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

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.