the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Multisensor monitoring and data integration reveal cyclical destabilization of Äußeres Hochebenkar Rock Glacier
- 1Institute for Interdisciplinary Mountain Research, Austrian Academy of Sciences, Innrain 25, 3. OG 6020 Innsbruck, Austria
- 2Department of Geography, University of Innsbruck, Innrain 52f, 6020 Innsbruck, Austria
- 3Institute of Geography, Heidelberg University, Im Neuenheimer Feld 368, 69120 Heidelberg, Germany
- 4Interdisciplinary Center for Scientific Computing (IWR), Heidelberg University, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
- 5Department of Geography, University of Zurich, Winterthurerstr. 190, 8057 Zurich, Switzerland
- 6Alaska Climate Research Center, Geophysical Institute, University of Alaska Fairbanks, 2156 Koyukuk Dr, Fairbanks, AK 99775, United States
- 1Institute for Interdisciplinary Mountain Research, Austrian Academy of Sciences, Innrain 25, 3. OG 6020 Innsbruck, Austria
- 2Department of Geography, University of Innsbruck, Innrain 52f, 6020 Innsbruck, Austria
- 3Institute of Geography, Heidelberg University, Im Neuenheimer Feld 368, 69120 Heidelberg, Germany
- 4Interdisciplinary Center for Scientific Computing (IWR), Heidelberg University, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
- 5Department of Geography, University of Zurich, Winterthurerstr. 190, 8057 Zurich, Switzerland
- 6Alaska Climate Research Center, Geophysical Institute, University of Alaska Fairbanks, 2156 Koyukuk Dr, Fairbanks, AK 99775, United States
Abstract. This study investigates rock glacier destabilization based on the results of a unique in situ and remote sensing-based monitoring network focused on the kinematics of the rock glacier in Äußeres Hochebenkar (Austrian Alps). We consolidate, homogenize, and extend existing time series to generate a comprehensive dataset consisting of 14 digital surface models covering a 68 year time period, as well as in situ measurements of block displacement since the early 1950s. The digital surface models are derived from historical aerial imagery and, more recently, airborne and uncrewed aerial vehicle-based laser scanning (ALS, ULS). Since 2017, high-resolution 3D ALS and ULS point clouds are available at annual temporal resolution. Additional terrestrial laser scanning data collected in bi-weekly intervals during the summer of 2019 is available from the rock glacier front. Using image correlation techniques, we derive velocity vectors from the digital surface models, thereby adding rock glacier-wide spatial context to the point scale block displacement measurements. Based on velocities, surface elevation change, analysis of morphological features, and computations of the bulk creep factor and strain rates, we assess the combined datasets in terms of rock glacier destabilization. To additionally investigate potential rotational components of the movement of the destabilized section of the rock glacier, we integrate in situ data of block displacement with ULS point clouds and compute changes in the rotation angles of single blocks during recent years. The time series shows two cycles of destabilization in the lower section of the rock glacier. The first lasted from the early 1950s until the mid 1970s. The second began around 2017 after approximately two decades of more gradual acceleration and is currently ongoing. Both destabilization periods are characterized by high velocities and the development of morphological destabilization features on the rock glacier surface. Acceleration in the most recent years has been very pronounced, with velocities reaching 20–30 m/a in 2020/21. These values are unprecedented in the time series and suggest highly destabilized conditions in the lower section of the rock glacier, which shows signs of translational as well as rotational, landslide-like movement. Due to the length and granularity of the time series, the cyclic destabilization process at Äußeres Hochebenkar rock glacier is well resolved in the dataset. Our study highlights the importance of interdisciplinary, long-term and continuous, high-resolution 3D monitoring to improve process understanding and model development related to rock glacier rheology and destabilization.
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Lea Hartl et al.
