Temporal variations in rockfall and rock-wall retreat rates in a deglaciated valley over the past 11 k.y.

Marco B Ott,Matthias Nettesheim,Todd A Ehlers,Solmaz Mohadjer,Christoph Glotzbach,Reinhard Drews

doi:10.1130/g47092.1

Abstract

Abstract This study addresses the temporal variations in rockfall activity in the 5.2 km2 calcareous cliffs of the deglaciated Lauterbrunnen Valley, Switzerland. We did this using 19 campaigns of repeated terrestrial laser scans (TLS) over 5.2 yr, power-law predicted behavior from extrapolation of the TLS-derived frequency-magnitude relationship, and estimates of long-time-scale (∼11 k.y.) activity based on the volume of preserved postglacial rockfall talus. Results from the short-time-scale observations indicate no statistically significant difference between TLS observations averaging over 1.5 versus 5.2 yr. Rock-wall retreat rates in both cases are 0.03–0.08 mm/yr. In contrast, the power-law predicted rock-wall retreat rates are 0.14–0.22 mm/yr, and long-term rates from talus volumes are 0.27–0.38 mm/yr. These results suggest (1) short (1.5 yr) TLS inventories of rockfalls provide (within uncertainties) similar frequency-magnitude relationships as longer (5.2 yr) inventories, thereby suggesting short 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 and stress relaxation effects after glacial retreat, and/or enhanced rockfall activity under periglacial (climatic) conditions.

Highlights

Rockfalls are efficient agents of erosion, controlling the development of rock slopes, and they can pose a threat to settlements and infrastructure
Rockfall data sets derived from direct measurements cover time scales from hours to years and are often used in modeling rock-wall retreat rates based on rockfall frequency-magnitude distributions over decadal to centennial time scales (Dussauge et al, 2003; Rosser et al, 2005; Barlow et al, 2012; Santana et al, 2012)
This study investigated temporal variations in rock-wall retreat rates in the 5.2 km2 calcareous cliffs of the Lauterbrunnen Valley, Switzerland

Summary

Introduction

Rockfalls are efficient agents of erosion, controlling the development of rock slopes, and they can pose a threat to settlements and infrastructure. Rockfalls occur frequently in deglaciated alpine valleys where rock walls are oversteepened, exposed, and more susceptible to failure after glacial retreat. Rock-wall retreat rates under present-day conditions and their temporal change since deglaciation remain less understood. We investigated rock-wall retreat rates over different time scales (∼5 yr to ∼11 k.y.) in the deglaciated Lauterbrunnen Valley of the Bernese Alps, Switzerland (Fig. 1). Indirect measurements based on volumetric calculation of talus deposits have been used to estimate rock-wall retreat rates over millennial time scales (Curry and Morris, 2004; Sass and Krautblatter, 2007; Siewert et al, 2012). Alpine rock-wall retreat rates vary between both methods. Various factors contribute to this discrepancy, including joint spacing and orientation, and rockfall triggering processes such as paraglacial unloading after deglaciation (Hinchcliffe and Ballantyne, 1999; Arsenault and Meigs, 2005) and periglacial conditions (e.g., enhanced freeze-thaw activity and permafrost degradation)

Results

Discussion

Conclusion