Lawinenbeobachtungen im Parsenn-Gebiet,
19. Dezember 2005

Avalanche Observations in the Parsenn Area on December 19, 2005

General meteorological situation

Snowfalls of 30–50 cm occurred in the Parsenn area in November. An extended period of dry, clear weather during November and the first half of December led to extensive formation of depth hoar. On December 17 and 18, close to 1 m of fresh snow fell on this unstable substratum; the temperatures were significantly below 0°C during and after the snowfall, which was moreover accompanied by tempestuous winds. Many upwind slopes (exposed in northerly to westerly directions) were barren afterwards whereas downwind slopes were heavily loaded.
Parsenndienst, the security service of the Parsenn skiing area, secured a large number of slopes above the ski runs by means of artificial avalanche releases in the mornings of December 18 and 19. A great many of these release attempts were successful, and the avalanches often comprised a large fraction of the maximum possible release area.
Due to the high avalanche danger and multiple time constraints, A. Errera, S. Priano and D. Issler could only make a quick survey from the ski runs that were open or at least secured by the Parsenn security service on December 19, 2005. More detailed investigations were attempted from January 12 to 17, 2006, but extensive snowdrift, moderate snowfalls of 10–20 cm shortly after New Year’s and uninhibited off-piste skiing had obliterated the traces of these avalanches so that little information useful for understanding their dynamics or statistical analysis of runout distances could be gained beyond what is described in this document.

Dorfbachtobel

Several releases occurred in the Dorfbachtobel area. On the left flank of the stream, only one release was visible. It occurred only a little downhill of Höhenweg. Its extents could not be estimated well from the cablecar. A very small release on the opposite side was stopped by the bushes and shrubs after a few meters.
A significant avalanche started from the slope below the Schafläger dam. It was quite wide, and from the photos it is possible that the release zone consisted of up to three separate areas. The avalanche flowed down as two branches separated by a gentle shoulder. Both branches crossed the ski run just after the latter had crossed the Dorfbach stream. The left branch (as seen in the flow direction) continued across the slope below the ski run and entered the Dorfbachtobel gully. Its edges were not sharp, indicating the presence of a fluidized part. Snow deposits plastered onto the trunks of small trees near the edge of the gully also confirmed this inference. The height of the suspension (or fluidized) layer was between 1 and 2 m.

Abb. 1. Lawinenanriss im Hang unterhalb des Auffangdammes im Schafläger. Niedergang in zwei Armen, die beide die Skipiste (im Vordergrund) überquerten. Relase area below the Schafläger catching dam. The avalanche flowed in two branches, both of which crossed the ski run (visible in the foreground).

Abb. 2. Die Lawine vom Schafläger-Damm entwickelte eine fluidisierte Schicht mit einer Fliesshöhe von mindestens 2 m, wie der an dei Baumstämme gepflasterte Schnee zeigt (Blick von der Piste ins Dorfbachtobel). The avalanche from the Schafläger catching dam developed a fluidized layer with a flow depth of at least 2 m, as evidenced by the snow plastered onto the tree trunks (view from the ski run down the Dorfbach gully).

Dorftälli

Abb. 3. Die sogenannte "Hundeführerhaldenlawine" vom Mittelgrat nahe dem Salezerhorn ins Dorftälli hinunter, im Vordergrund die Dorftällipiste und die Trasse der Parsennbahn. The "Hundeführerhalde" avalanche starting near the crest ("Mittelgrat") between Weissfluhjoch and Salezerhorn. The Dorftälli ski run and the track of the Parsenn cable car are visible in the foreground.

On the right side, at least five releases could be observed. Even though they originated under apparently similar topographic and nivological conditions, they showed quite a wide spectrum of properties.
The path passing by the old tripod instrument carrier (Fig. 4) featured a release depth estimated to about 1 m, with signs of significant snowdrift effects. The path immediately up-valley from it also released, with quite similar characteristics, but somewhat smaller mass and release depth. Seen from the ski run, the deposit conspicuously lacked large blocks, looked quite homogeneous, showed a sharp boundary and seemed quite shallow. In particular, there was no indication of a saltation layer.

