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 knowledge of it is still not sufficient due to the lack of exhaustive and precise rockfall databases. In this study, rockfalls have been detected in a limestone cliff by annual terrestrial laser scanning, and dated by photographic survey over a period of 2.5 years. A near-continuous survey (one photo every 10 min) with a wide-angle lens has made it possible to date 214 rockfalls larger than 0.1 m 3 , and a monthly survey with a telephoto lens has dated 854 rockfalls larger than 0.01 m 3 . Analysis of the two databases 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 three-level scale has been proposed for predicting the temporal variations of rockfall frequency. The more precise database 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 melts, because the cohesion of the ice-rock interface can be sufficient to hold the rock compartment which has been cut.

The Importance of Monitoring Interval for Rockfall Magnitude‐Frequency Estimation

“…Given the apparent difficulty in identifying rockfall triggers, some have explored the possibility of lagged effects (e.g., Lim et al, 2010;Strunden et al, 2015). Our findings suggest that monitoring at higher frequencies has the potential both to capture such lagged responses and to observe indicators of the underlying mechanisms, such as small-scale precursory rockfalls, prefailure deformation, or fracture dilation (Carlà et al, 2016;D'Amato et al, 2016;Rosser et al, 2007;Royán et al, 2015). While broad seasonal rockfall patterns or the rockfall response to distinct events can be obvious, even in low-frequency monitoring data, without data captured at a high temporal resolution it will remain difficult to distil a mechanically meaningful understanding of rockfall triggers from time-averaged data alone.…”

“…Lim et al, 2010). Importantly, however, T int is considerably longer in most studies (e.g., weeks to months) than the timescales over which potential triggers fluctuate (D'Amato et al, 2016) and over which successive rockfall releases can occur from the same location. As a result, it remains difficult to attribute individual rockfall events to discrete triggers due to ambiguities within our measurements of both rockfall volume and timing (e.g., Dietze, Mohadjer, et al, 2017;Strunden et al, 2015).…”

Seismic Analysis of the Detachment and Impact Phases of a Rockfall and Application for Estimating Rockfall Volume and Free‐Fall Height

“…Mount Saint-Eynard has been monitored since 2013 by several methods. The south of the Mount Saint-Eynard has been yearly surveyed by TLS since 2009, using an Optech Ilris-LR laser scanner, along a 750-m zone of interest (D'Amato et al, 2016). In this study, we focus on rockfalls detected at the Mount Saint-Eynard cliffs between November 2013 and December 2015.…”

“…They thus provide volume-frequency relationships. However, temporal information is often limited as they rely on the survey lapse times (respectively 2.5, 0.5, and 1 year interval for Dewez et al, 2013;Kuhn & Prüfer, 2014;D'Amato et al, 2016). Hence, with these approaches, it is impossible to resolve the gradual collapse of blocks released from the same location or to determine the relation between rockfalls occurrence and external triggers.…”

Spatiotemporal patterns and triggers of seismically detected rockfalls

“…The lag time between a recorded freeze-thaw or thaw-freeze transition and a rockfall can be transgressive, as the trigger process itself is transgressive, too. D'Amato et al (2016) find the most common response times at their highest resolution level (< 20 h), which means that slope reaction to free-thaw transitions can be expected to be 20 hours or less.…”