Temporal Variations in Rockfall and Rock-Wall Retreat Rates in a Deglaciated Valley Over the Past 11 K.Y
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https://doi.org/10.1130/G47092.1 Manuscript received 10 October 2019 Revised manuscript received 7 January 2020 Manuscript accepted 19 February 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 27 March 2020 Temporal variations in rockfall and rock-wall retreat rates in a deglaciated valley over the past 11 k.y. Solmaz Mohadjer, Todd A. Ehlers, Matthias Nettesheim, Marco B. Ott, Christoph Glotzbach and Reinhard Drews Department of Geosciences, University of Tübingen, 72074 Tübingen, Germany ABSTRACT uniform lithology (e.g., Guzzetti et al., 2003). This study addresses the temporal variations in rockfall activity in the 5.2 km2 calcareous Many studies compare rock-wall retreat rates for cliffs of the deglaciated Lauterbrunnen Valley, Switzerland. We did this using 19 campaigns localities in different environments (Siewert et al., of repeated terrestrial laser scans (TLS) over 5.2 yr, power-law predicted behavior from 2012; Curry and Morris, 2004; Hinchliffe and extrapolation of the TLS-derived frequency-magnitude relationship, and estimates of long- Ballantyne, 1999). This comparison may identify time-scale (∼11 k.y.) activity based on the volume of preserved postglacial rockfall talus. factors for differing rates, but it provides limited Results from the short-time-scale observations indicate no statistically significant difference information on the long-term behavior of a given between TLS observations averaging over 1.5 versus 5.2 yr. Rock-wall retreat rates in both rock mass in one location. This study comple- cases are 0.03–0.08 mm/yr. In contrast, the power-law predicted rock-wall retreat rates are ments previous work by calculating rock-wall 0.14–0.22 mm/yr, and long-term rates from talus volumes are 0.27–0.38 mm/yr. These results retreat rates over time scales from years to tens suggest (1) short (1.5 yr) TLS inventories of rockfalls provide (within uncertainties) similar of thousands of years for the 5.2 km2 limestone frequency-magnitude relationships as longer (5.2 yr) inventories, thereby suggesting short rock walls of a deglaciated valley. observation periods may be sufficient for hazard characterization from TLS, and (2) higher rock-wall retreat rates over long time scales (Holocene averaged) may reflect debuttressing STUDY AREA and stress relaxation effects after glacial retreat, and/or enhanced rockfall activity under The Lauterbrunnen Valley is a deglaciated periglacial (climatic) conditions. valley with near-vertical walls consisting of Hel- vetic limestone (Fig. 1; Fig. DR4 in the GSA INTRODUCTION to centennial time scales (Dussauge et al., 2003; Data Repository1). Data on historic rockfalls Rockfalls are efficient agents of erosion, con- Rosser et al., 2005; Barlow et al., 2012; Santana show large slope failures since 1750 CE, includ- trolling the development of rock slopes, and they et al., 2012). In contrast, indirect measurements ing the 1889 landslide that released >104 m3 of can pose a threat to settlements and infrastruc- based on volumetric calculation of talus deposits debris (Michel, 1979). Using TLS data, Strun- ture. Rockfalls occur frequently in deglaciated have been used to estimate rock-wall retreat rates den et al. (2015) detected 122 rockfalls in the alpine valleys where rock walls are oversteep- over millennial time scales (Curry and Morris, valley over an 18 month period. These events ened, exposed, and more susceptible to failure 2004; Sass and Krautblatter, 2007; Siewert et al., ranged in volume from 0.06 m3 to 119.34 m3, after glacial retreat. Rock-wall retreat rates under 2012). Alpine rock-wall retreat rates vary be- with those less than 1 m3 being most common present-day conditions and their temporal change tween both methods. Present-day retreat rates and associated with freeze-thaw cycles. Using since deglaciation remain less understood. Here, for alpine environments range from 0.01 mm/yr seismic signals, Dietze et al. (2017) detected 49 we investigated rock-wall retreat rates over dif- to 0.84 mm/yr, while the Holocene retreat rates rockfalls over a 6 month period, 10% of which ferent time scales (∼5 yr to ∼11 k.y.) in the de- range from 0.2 mm/yr to 2.5 mm/yr (Curry and were influenced by freeze-thaw cycles. They glaciated Lauterbrunnen Valley of the Bernese Morris, 2004, and references within). Various inferred a systematic lowering of a rock mass Alps, Switzerland (Fig. 1). An improved under- factors contribute to this discrepancy, including release zone driven by a lowering of the water standing of these rates is motivated by the need joint spacing and orientation, and rockfall trig- table in the rock