Abb. 4. Mehrere der Lawinen vom Grat zwischen Weissfluhjoch und Schiahorn zeigten scharf begrenzte Ränder und relativ dünne Ablagerungen. Waren diese Lawinen nahe dem Übergang zum fluidisierten Fliessregime? Several avalanches that released from the crest betweenWeissfluhjoch and Schiahorn produced sharply delimited,but fairly shallow deposits. Were these avalanches close to the transition to the fluidized flow regime?

Further down the valley, to the side of a plateau with a mast of the Parsenn Rapid skilift, a path with a fairly long, regular starting zone that is only visible from certain locations released with a large fracture depth (> 1 m?). In the upper track, the avalanche was clearly channeled; due to the topography, it made a sharp turn (about 45°). At several locations along the main track, secondary release zones could clearly be seen. In the deposition area, the avalanche widened by an estimated factor of 3. The topographic map 1:10’000 suggests that this is mostly due to the transverse curvature of the debris cone that forms the runout area.

A little further down the crest, a much smaller avalanche was observed that stopped in the slope. No significant widening was visible.

Weissfluh

A fresh release area was visible on the south face of Weissfluh, near the summit station of the telepherique. A little south of the Summit East ski run, an avalanche could be recognized even though it was partly covered by fresh snow or snow drift. On the hummocky north-east slope, a small avalanche not much above the Derby run featured a strongly curved track due to outcrops (Fig. 5). Deposition must have started immediately after release—large blocks from the early break-up phase were scattered along the entire path.

Meierhofertälli

The ski run in the Meierhofertälli was not yet prepared for lack of snow (snow drift!), hence artificial releases were attempted (and achieved) only where the avalanches could hit the masts of the ski lifts to the north and north-east of the Weissfluhjoch summit. Several fairly large releases could be observed. They varied significantly in their characteristics, probably because of differences in wind speeds and directions, slope angle and ground roughness. In one case, jumps of the sliding horizon occurred.

Schwarzhorn

No releases occurred on the north-west slope of Totalphorn. Opposite to it, the Schwarzhorn crest produced two avalanches of different sizes and characteristics. The slope investigated in February 2005 by Bernhard Krummenacher was not as strongly loaded as then and produced a somewhat smaller avalanche; it was clearly narrower than its predecessor and did not flow to the north-east of a conspicuous outcrop. The most interesting observation concerned a rather clear and regular boundary between a thicker proximal deposit area and a thinner distal one. Several interpretations are possible without further data on the avalanche, but a particularly attractive one is that the thin deposit is due to the fluidized part that just began to form after a distance of less than 150 m.
The situation concerning the fluidized layer was less clear with the next avalanche a little further to the north-east. The release area began just below the steep, very rough rocks underneath the summit.

Abb. 5. Kleine Lawine mit stark gekrümmter Bahn am Nordost-Hang der Weissfluh. Small avalanche with strongly curved track on the north-eastern slope of Weissfluh.		Abb. 6. Mehrere Lawinen gingen im Hang nördlich des Furka-Skilifts nieder. Several avalanches occurred in the slope to the north of the Furka ski lift.

Parsennfurka

Four distinct avalanches could be observed to the right when riding up the Furka skilift, from the crest between Parsennfurgga and Seetälli (Fig. 6). We photographed and mapped them on the basis of the photos, but it was not safe to investigate them more closely. A fifth avalanche occurred to the right of the main ski run down to Parsennhütte; it was quite small and deposited fragments of the slab along the entire track, see Fig. 7.

Abb. 7. Kleine Lawine am Nordhang des Schwarzhorns, nahe dem Furka-Skilift. Small avalanche on the north face of Schwarzhorn, close to the Furka ski lift.

Casanna

On Grünhorn, two detonation masts allow artificial releases that help to secure the ski runs under­neath. Slightly to the side and below the lower of the two masts, a small slide did not reach the ski area but stopped in the middle of the slope.