Status: open (until 06 Nov 2022)
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RC1: 'Comment on esurf-2022-48', Wilfried Haeberli, 29 Sep 2022
reply
Comments by Wilfried Haeberli
on
Multisensor monitoring and data integration reveal cyclical destabilization of Äußeres Hochebenkar Rock Glacier
Paper submitted to Earth Surface Dynamics by
- Hartl, T. Zieher, M. Bremer, M. Stocker-Waldhuber, V. Zahs, B. Höfle, C. Klug and A. Cicoira
General
The spectacular active rock glacier in the Äusseres Hochebenkar of the Ötztal Alps, Austria, is among the best documented viscous creep features in warm and warming mountain permafrost. The submitted paper compiles, homogenizes and interprets geodetic/photogrammetric observations of extraordinary length (68 years) complemented by modern airborn-uncrewed laser scanning with extreme spatio-temporal resolution. Emphasis is on technical innovation and on the analysis of flow acceleration to destabilization. The unique and extraordinarily rich data set including such novel aspects as, for instance, subseasonal effects and phenomena of block rotation enables detailed insights concerning the landslide-type evolution of the steep frontal part with an earlier, weaker destabilization between the 1950s and 1970s, followed by more stable conditions until the most recent, ongoing and extreme acceleration and destabilization. The text is well written and illustrated, follows a clear/logical structure and discusses the applied techniques and observed phenomena at the forefront of the rapidly growing research field and scientific literature. The paper can essentially be recommended for publication in its present form. The authors may, nevertheless, wish to consider the following reflections.
More could be said about local permafrost conditions as the decisive environmental aspect related to the analyzed creep phenomena. The high-resolution, Alpine-wide permafrost map by Boeckli et al. (2012) should at least be mentioned. A superposition using the publicly available kmz file of permafrost occurrence onto a Google-Earth image would be instructive (Figure 1 in Haeberli 2013 is an example from the Oetztal region). The excellent meteo-data in the region can be used to calculate mean annual air temperatures at the investigated site. This would immediately make clear that permafrost is quite warm despite the cooling effects of the ventilated surface layer consisting of coarse blocks. Mention ongoing permafrost warming trends as documented by borehole measurements (Etzelmüller et al. 2020) and related paleo-effects at depth (thermal anomaly down to > 50m). The perennially frozen, ice-rich condition of the creeping mass is documented by high but variable P-wave velocities and by electrical D.C. resistivities in the medium to high kOhmm range.
The geometry and kinematics should strictly relate to the moving mass of perennially frozen talus rather than to the landform “rock glacier” (as mental constructs and conventions, landforms can – strictly speaking – not flow). It is important to make clear that the thickness of the landform rock glacier (sediment above bedrock?) can differ from the depth of the thermally defined permafrost, and that the thickness of the moving mass may depend on the occurrence of shear horizons rather than on landform thickness or permafrost depth. While the term “acceleration” is self-explaining, the term “destabilization” is more complex and needs a precise definition. Critical strain rates for crevasse formation as an indication of tensile strengths had already been discussed at Gruben rock glacier (Haeberli et al 1979) and may relate to the onset of rapid local sliding. Interesting destabilization phenomena have also been documented in creeping subarctic permafrost (Daanen et al. 2012).
Minor remarks
Line 25: In connection with the fully justified statement that “… rock glaciers are … generated by … creep of frozen ground …”, the reference to the PhD thesis of Whalley (1974) is astonishing. Whalley had postulated that rock glaciers are debris-covered glaciers and that permafrost is unlikely to occur in the Alps. Still today and against all measured evidence, this author keeps fighting against what he calls “the permafrost model” of rock glacier formation (see the recent discussion in The Cryosphere). The authors should either mention this fact or skip this reference. The latter may be more adequate as long-outdated beliefs from intuitive landform interpretation are at best of historical interest and hardly relate to the excellent quantitative material presented in the submitted paper.
Line 66: The paper by Daanen et al. about permafrost destabilization in the Brooks Range could be cited here.
