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Dry-Snow Avalanche at Alp Drusatscha on 14 February 2006

Stefano Priano and Dieter Issler

On February 14, 2006 a dry-snow avalanche was released on the north-western slope of Hüreli above the Drusatscha mountain pasture (Fig. 1). The GPS measurements indicated the topmost point of the crown to be at an altitude of 2238 m a.s.l. and the lowermost point of the deposit at 1758 m a.s.l. on the plateau of Alp Drusatscha. When the measured X and Y coordinates of these two points were put onto the 1:10,000 scale map, the corresponding Z coordinates were 2230 and 1743 m a.s.l., respectively.

View down the avalanche path to Alp
              Drusatscha  
Fig. 1. View down the path of the 2006-02-14 Drusatscha avalanche, with the flow direction and the deposit area indicated. Alp Drusatscha is in the middle ground, the Lake of Davos can be seen at the upper left edge of the picture. More extreme events turn to the left where this avalanche stopped and may reach the lake.

The projected length from the crown to the toe of the deposit was slightly less than 800 m, giving a run-out angle α of approximately 31°. The avalanche stopped in terrain that was still slightly steeper than 10°. Correspondingly, the β angle characterizing the steepness of the path is smaller than α by about 1°. This indicates that the 2006 event was far from being an extreme event. Indeed, much larger avalanches have occurred in this path before, one avalanche around 1910 turning left at Alp Drusatscha, descending towards the lake, hitting a train and killing several persons.

The 2006 avalanche was probably triggered by a skier crossing the slope (some traces were visible at the upper right side of the crown), but there is no report of anybody having been caught in the avalanche.

Investigations in the release zone

In the release zone there were several big slabs of the original snowcover that had started sliding on the failure plane but stopped after a few meters (Fig. 2).

Snow blocks that remained near the
              fracture line
 
Fig. 2. Partial view of the fracture line, showing several snow blocks that broke off, but did not slide down.

Partial view of fracture line   
Detail of fracture surface 
Fig. 3. Measurements at the fracture line.

The first phase of our field work was dedicated to carefully mapping the release zone and investigating the fracture crown.

The crown had a semicircular shape with a diameter of 13 m and a tensile fracture surface at a right angle to the original snowcover surface and the sliding plane (Fig. 3). The latter consisted of a hard surface with a variable depth between 30 cm (in the central part) to 15 cm (along the sides), overlaid by the weak stratum in which the failure occurred (see the profile in Fig. 4).

We could also observe several fissures to the side of the avalanche crown, probably due to the tension forces at the moment of failure.

Standard snow profile near fracture
              line 
Fig. 4. Standard snow profile taken at the fracture line near its maximum height. The red line indicates the snow (full line) and air (broken line) temperature profiles, the blue bar diagram is the ram resistance. The failure occurred at the base of an approximately 10 cm thick layer of very low ram resistance. The thin layer at 28 cm above ground is a melt crust. A compression test was performed and led to fracture at the sixth tap from the elbow, indicating moderate stability.

Above the release area there is a gently inclined ledge between 2250 and 2260 m a.s.l. It appears, however, that the distance between its edge and the fracture line is too far for the stress concentration induced by the convexity of the terrain to be a viable explanation for the location of the fracture line.

In order to evaluate the mass balance, we made a series of measurements of the snow heights along the fracture line (every 2 m), and we also made density measurements at one location in the tensile fracture surface (every 10 cm vertically). The maximum fracture height was found in the middle of the crown (87 cm), while it diminished towards the sides (40–50 cm).

Overview of release area

Fig. 5. Overview of the release area (left) and variability of the fracture height along the crown (right). 
Variability of fracture height
              along crown

Table 1. Density profile near the middle of the fracture line

Height above ground
(cm)
Density
(kg/m³)
108 157±10
98 300±10
88 231±10
78
241±10
68 268±10
58 315±10
48 257±10
38 304±10 Failure plane
28 325±10
18 367±10
8 362±10

Observations in the avalanche track

The avalanche track is a little less than 700 m long (horizontal extent), essentially straight and largely unchanneled. The slope angle fluctuates between 22° and 63° (according to the digital terrain model derived from the map at a scale of 1:10,000), the steepest part being a rockface roughly half way in the track at 1900 m a.s.l. However, in the terrain the slope appears not as hummocky as the calculated slope angles indicate (Fig. 1).

