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| Titel |
Is rock slope instability in high-mountain systems driven by topo-climatic, paraglacial or rock mechanical factors? - A question of scale! |
| VerfasserIn |
Karoline Meßenzehl, Richard Dikau |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250124780
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| Publikation (Nr.) |
 EGU/EGU2016-4267.pdf |
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| Zusammenfassung |
| Due to the emergent and (often non-linear) complex nature of mountain systems the
key small-scale system properties responsible for rock slope instability contrast to
those being dominant at larger spatial scales. This geomorphic system behaviour
has major epistemological consequences for the study of rockfalls and associated
form-process-relationships. As each scale requires its own scientific explanation, we cannot
simply upscale bedrock-scale findings and, in turn, we cannot downscale the valley-scale
knowledge to smaller phenomena.
Here, we present a multi-scale study from the Turtmann Valley (Swiss Alps), that
addresses rock slope properties at three different geomorphic levels: (i) regional valley scale,
(ii) the hillslope scale and (iii) the bedrock scale. Using this hierarchical approach, we
aim to understand the key properties of high-mountain systems responsible for
rockfall initiation with respect to the resulting form-process-relationship at each
scale.
(i) At the valley scale (110 km2) rock slope instability was evaluated using a GIS-based
modelling approach. Topo-climatic parameters, i.e. the permafrost distribution and the time
since deglaciation after LGM were found to be the key variables causative for the
regional-scale bedrock erosion and the storage of 62.3 - 65.3 x 106 m3 rockfall sediments in
the hanging valleys (Messenzehl et al. 2015).
(ii) At the hillslope scale (0.03 km2) geotechnical scanline surveys of 16 rock slopes and
one-year rock temperature data of 25 ibuttons reveal that the local rockfall activity and the
resulting deposition of individual talus slope landforms is mainly controlled by the specific
rock mass strength with respect to the slope aspect, than being a paraglacial reaction.
Permafrost might be only of secondary importance for the present-day rock mechanical state
as geophysical surveys disprove the existence of frozen bedrock below 2600 m asl.
(Messenzehl & Draebing 2015).
(iii) At the bedrock scale (0.01 mm - 10 m) the spacing, persistence and orientation of
joints turned out to be the most causative bedrock properties for the higher-scale
rock mass strength. Rock temperature data suggest that high-frequent, surficial
thermal processes, daily freeze-thaw cycles and seasonal ice segregation coupled with
a winter snow cover are the major rock breakdown mechanisms. By linking the
rockwalls’ joint geometric pattern to the size and shape of rockfall blocks lying on the
corresponding talus slopes, different rockfall magnitudes and frequencies were
identified.
Here we show, that the decrease in spatial scale is linked with a shift in variable
importance, from topo-climatic and paraglacial factors at the largest scale to rock mechanical
parameters at the smallest scale. Therefore, to understand the key destabilising factors of
rock slopes in mountain systems and the resulting landforms, a holistic research
approach is needed which considers the nested, hierarchical structure of geomorphic
systems.
Messenzehl, K., Meyer, H., Otto, J.-C., Hoffmann, T., Dikau, R., 2015. Regional-scale
controls on the spatial activity of rockfalls. (Turtmann valley, Swiss Alps) - A multivariate
modelling approach. In: Geomorphology.
Messenzehl, K., Draebing, D., 2015. Multidisciplinary investigations on coupled rockwall
talus-systems (Turtmann valley, Swiss Alps). Geophysical Research Abstracts, 17
(EGU2015-1935, 2015). |
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