wall. Other potential triggers, to understand postglacial erosion and the role of gering processes such as paraglacial unloading such as earthquakes and anthropogenic activity rockfalls in the evolution of alpine landscapes. after deglaciation (Hinchcliffe and Ballantyne, (as shown in Mackey and Quigley, 2014), are Rockfall data sets derived from direct mea- 1999; Arsenault and Meigs, 2005) and peri- unlikely to influence rockfalls in this study area. surements (e.g., terrestrial laser scans [TLS] glacial conditions (e.g., enhanced freeze-thaw and photogrammetry) cover time scales from activity and permafrost degradation). METHODS hours to years and are often used in model- Although previous studies have reported rock- TLS Data Collection and Processing ing rock-wall retreat rates based on rockfall wall retreat rates, few have provided a complete A long-range terrestrial light detection frequency-magnitude distributions over decadal and continuous coverage of large rock walls with and ranging scanner was used to acquire 1GSA Data Repository item 2020174, additional details on methods and results, including rock-wall retreat rate calculations and return period for large rockfall events, Figures DR1–DR4, and Tables DR1–DR3, is available online at http://www.geosociety.org/datarepository/2020/, or on request from [email protected]. CITATION: Mohadjer, S., et al., 2020, Temporal variations in rockfall and rock-wall retreat rates in a deglaciated valley over the past 11 k.y.: Geology, v. 48, p. 594–598, https://doi.org/10.1130/G47092.1 594 www.gsapubs.org | Volume 48 | Number 6 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/6/594/5051223/594.pdf by guest on 01 October 2021 3 A dividing the total rockfall volume (1610.87 m ) 7°55’ E WT1 from the west wall in the 5.2 yr period by its area (3.7 km2). Similarly, the retreat rate for the east Figure 1. Shaded relief 3 N A wall was obtained using 229.65 m of rockfall vol- ’ digital elevation model 2 of Lauterbrunnen Valley, ume and a wall area of 1.5 km . The short-term N Switzerland (digital ele- wall retreat rates for the west (0.08 mm/yr) and E vation model courtesy east walls (0.03 mm/yr) agree with those obtained 46°35 B B WT2 of Swisstopo, https:// over an 18 month period by Strunden et al. (2015). www.swisstopo.admin. N ch/). Black circle marks west location of Lauterbrun- Power-Law Predicted Wall Retreat Rates wall C WT3 nen Village, and the The empirical log-frequency and log-mag- east wall river Weisse Lütschine nitude distributions for the 5.2 yr rockfall data is shown in blue. Boxes set show a rollover and power-law tail (Fig. 3). C represent location of investigated talus cones. A nonlinear least-squares regression and MLE N 0 Talus surfaces are shown were used to determine the frequency-magni- D WT4 in yellow, with boundaries tude relationship. The inferred rollover volumes, marked with dashed lines. based on optimization of the R2 value and the D Red dashed line marks the scarp of the 1889 Kolmogorov-Smirnov statistic, differed for N CE mass-wasting event. the two methods (Fig. DR1). We observed de- 46°N Switzerland E ET1 WT1–4 and ET1 are talus viations from pure power-law behavior and a names. systematic trend in fit parameters for rollover 3 0 0.5 km volumes of 0.29–0.92 m . For these values, the 0 100 MLE of the scaling exponent b ranged from 0.61 N m to 0.72. The power-law fit parameters agree with previous values obtained for a shorter observa- three-dimensional (3-D) point clouds from 22 rates over the last 11 k.y. were inferred from tion duration (1.5 yr; Table DR2, Fig. DR2C). scan positions during each campaign. Nineteen the talus volumes using TLS and a digital el- However, the frequency of rockfalls in the 5.2 yr (19) campaigns were conducted over 5.2 yr (Feb- evation model on a 2 m grid (swissALTI3D; data set is lower compared to the 1.5 yr data set ruary 2012 to April 2017). Scans collected from Fig. DR3 and Section DR2). To compensate (Fig. 3). This is particularly evident for rockfalls similar positions at different times were aligned for the density difference between talus bulk smaller than 2 m3. using an iterative closest point algorithm. To pro- and intact bedrock, we used a density correction Using the above parameters, the pow- duce a continuous surface, triangle meshes of factor of 0.77 for limestone (from Krautblat- er-law predicted eroded volume per year is reference scans were computed. Surface mesh- ter et al., 2012; Sass and Wollny, 2001). The (0.70–1.13) × 103 m3, and wall retreat rates es were compared with point clouds to identify density-corrected volume was divided by the are 0.14–0.22 mm/yr. Using observations from rockfalls between successive campaigns. Rock- rock-wall surface area above each talus fan and Strunden et al.