Influence of meteorological factors on rockfall occurrence in a middle mountain limestone cliff

Abstract

Abstract. The influence of meteorological conditions on rockfall occurrence has been often highlighted, but its knowledge is still not sufficient due to the lack of exhaustive and precise rockfall data bases. In this study, rockfalls have been detected in a limestone cliff by annual terrestrial laser scanning, and dated by photographic survey during 2.5 years. A near-continuous survey (1 photo each 10 mn) with a wide-angle lens have allowed dating 214 rockfalls larger than 0.1 m3, and a monthly survey with a telephoto lens, dating 854 rockfalls larger than 0.01 m3. The analysis of the two data bases shows that the rockfall frequency can be multiplied by a factor as high as 7 during freeze–thaw episodes and 26 when the mean rainfall intensity (since the beginning of the rainfall episode) is higher than 5 mm h−1. Based on these results, a 4-level scale has been proposed for predicting the temporal variations of hazard. The more precise data base and freeze–thaw episode definition make it possible to distinguish different phases in freeze–thaw episodes: negative temperature cooling periods, negative temperature warming periods and thawing periods. It appears that rockfalls occur more frequently during warming and thawing periods than during cooling periods. It can be inferred that rockfalls are caused by thermal ice dilatation rather than by dilatation due to the phase transition. But they may occur only when the ice melt, because the cohesion of the ice–rock interface can be sufficient to hold the rock compartment which has been cut.

Spatiotemporal patterns, triggers and anatomies of seismically detected rockfalls

“…Other studies found similarly strong links between rockfall (Helmstetter and Garambois, 2010;Hibert et al, 2014;D'Amato et al, 2016) and other mass-wasting process (e.g., Burtin et al, 2013) activity and precipitation. However, the small lag time implies that a temporal resolution of several hours (D'Amato et al, 2016) is still insufficient to constrain precipitation as a trigger. Perhaps even the hourly aggregated meteorological data used in this study are not detailed enough.…”

“…As a consequence, precise constraints on timing, location and triggers are hard to come by. There are many established approaches to detect rockfall activity spatially, for example surveys of talus slopes, dendrometric and lichenometric approaches (Matsuoka and Sakai, 1999;Stoffel et al, 2005;Krautblatter et al, 2012), and more recently image-based mapping and terrestrial and airborne laser scanning (Stock et al, 2011;Strunden et al, 2014;D'Amato et al, 2016). The temporal information delivered by these methods is not very precise as it is bound to the survey lapse times, which are typically on the order of weeks to years.…”

“…However, rockfall activity has mainly received attention as a "by-product" of seismic observatories with different scopes (e.g., Dammeier et al, 2011;Hibert et al, 2011;Burtin et al, 2014) and research has focused on linking seismic properties with geometric and kinetic characteristics, such as mobilised volume, run-out length or fragmentation (Dammeier et al, 2011;Ekström and Stark, 2013;Hibert et al, 2014). Systematic linking of events to more than one potential environmental trigger has received only marginal attention (Helmstetter and Garambois, 2010;Burtin et al, 2013;D'Amato et al, 2016).…”

Seismic constraints on rock damaging related to a failing mountain peak: the Hochvogel, Allgäu

“…Its suddenness, velocity and limited predictability renders gravitational mass wasting hazardous yet hard to constrain in terms of drivers and triggers, and mechanisms and their temporal evolution. Classic approaches to studying mass wasting processes are either ex post investigations of the failure mechanism (Frayssines and Hantz, 2006), volume and environmental conditions before and during the event (D'Amato et al ., 2016), long‐term monitoring, often with remotely sensed imagery, or, increasingly, point measurements from dedicated sensors at sites with known or expected activity (Collins et al ., 2018; Di Maio et al ., 2010; Dixon et al .,2018; Lévy et al ., 2010; Walter et al ., 2020). None of these approaches can provide detailed continuous insight into the activity with high temporal resolution, beyond the “point dimension” and throughout the wider process domain, including the downslope pathway of a failing rock mass.…”

Remote thermal detection of exfoliation sheet deformation

“…A wide range of external environmental factors, including precipitation, freeze-thaw cycles, and thermal effects associated with temperature and insolation, can trigger the failure of unstable rock slopes (Wieczorek and Jäger 1996 ; Matsuoka and Sakai 1999 ; Ishikawa et al 2004 ; Gunzburger et al 2005 ; Frayssines and Hantz 2006 ; D'Amato et al 2016 ; Collins and Stock 2016 ; Dietze et al 2017 ). Regarding daily and seasonal temperature fluctuations in particular, many authors (Vlcko et al 2009 ; Gischig et al 2011 ; do Amaral Vargas et al 2013 ; Bottelin et al 2013 ; Collins and Stock 2016 ; Draebing et al 2017 ; Leith et al 2017 ; Villarraga et al 2018 ; Collins et al 2018 , 2019 ) have shown that repeated cycles of heating and cooling can generate stresses capable of propagating cracks in both fractured and competent rocks.…”