Ground thermal and geomechanical conditions in a permafrost-affected high-latitude rock avalanche site (Polvartinden, northern Norway)

Frauenfelder, Regula; Isaksen, Ketil; Lato, Matthew J.; Noetzli, Jeannette

doi:https://doi.org/10.5194/tc-12-1531-2018

Articles | Volume 12, issue 4

https://doi.org/10.5194/tc-12-1531-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

The evolution of permafrost in mountain regions

https://doi.org/10.5194/tc-12-1531-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 4

Research article

|

27 Apr 2018

Research article |

| 27 Apr 2018

Ground thermal and geomechanical conditions in a permafrost-affected high-latitude rock avalanche site (Polvartinden, northern Norway)

Regula Frauenfelder, Ketil Isaksen, Matthew J. Lato, and Jeannette Noetzli

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Final revised paper (published on 27 Apr 2018)
Preprint (discussion started on 15 Nov 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Reviewer Comments to „Ground thermal and geomechanical conditions in a permafrost affected high-latitude rockslide site (Polvartinden, Northern Norway)” by Frauenfelder et al.', Anonymous Referee #1, 29 Nov 2016
- AC1: 'Response to referee #1', Regula Frauenfelder, 29 Jun 2017
RC2: 'Review: Frauenfelder et al «Ground thermal and ….»', Anonymous Referee #2, 15 Feb 2017
- AC2: 'Response to referee #2', Regula Frauenfelder, 29 Jun 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Regula Frauenfelder on behalf of the Authors (29 Jun 2017) Manuscript

ED: Referee Nomination & Report Request started (04 Jul 2017) by Christian Hauck

RR by Anonymous Referee #3 (01 Sep 2017)

Suggestions for revision or reasons for rejection

Comments to the paper:
Ground thermal and geomechanical conditions in a permafrost-affected high-latitude rock avalanche site (Polvartinden, northern Norway)

by Regula Frauenfelder, Ketil Isaksen, Jeannette Noetzli, Matthew J. Lato

This manuscript describes a rock fall event happened in 2008 at Polvartinden, Northern Norway. The authors estimate the volume of the rock fall and they investigate possible permafrost conditions at the detachment site of the rock avalanche. They use methods like ground based laser scanning to establish a digital terrain model and ground temperature measurements to determine the ground thermal regime. The measurements are also used to drive different models such CryoGrid2 (by Westermann 2013) and a 3D-heat conduction model (by Noetzli 2007).

General comments:
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This study is elaborated very carefully using a minimum amount of measurements. Although having not a lot of direct measurements available, I believe that the authors got most out of the data using them in a very smart combination with other existing data (like meteorological information of metno) and numerical models (like CryoGrid2).

In general, the paper is written very concise and clear. I have only some minor points, where I suggest some changes in the paper. Most of them are directly related to the section below ‘specific comments’.
I suggest to introduce a short chapter ‘Site Description’, where the authors should collect all information from the Introduction and Method section related to the site description. This would make the structure of the paper more clear.

Specific comments:
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Abstract:

Page 1, Line 24: change this by these

Introduction:

Page 2, Line 4: it would be interesting to now at which depth the ice vertical to the surface BEFORE the rock fall was situated. This should be easy knowing the surface before and after the rock fall. It can tell us may be also something about the heat and water fluxes. Was the ice visible in Figure 2 not dated? If yes, this information would be of outstanding interest.

Page 3, Line 3: what means rock avalanche deformation … Do you mean here rock avalanche formation?

Page 3, Line 18: I was wondering if a 10 m active layer corresponds with the observation of the ice in the rock avalanche detachment zone. Therefore, it is very interesting to know the rock depth above the ice at the detachment zone before the rock avalanche happened.

Page 3, Lines 23-26: I would rewrite the sentences accordingly: For the same mountain Myhra et al. (2015) modelled, a bedrock warming at 20 and 50 m depth of about 1.0 and 0.5°C, respectively, from the end of the LIA and using a present lower limit of permafrost of about 650 m asl. A new Nordic permafrost map based on modelled results confirm ongoing degradation of mountain permafrost in the fjord areas in Troms (Gisnås et al., 2016).

