II – Les Drus IV – Jungfrau East Ridge 3-dimensional analysis of the thermal conditions in recent periglacial rock fall detachment zones Since the end of the LIA the Drus West face has repeatedly been affected by On October 6, 1937 ca.150’000 m3 rock detached from rock avalanches. In the last 20 years three bigger events occurred (Ravanel, the Jungfrau East ridge at an elevation of 3800 m a.s.l. Jeannette Noetzli, Luzia Fischer and Stephan Gruber 2006): 1997 (25’000-30’000 m3), 2003 (6’000-7’000 m3) and 2005 (255’000 and fell onto the Aletschglacier below. Glaciology and Geomorphodynamics Group, Department of Geography, University of Zurich, Switzerland -275’000 m3). Contact: firstname.lastname@example.org Surface temperatures are modelled based on climate Surface temperatures are calculated based on climate time series 1990- time series 1990-1999 from Jungfraujoch and DHM25 1999 from Jungfraujoch. Level2 (Source: Swisstopo). Surface temperatures in the area of the starting zones are in the area of -2 In the area of the starting zone permafrost temperatures Introduction and modelling approach to -5 °C and indicate permafrost conditions. are ca. -3 °C on the southern and ca. -7 °C on the north- ern side of the ridge. These modelled temperatures cor- Figure 4. West face of the Drus (3778 Climate time series L in/out respond well to measurements from boreholes situated m a.s.l) near Chambéry, F. The de- Permafrost is regarded as one of the crucial factors influencing the stability of Topography (DEM) tachment zone is still visible in grey. nearby (Wegmann, 1998). Picture taken in September 2006. B steep bedrock in alpine areas. Instabilities are expected to originate preferen- (Sub)surface information S in/out A ? tially in locations with temperatures little below the melting point. Detailed pro- Surface energy balance Figure 8: Starting zone of 1937 event at the Jung- -10-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 temperature [°C] cess understanding and knowledge on the thermal conditions under which such B frau East ridge just above the Swisscom station on Surface temperatures Q h , Q le E B -10-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 two recent photos. Photo below: S. Bircher. instabilities develop is still lacking. New information may be gained e.g. from the S temperature [°C] Topography (DEM) N A analysis of the thermal conditions of recent periglacial detachment zones. W B Q g (+le) Subsurface information N W Based on an inventory of more than 50 recent rock fall events, we analyse high Subsurface heat conduction E S alpine rock fall detachment zones with focus on their thermal condition and 3- B Subsurface temperatures dimensional situation. Surface temperatures will be determined for all events Q dg collected and a detailed 3D-study is conducted for a smaller number of selected Modelling details: events. First results of the 3D-studies are presented here. Surface temperatures are modelled based on a DEM using an energy balance model (TEBAL, Gruber 2005) driven by hourly climate time series. Climate series 1990-1999 are taken from the high elevation sites, Corvatsch (Upper Engadine, 3330 m a.s.l., for central Alpine, radiation dominated locations) and For the temperature simulation we use a surface energy-balance model together Jungfraujoch (Bernese Alps, 3500 m a.s.l., for other locations; Data source: Meteoswiss). with a 3-dimensional ground heat-conduction scheme (see on the right for more The 3-dimensional subsurface heat conduction is calculated with the software COMSOL Multiphysics. Figure 5. Modelled mean annual surface temperatures for the Drus 1990-1999 Figure 9: Modelled mean annual surface temperatures for the Jungfraujoch area details). Modelled surface temperatures are imposed as upper boundary condition, as lower boundary condition (A). Slices taken along and perpendicular to the ridge in the area of the starting 1990-1999 (A). Slices taken along and perpendicular to the ridge in the area of the a heat flux of 0.08 W/m2 is set. In this first step, below ground temperatures are simulated stationary con- zones (B, red arrow) show the 3D pattern of the subsurface temperatures, that starting zones (B, red arrow) show the 3D pattern of the subsurface temperatures. sidering only conduction and homogeneous subsurface conditions. is strongly influenced by the warmer South-East face. First conclusions I – Punta Thurwieser III – Matterhorn V – Dents Blanches All starting zones presented are located in permafrost condi- The Thurwieser rock avalanche occurred on September 18, 2004 On July 15, 2003 around 1000 m3 detached from the Matterhorn East Following a rock avalanche at Dent du Midi on tions and most of them are near the permafrost boundary or in the Zebru Valley, Italy. An estimated volume of 2.5 Mio m3 rock Ridge at an elevation of approx. 