Along both sides of the path we found deposits, the texture of which differed from the texture of the deposit along the centerline of the path. The lateral deposits were clearly less deep, less dense and consisted of smaller particles embedded in a matrix of fine-grained snow. The texture of the latter was oriented in the flow direction, presumably as a result of air drag on the surface of the flow. All these observations lead us to interpret the lateral deposits as snow that was fluidized until briefly before it deposited. The fluidized deposit was quite clearly delineated on the right-hand side, while it was wider (6–9 m) on the left-hand side and could be traced along the entire avalanche path.

 

Fig. 6. Partial views of the avalanche tracks, showing the difference between the area affected by the dense flow and the sides where only the fluidized portion of the avalanche passed. Note that there are exgtended areas where the avalanche eroded and entrained the snow cover to the ground.

As mentioned above, the slope inclination changes rather abruptly at several points along the path. In the more gently inclined segments, the avalanche slows down and deposition increases (we already found big snowballs), while the velocity increases and the avalanche erodes the snowcover to the bottom where the slope steepens.

However, the erosion depth varied not only along the flow direction, but also in the transverse direction. Only in the central part of the flow, where the velocity and thus the shear stress most certainly were highest, was the avalanche capable of eroding the entire snowcover, whereas part of the original snow cover always remained in the lateral zones.

There was an exception to the rule that erosion increases with slope inclination: At the edge of the cliff near 1900 m a.s.l, the slope angle increases abruptly from about 25° to over 60°. Below the foot of the cliff, the slope is more uniform with an inclination close to 30°. At this cliff the flow jumped a distance of about 40 m. This could be inferred from the absence of pronounced flow traces, which should be present if the entire avalanche mass and in particular the fast head had flowed though this area. (The slow tail of the avalanche seems to have stayed on the ground, however).

Cliff at 1900 m a.s.l. with jump
              line of the avalanche front indicated. 
Fig. 7. Cliff at 1900 m a.s.l. with jump line of the avalanche front indicated. The horizontal jump length is approximately 40 m, the vertical drop about 35 m.

This jump offers an opportunity to estimate—rather crudely—the velocity of this avalanche: Lift-off requires that the centripetal force be larger than the slope-normal component of the gravitational acceleration. We do not know the curvature at the upper side of the cliff precisely, but from the profile data (avalanche #33 in the table of recorded avalanches), we infer an order-of-magnitude of 25–30 m, which appears reasonable for this terrain. From this follows a critical velocity of the order of 15 m/s. This value appears plausible as well: The head of the avalanche certainly exceeded this threshold whereas the very tail may have been slower than this. Furthermore, a jump width of 40 m, as estimated in the field, corresponds to a fall height during the jump of approximately 37 m according to the available terrain data. Taking off at an angle of 25°, the required velocity is approximately 23 m/s. This value is roughly double the critical velocity for lift-off and lends support to the inference from velocity measurements at avalanche test sites and other observations, made in the course of this project (Issler et al., 2008), that the dense tail of an avalanche moves at roughly half the speed of the fluidized front.

Somewhat below the reattachment point, the flow width was measured as 42 m, of which 6 m and 9 m on the right and left sides, respectively, are to be ascribed to the fluidized layer. At this location we dug a 10 m long trench, transverse to the flow direction, in order to estimate the snow entrainment depth. The trench started outside the area affected by the avalanche and extended across the lateral strip with fluidized deposits towards the inner region with the main deposits. At the outer end of the trench, we found the snowcover to be 1 m deep. It consisted of 50 cm of depth hoar at the base, overlaid by a fairly hard layer (probably the same as in the release zone). The stratification near the surface became clearly visible upon dyeing the vertical profile face with ink in the same way as for the Inneralp avalanche in 2004. At several locations, density profiles were taken.



Fig. 8. Transverse snow pit in the avalanche track, with the snow texture made visible by means of dyeing with ink.