Page 3, Line 29: … subsequent modelling of the temperature regime …

Page 3, Lines 32-33: You certainly discuss the influence of solar radiation very good in the discussion section. However, as you state it here in the Introduction as a main goal of the paper, you definitively do not contribute a lot with measurements or modelling to better understanding of solar radiation. Therefore, I would skip this sentence. Although, I do full agree with your comments about this topic in the Discussion chapter.

Between Introduction and Methods chapter, please insert a chapter Site Description.

Methods:

Page 4, Line 5: Insert: … subsequent ground temperature modelling …

Discussion:

Page 13, Line 11: Please mention also study of Heggem et al. 2005 showing the influence of of radiation on permafrost in Southern Norway from west to east. Heggem, E. S. F., Juliussen, H., and Etzelmüller, B. (2005). "Mountain permafrost in Central-Eastern Norway." Norwegian Journal of Geography 59(2): 94-108.

Figures:

Page 29, Figure 6: please change colours of the lines fitting them better thematically to the different loggers put at rock and soil sites.

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RR by Michael Krautblatter (12 Oct 2017)

Suggestions for revision or reasons for rejection

Dear Dr. Frauenfelder,

I have carefully read your paper also in comparison to the first submitted version and I have found a large number of issues that I would have commented in the earlier version now carefully revised in the present Version 4. Earlier reviews have commented a lot on the thermal permafrost content in this paper, so I focussed a little bit more on the geological and slope instability content of this paper.

I made comments on a number of issues relating to
>the introduction of the structural and geological controls of the rock slope failure
>the mechanical explanation of the failure
>the terminology
>the structural interpretation
>and the way permafrost could have played a role.
However, even if there is quite a number of these comments, they are all minor revisions in my view. I hope they will help to make some statements including the key statement of a potentially permafrost-affected rock slope failure clearer. I also added some comments on thermal conditions in steeo structured rock faces.

Including these comments, I can see that this paper is a valuable contribution to better understanding preparatory conditions of rock slope failures in permafrost-affected rock walls in a setting that is quite different from previously reported Alpine conditions.

Thus, I suggest to publish this paper subsequent to a number of minor revisions revisions listed below

Good luck with the revision
Michael Krautblatter

P2, L25: >I think you should include a statement that the evidence of permafrost-related destabilisation is not only derived (abductively) from observation, where the relative importance of permafrost degradation remains difficult to prove, but also supported by (inductive/deductive) mechanical studies that explain the underlying physics and processes of destabilisation. Without mechanical papers by Mellor, Davies, Guenzel, Dviwedi, Arenson, this statement could not be validated physically.
P3, L1: “Large-scale rock-slope failures pose a significant geohazard in the fjords and valleys of western and northern Norway (Blikra et al., 2006).”
>I think you should update and specify this comment since, especially as Norway has a very elaborated ranking of rock slope failure hazards now.

