3-dimensional analysis of the thermal conditions in recent periglaci by awt10412

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									                                                                                                                                                                                                                                  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: jnoetzli@geo.unizh.ch                                                                                                                                                                                                                                                                                                                                                                                                                   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.
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     steep bedrock in alpine areas. Instabilities are expected to originate preferen-                     (Sub)surface information                       S in/out
                                                                                                                                                                                                                                       A




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     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-
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                                                                                                                                                                                                                                                                                                                        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

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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        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
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     analysis of the thermal conditions of recent periglacial detachment zones.                                                                                                                                                                                                                                          W                                                                                                                                                  B
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                                                                                                          Subsurface information                                                                                                                                                                                                                                                                                                                                                                                       N

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     Based on an inventory of more than 50 recent rock fall events, we analyse high                       Subsurface heat conduction                                                                                                                                                                                                                                                                                                                                                                                         E

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     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
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     future step the model runs will be done time-dependent.
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                                                                                                                                                                                  Figure 1. Starting zone of the 2004 event
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     Permafrost degradation cannot be excluded as an important
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                                                                                                                                                                                  at Punta Thurwieser during (below) and
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     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.
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                                                                                                                                                                                   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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