At the edge of the avalanche, the weak layer that had failed in the release area could be identified. On top of it, we found approximately 20 cm of the original new-snow layer, but compressed and of inhomogeneous texture. This layer was covered by about 10 cm of deposits with the properties of a flow of intermediate density. These observations seem to indicate that the flow passing over this area was fluidized and quite mobile, only its rear part depositing here. Moreover, the flow was strong enough to entrain a substantial part of the new-snow layer, but not all of it; the remainder was, however, subjected to a shear stress that caused some internal shearing in the layer.

Towards the southern end of the trench, i.e., in the central part of the flow, the basal layer of depth hoar was overlaid by a thick deposit made up of hard snowballs, covered by soft snowballs and light snow. At this location, where the slope is inclined at less than 30°, the avalanche did not have enough erosive power to entrain the entire snow cover, as it had done higher up. However, the glide plane in the central part of the flow was very hard, and we found small snow balls sintered to it, their orientation preserving the direction of the flow. The presence of the hard glide plane allowed us to measure the depths of the remaining snow cover and of the deposit above it across the entire width of the avalanche using an avalanche probe, without the need of snow pits (Fig. 9).

  Fig. 9. Snow depth profiles across the snow pit, showing the remaining thickness of the original snow cover (mostly depth hoar and angular crystals), the thickness of the dense deposits attributed to the dense-flow part of the avalanche, and the lighter deposits from the fluidized part on the top.

The deposit area

The main deposition area extended from 1850 m a.s.l. to about 1760 m (according to the GPS measurements, whereas the map at scale 1:10,000 indicates approximately 30 m lower altitudes). It covered an elongated area of about 2 ha.

In the run-out area, the avalanche split into two main branches that followed two shallow gully-like depressions in the terrain on the right-hand and left-hand side of the path. However, the area in between was also covered by avalanche debris.

The entire deposit was seamed by soft deposits from the fluidized layer (generally only a few centimeters deep), sometimes with interspersed snowballs of some 10 cm in diameter. With a GPS we followed the avalanche margin to evaluate the lateral and frontal spreading of the avalanche. On the surface of the deposit we found tracers (branches and pieces of the trunks of small trees) that were picked up by the flow near the cliff at 1900 m a.s.l. and carried to the deposit area.

Slightly below the slope change that marks the beginning of the depostion zone, we found two elongated, symmetrical, drop-shaped ridges (i.e., their rear part was thinner than their frontal part). These ridges resembled those observed in the Gotschnawang avalanche investigated about three weeks earlier, but they were significantly smaller. As at the Gotschnawang, they seemed to originate at an abrupt decrease of the slope angle, but we were not able to identify topographic controls that would explain their shape and precise location relative to the flow axis. Another (more readily explained) common feature of the ridges in these two events are the dense sets of longitudinal normal and parallel inverse failures.

We dug a cross-sectional snow pit across one of these flow ridges to better understand their main aspects. The bottom layer consisted of depth hoar, as in the other snow pits of this avalanche. Above it, we found a hard and compact, lens-shaped deposit without any apparent internal structure. This enigma thus still awaits its resolution.

 
Fig. 10. Cross-sectional snow pit across one of the flow ridges.

Our preliminary explanation for the shape and orientation of the failures observed in the flow ridges is as follows: The collisions between snow particles become less frequent and less energetic when the velocity decreases so that the snowballs can aggregate. At this stage, the snow behavior changes from mostly fluid to mostly solid. If the front encounters an area of reduced friction or increased gravitational pull, it accelerates ahead of the masses at the rear. This leads to a normal fault perpendicular to the flow direction. (Such normal faults were observed in the Taferna avalanche of February 2005.) If the flow resistance varies in the lateral direction, steep-walled failure planes approximately parallel to the flow direction are expected to form. They are commonly observed in humid avalanches, an example being the Dorfberg avalanche of March 2005. The third possibility, expected to occur at concave slope breaks, are inverse normal faults due to the compressive stresses that arise when the front encounters stronger resistive forces than the masses behind, which then push the stopped mass at the front.




Verantwortlich für diese Webseite / Responsible for this webpage:  Dieter Issler
Letzte Änderung / Most recent changes:  2014-01-17

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