Line 100: Zahs et al. (2019) report resistivities (medium to high kOhmm range) at the rock glacier margin which indicate ice-rich frozen ground with variable ice content, not just “isolated ice-lenses”).
Table 1: Take care of “Umlaute” and be consistent: Ladstädter/Ladstaedter. Check throughout the paper and the reference list.
Figure 1: The top-left graph could include the regional permafrost occurrence after Boeckli et al. (2012). The longitudinal profile and the central flowline in the lower-rigth graph cannot easily be discriminated and are not identical – flow directions in the root zone deviate from the given black line.
Bulk creep factor (BCF): This is a useful but rather abstract concept. At an individual location, where surface slope remains nearly constant and changes in flow depth cannot realistically be accounted for, changes in BCF directly reflect changes in surface velocities. The authors rightly emphasize that changes in space and time of both, BCF and strain rates combined, may most significantly indicate the onset of rapid sliding-type movements.
Line 279: Be more precise concerning depth values for landforms, permafrost and flowing mass as explained above.
Line 296: Perhaps better “ the frozen mass of the rock glacier entered …”
Line 381: what does “often by a substantial margin” mean?
Figure 8: The important information is hard to deciffer – enlarge.
Lines 400-406: Be consistent with the times (present – past) used in writing.
Line 415: These values may be compared with the “critical strain rates for crevasse formation” discussed for Gruben rock glacier by Haeberli et al.(1979).
Line 568: Perhaps better “rheology of perennially frozen materials in rock glaciers”
Line 598: Perhaps better “permafrost creep” or “premafrost creep in rock glaciers”
Line 821: Check “Umlaute”: Blockströmen, Ötztaler
I congratulate the authors for their outstanding work and hope their important contribution to be published soon.
References:
Boeckli, L., Brenning, A., Gruber, S. Noetzli, J., 2012. Permafrost distribution in the European Alps: calculation and evaluation of an index map and summary statistics. The Cryosph., 6, 807-820. doi:10.5194/tc-6-807-2012
Daanen, R.P., Grosse, G., Darrow, M.M., Hamilton, T.D., Jones, B.M., 2012. Rapid movement of frozen debris-lobes: implications for permafrost degradation and slope instability in the south-central Brooks Range, Alaska. Nat. Haz. Earth Syst. Sc. 12, 1521-1537. doi:10.5194/nhess-12-1521-2012
Etzelmüller, B., Guglielmin, M., Hauck, C., Hilbich, C., Hoelzle, M., Isaksen, K., Noetzli, J., Oliva, M. and Ramos, M. (2020): Twenty years of European mountain permafrost dynamics – the PACE legacy. Environmental Research Letters 15, 104070. doi.org/10.1088/1748-9326/abae9d
Haeberli, W. (2013): Mountain permafrost — research frontiers and a special long-term challenge. Cold Regions Science and Technology 96, 71-76. http://dx.doi.org/10.1016/j.coldregions.2013.02.004
Haeberli, W., King, L. and Flotron, A. (1979): Surface movement and lichen cover studies at the active rock glacier near the Grubengletscher, Wallis, Swiss Alps. Arctic and Alpine Research, 11/4, 421-441.
Lea Hartl et al.
Data sets
Flow velocity records at Rock Glacier Outer Hochebenkar (Äußeres Hochebenkar), Ötztal, Tyrolian Alps, Austria, 1997 et seq. Stocker-Waldhuber, Martin; Fischer, Andrea; Hartl, Lea; Abermann, Jakob; Schneider, Heralt https://doi.pangaea.de/10.1594/PANGAEA.928244
Correspondence-driven plane-based M3C2 for quantification of 3D topographic change with lower uncertainty [Data and Source Code] Zahs, Vivien; Winiwarter, Lukas; Anders, Katharina; Williams, Jack G.; Rutzinger, Martin; Bremer, Magnus; Höfle, Bernhard https://doi.org/10.11588/data/TGSVUI
Lea Hartl et al.
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