Multi-method monitoring of rockfall activity along the classic route up Mont Blanc (4809 m a.s.l.) to encourage adaptation by mountaineers

Abstract

Abstract. There are on average 35 fatal mountaineering accidents per summer in France. On average, since 1990, 3.7 of them have occurred every summer in the Grand Couloir du Goûter, on the classic route up Mont Blanc (4809 m a.s.l.). Rockfall is one of the main factors that explain this high accident rate and contribute to making it one of the most accident-prone areas in the Alps for mountaineers. In this particular context, the objective of this study is to document the rockfall activity and its triggering factors in the Grand Couloir du Goûter in order to disseminate the results to mountaineers and favour their adaptation to the local rockfall hazard. Using a multi-method monitoring system (five seismic sensors, an automatic digital camera, three rock subsurface temperature sensors, a traffic sensor, a high-resolution topographical survey, two weather stations and a rain gauge), we acquired a continuous database on rockfalls during a period of 68 d in 2019 and some of their potential triggering factors (precipitation, ground and air temperatures, snow cover, frequentation by climbers). At the seasonal scale, our results confirm previous studies showing that rockfalls are most frequent during the snowmelt period in permafrost-affected rockwalls. Furthermore, the unprecedented time precision and completeness of our rockfall database at high elevation thanks to seismic sensors allowed us to investigate the factors triggering rockfalls. We found a clear correlation between rockfall frequency and air temperature, with a 2 h delay between peak air temperature and peak rockfall activity. A small number of rockfalls seem to be triggered by mountaineers. Our data set shows that climbers are not aware of the variations in rockfall frequency and/or cannot/will not adapt their behaviour to this hazard. These results should help to define an adaptation strategy for climbers. Therefore, we disseminated our results within the mountaineering community thanks to the full integration of our results into the management of the route by local actors. Knowledge built during this experiment has already been used for the definition and implementation of management measures for the attendance in summer 2020.

Estimating surface water availability in high mountain rock slopes using a numerical energy balance model

“…8). A high rockfall frequency in such a permafrost-affected site was recently demonstrated by Mourey et al (2022) in the Mont-Blanc massif. We compared the influence of elevation on water balance in N and S facing hillslopes (Fig.…”

Go or grow? Feedbacks between moving slopes and shifting plants in high mountain environments

“…Monitoring and reconstructing linked geomorphic and vegetation changes can help us to better understand biogeomorphic feedbacks and balances. Combining ecological and geomorphic field techniques, such as repeated vegetation surveys (Cannone and Piccinelli 2021; Grabherr et al, 2000) and continuous geomorphic monitoring (Belli et al, 2022; Mourey et al, 2022) can determine decadal scale biogeomorphic dynamics on local scales. Especially UAV surveys and dendroecology are of great value for biogeomorphic research as they can monitor or reconstruct geomorphic (De Haas et al, 2021; Favillier et al, 2018; Stoffel, 2010) and vegetation changes (Francon et al, 2020; Wei et al, 2021) at the same time.…”

Climbing Mont Blanc (4808 m a.s.l.): What Are the Risks in the Grand Couloir Du Goûter and On the North Face of Mont Blanc Du Tacul?

“…As these two sections are regularly the scene of tragedies (Mourey et al, 2018), this work aims to objectify the associated hazards and the risks taken by climbers. Thanks to innovative monitoring of the Grand Couloir during the summer of 2019 (Mourey et al, 2022), the frequency and intensity of rockfalls were measured using a seismic device, while numbers of climbers were quantified using a pyroelectric sensor. At Tacul, five years of automatic photography (2016)(2017)(2018)(2019)(2020) was used to determine the frequency and volume of serac falls (large blocks of ice) and to study the tracks left by climbers, as well as their dangerousness, and to quantify the number of climbers in summer 2017.…”

“…This is the first time that sections of mountaineering routes and their hazards are precisely studied, and that the risks experienced by mountaineers are quantified. To date, studies have mainly focused on changes in practice and routes due to global warming (e.g., Ritter et al, 2012;Temme, 2015;Purdie and Kerr, 2018;Mourey et al, 2019Mourey et al, , 2022. In addition to its societal dimension, this work tests two purely physical hypotheses: the close links between thermal conditions and rockfalls, and the absence of a direct link between air temperatures and serac falls.…”