3500 m a.s.l. During the extraordinary October 29, 2006 in the same region, more than warm permafrost. and ice detached at an elevation of ca. 3600 m a.s.l. hot summer 2003, similar events occured near the South-West (3800 m 1 Mio. m3 detached from Les Barmes/Dents a.s.l.) and North-West ridges (3650 m a.s.l.). In at least two starting Blanches on November 8 near Champéry, Va- As many starting zones are located in ridge situation it is im- Surface temperatures are modelled based on climate time series zones massive ice was visible after the event confirming the relation of lais, CH. portant to analyse the 3-dimensional temperature distribu- 1990-1999 from Corvatsch and a 20 m resolution DTM (extended instability and thawing permafrost. tion. When only looking at the surface, the occurrence of per- by SRTM data). Surface temperatures are modelled based on mafrost in the subsurface may not be detected (e.g. Punta Surface temperatures are modelled based on climate time series 1990- climate time series 1990-1999 from Corvatsch Surface temperatures in the area of the starting zone are mainly Thurwieser) as it is induced by the colder mountain side op- 1999 from Jungfraujoch and DHM25 Level2 (Source: Swisstopo). and DHM25 Level2 (Source: Swisstopo). positive. However, only few decametres below the surface perma- posite. Likewise permafrost temperatures can be warmer frost occurrs that is induced by the much colder northern side of Surface temperatures in the area of the starting zone are in the area of In the area of the starting zone temperatures are Figure 10: The Dents Blanches on the map of potential per- than expected due to a heat flux from a warmer mountain mafrost distribution (published by the Federal Office for the the peak. The warming of the surface of ca. 1 °C during the past -1 to 1 °C and indicate warm permafrost conditions. The other starting just around the melting point and shallow perma- Environment (FOEN), 2006). Yellow/purple shaded areas in- side (e.g. Drus). century has not yet penetrated to greater depth (Figure 3) and zones on Matterhorn from 2003 are also located in permafrost. frost is indicated. dicate warmer/ colder permafrost. The area of the starting zone is marked by the red arrow. The recent warming of surface temperatures has not yet pen- modelled surface temperatures tend to be too warm. Considering Figure 6. Starting zone of the 2003 event at the Matterhorn South-East Ridge etrated to greater depth. Here, temperatures are modelled this, permafrost is likely present just below the surface. (Hörnligrat). Photo: B. Jelk. stationary without a corresponding model spin-up. The sub- surface temperatures her therefore tend to be too warm. In a -10-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 temperature [°C] future step the model runs will be done time-dependent. -10-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 temperature [°C] W W W S N S Figure 1. Starting zone of the 2004 event N S Permafrost degradation cannot be excluded as an important N at Punta Thurwieser during (below) and A E E E B A factor contributing to the destabilisation of the rock. However, after the event (above). Upper photo: Fos- B sati; lower photo: J. Rozman. other factors such as the geological and hydrological condi- B tions should be considered in conjunction. B Figure 3. Modelled subsurface tempera- tures for crosssection of an idealised ridge that corresponds to the elevation and ex- position of Punta Thurwieser. Above: Temperatures simulated for steady state Acknowledgements conditions. Below: The recent warming of the surface during the past century is Figure 7. Modelled mean annual surface temperatures for the Matterhorn, Figure 11. Modelled mean annual surface temperatures for the This study is funded by the Swiss National Science Foundation: 1990-1999 (A). Slices taken along and perpendicular to the ridge in the area Dents Blanches 1990-1999 (A). Slices taken along and perpendicu- Figure 2. Modelled mean temperatures 1990-1999 taken into account: Transient simulation Project numbers 200021-10796/1 and 200021-111967/1 of the starting zones (B, red arrow) show the 3D pattern of the subsurface lar to the ridge in the area of the starting zones (B, black arrow) shown in slices taken in the area of the starting with a model spin up of 100 y and +1 °C Technical support for the DTM import into COMSOL Multiphysics by FEMLAB AG, Swit- temperatures. The starting zone is located near the permafrost boundary. show the 3D pattern of the subsurface thermal field. zones. during this time. zerland is acknowledged.
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