2. March 1964. 64 pp., illus.).
This model assumes that the truck and the two tractor attachments moved from A to B at the same time. The two attachments did not attain as high velocity as the truck because of their shape and the protection offered by the truck; therefore, they stopped relatively close to B and the truck went over them to land in the streambed.
Fig. 4 - Sketch of truck location before (A) and after
(C) the avalanche.
The horizontal component of the velocity of the snow-free air needed to accelerate the truck from A to B against sliding friction would be 71.5 m sec-1 (160 mph) (appendix II C(1)). The velocity of a snow-air mixture with a specific weight of 5.6 kg m-3 such as might exist in the dust cloud of a powder avalanche (appendix III C) would need to be only 28.7 m sec-1 (64 mph) to do the same thing (appendix II C(2)).
It is now necessary to go to the empirical expressions of Voellmy to see if such velocities could have been reached. Before doing so, it should be pointed out that Salm (1965) (3) considers some of the results of Voellmy's theory "debatable", and that it has not been possible for us to confirm the derivation of his equation for the velocity of a powder avalanche. In spite of these reservations, his work offers the best and most convenient theoretical check available.
The equation for the velocity of a ground avalanche shows the avalanche in question reached speeds as great as 32 m sec-1 (72 mph) in the upper track (appendix III A). If we accept Voellmy's statement that "on slopes of 300 or greater even unloosened snow can form powder avalanches after velocities of 15 to 20 m sec-1 (34 to 45 mph) are reached " there is little doubt this avalanche reached the runout zone as a combination powder and flowing avalanche.
The velocity of powder avalanches is considered to be a function of the thickness of the layer of snow that avalanched, the density of this snow, and the amount of avalanching snow that becomes airborne; velocity is considered to be independent of slope gradient (Voellmy 1955). On this basis, the powder avalanche had a velocity of at least 80 m sec-1 (180 mph) (appendix III B(1)). The velocity could have been as much as 100 m sec-1 (224 mph) if half of the avalanching layer became airborne (appendix III B(2)).
Further calculations show the velocity of the air following the avalanche blast was about 60 m sec-1 (135 mph); the pressure of the powder avalanche striking still air was 162 kg m-2; and the height of the dust cloud was about 52 m in the lower track (appendix III D, E, and F).
From these data, it is impossible to say whether the truck was hit by a blast of fast-moving, snow-free air or by a heavier, slower moving mixture of snow and air. Either could have done it. Field observers confirmed the tall plume of powder snow, but there is no way to check the pressure calculation.
This avalanche ran seven more times during the winter in response to explosive control. Although large masses of snow were involved in three of the subsequent avalanches, there were no more incidents of air blasts. The fact that this and 5 or 6 other avalanches of comparable size in the same immediate vicinity are under intensive explosive control opens the possibility for many very interesting studies of avalanche dynamics and movement.
(1) Personal communication: Gates Rubber Co. Denver, Colo.
(2) CD for a flat plate = 1.2. Page 407. ALBERTSON, Maurice L., James R. BARTON, and DARYL, B. Simons, 1960. Fluid Mechanics for Engineers. 567 pp., illus. Englewood Cliffs, N.J.
(3) SALM, Bruno. 1965. Contribution to Avalanche Dynamics paper presented at International Symposium on Scientific Aspects of Snow and Ice Avalanches. Davos, Switzerland, April 1965 (Proceedings to be published).
APPENDIX I: SYMBOLS AND VALUES USED
APPENDIX II: CALCULATIONS BASED ON DATA FROM THE SITE: See Figure 4.
(The distance A-B was estimated to be 6.1 m.)
APPENDIX III: CALCULATIONS USING FORMULAE GIVEN BY VOELLMY