P5, L9: “Low resolution data was collected to enable the visualization and modelling of a large area of the mountainside. Higher resolution data was collected at specific zones 10 near the failure zone and its surrounding areas to enable a geomechanical interpretation.”
>Specify the resolution of LiDAR especially for the term “Higher resolution data”, since this affects the geomechanical interpretation that can be achieved.
P5, L13: In the previous version you speak about “The extremely irregular surface as well as large amounts of snow and ice present resulted in the development of a mesh with numerous larger data holes”
>can Specify the range of dimensions of these ”numerous” holes, since they can camouflage/affect the geomechanical interpretation.
P5, L19 “The volume of the 2008 rock avalanche was computed using the 2012 TLS data by delineating a pre-failure surface based on adjacent slope topography bounding the scarp. The differential volume between the 2012 TLS data and the pre-failure surface was 20 computed using standard 3-dimensional techniques, as presented, e.g., by Lato et al. (2014)..”
>It is quite common that there is not sufficiently resolved 3D DEM before the failure, but please explain from where you took your constraints to interpolate the prior surface. Looking at Figure 4 it seems that the reconstruction of a former surface not straightforward.
P6, L2 “Where possible, a vertical distance of several meters to the flat terrain was chosen, however, this was not possible at all of the sites, which has some implications on the interpretation of the results (cf. chapter 4).”
>Please specify for which sensors this applies and whether these logger have been affected by snow accretion
>The specification for rock wall loggers also relies on a minimum steepness of rock faces (often >60°) where snow does not accumulate on the rock face – have you checked the steepness?
Figure 4: I would not use the term “rock avalanche” used for highly mobilised (low fahrboschung/ energy line angles <15°, which your event does not have) high-magnitude (>1 mio. m³) events with– rock slope failure is a more appropriate term.
Figure 5: “Figure 5. Kinematic analysis of the bedding planes for the sliding failure. The green line represents the orientation of the natural slope surface before failure, the white circle represents an estimated friction cone of 30° and the green cone represents the sliding daylight window for the associated pre-failure surface.”
>This Figure needs a more elaborated explanation
>It is difficult to read it without having any kind of geomechanical background, which should be described in at least in 4-5 lines in the introduction. Type of geology, discontinuity patterns, deglaciation history…
>For the readership of the Cryosphere you should explain your geomechanical analysis in a way they have a chance to understand it
>Please be careful with attributing the failure to a "bedding plane"
> From what I can see in Figure 1, there are persistent, vertical to subvertical, bended joint sets that predefine possible failure planes. These also seem to have at least partially a fine-grained infill.
>Pure or dirty ice infill can add certain limited (2 MPa) tensile strength to the vertical to subvertical, bended joint sets in the rock mass prior to failure and affect the sliding resistance.
>As it is presented now, I cannot really understand the point you want to make with this Figure 5
>I only have Fig. 1 to judge the geology and no background information is provided in your article, but your “complex wedge” is composed of an intersection of at least three sets of discontinuities and I strongly recommend you to give the reader some more background structural information on the rock face which must have certainly been recorded by the NGI.
P7, L8 Values defining the subsurface properties (heat capacity, thermal conductivity, porosity) were obtained from representative sites nearby
>Please state that they (especially porosity) can vary significantly and have quite an influence on the thermal behaviour
P7, L2: “Our results are valid for areas that are assumed not to be influenced by a snow cover, i.e., the steep rock-faces of Polvartinden. The main source of uncertainty for the three-dimensional modelling is related to the extrapolation of the MARST”
>Structured rock faces like yours have quite a significant snow cover an this can be a major error source for your model. Please check recent papers by A. Haberkorn.
P8, L4: “The discontinuity ordination data are plotted on a stereonet in Figure 5. 20 The bedding planes have been identified as the failure surface from the terrestrial laser scanning data and from interpretation of the GigaPan photography. The green line in Figure 5 represents the orientation of the natural slope surface, and the green circle represents the corresponding daylight window. The white cone depicts an estimated friction surface of 30°. Poles that are contained outside of the white circle but within the green circle are kinematically unstable and represent potential sliding failure planes (Goodman, 1995). The stereonet demonstrates that the bedding surface orientation is steeper than the estimated 25 friction angle, but shallow enough to daylight with respect to the slope face. The bedding surface meets, therewith, the kinematic requirements of a sliding failure posing a potential rockfall hazard (see e.g., Hasler et al., 2012).”
>I only have Fig. 1 to judge the geology and no background information is provided in your article, but your “complex wedge” is composed of an intersection of at least three sets of discontinuities and I strongly recommend you to give the reader some more background structural information on the rock face which must have certainly been recorded by the NGI.

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ED: Publish subject to minor revisions (Editor review) (12 Oct 2017) by Christian Hauck

AR by Regula Frauenfelder on behalf of the Authors (08 Feb 2018) Author's response Manuscript

ED: Publish subject to technical corrections (05 Mar 2018) by Christian Hauck

AR by Regula Frauenfelder on behalf of the Authors (20 Mar 2018) Author's response Manuscript

Short summary

On 26 June 2008, a rock avalanche with a volume of ca. 500 000 m³ detached in the north-east facing slope of Polvartinden, a high-alpine peak in northern Norway. Ice was observed in the failure zone shortly after the rock avalanche, leading to the assumption that degrading permafrost might have played an important role in the detaching of the Signaldalen rock avalanche. Here, we present a four-year series of temperature measurements from the site and subsequent temperature modelling results.