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| Titel |
Regional-scale controls of periglacial rockfalls (Turtmann valley, Swiss Alps) |
| VerfasserIn |
Karoline Meßenzehl, Thomas Hoffmann, Hanna Meyer, Richard Dikau |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101278
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| Publikation (Nr.) |
 EGU/EGU2015-391.pdf |
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| Zusammenfassung |
| Rockfalls are among the most hazardous processes in mountain regions and of major
importance for landform evolution and sediment budgets. The rockfall activity varies
significantly in space and time, driven by the complex interplay between locally
dynamic variables (i.e.Âdiscontinuities, freeze-thaw processes) as well as system
inherent predisposing factors dominating at a regional scale. Many studies focus on
small-scale triggering conditions for rockfalls, but the effects of regional-scale controls
leading to a basic instability of alpine rockwalls, such as topo-climatic settings,
lithology and i.e.Âtectonic structures as well as paraglacial adjustments, are poorly
understood.
In this study, we aim to understand the role of regional-scale controls of rockfalls in the
Turtmann Valley, which covers 110 km2 in the Swiss Alps. Based on an inventory of 220
talus slopes (Otto et al. 2009), rockfall source areas were determined and their causal
relationship between ten different prediction variables was assessed. By combining two
multivariate statistical models, we (i) explained the spatial pattern of rockfalls, (ii) evaluated
the relative importance of potential predisposing factors (iii) and discuss these regional-scale
controls in the light of the local-scale geomorphic and rock mechanical settings. Major
finding are:
(i) A stepwise logistic regression (LR) based on principal components and a random forests
(RF) model were performed and validated using a 75%-subset of the rockfall source areas.
Given the area under the ROC curves for both approaches, LR: 0.92, RF: 0.99, respectively,
the RF model performs slightly better to explain the large-scale variability of rockfalls in our
study area.
(ii) Both, the LR and RF model reveal that lithology and joint orientation have the strongest
causal influence on rockfalls at regional scale. In contrast, topo-climatic factors (elevation,
slope, solar radiation) might be of secondary importance. Additionally, the regional pattern of
rockfalls seems to be linked to the paraglacial adjustment time since LGM, conceptualised by
the ergodic principle (space-time-substitution).
(iii) The modelling results are in good agreement with local geomorphic and rock mechanical
surveys performed on different rockwalls. Rockfall events dominate locally on steep,
comparatively recently deglaciated, anaclinal slopes consisting of amphibolite rocks with
high rock mass strength (RMS: 50-70). Nevertheless, it must be supposed that the
regional-scale controls differ in their relative importance from small-scale factors, due to
emergent system behaviour and the increasing system complexity with increasing spatial
scale.
In contrast to many other large-scale statistical studies underestimating the importance of
rock mass properties, our results indicate that the rock mechanical parameters in combination
with the paraglacial response process of alpine rockwalls have to be considered as key
predisposing factors for rockfall activity at regional scales.
Otto, J. C., L. Schrott, M. Jaboyedoff, and R. Dikau, 2009, Quantifying sediment storage in a
high alpine valley (Turtmanntal, Switzerland), Earth Surface Processes and Landforms,
34(13), 1726-1742. |
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