Guerin Antoine, Ravanel Ludovic, Matasci Battista, Jaboyedoff Michel, Deline Philip
Risk Analysis Group, Institute of Earth Sciences, University of Lausanne, 1015, Lausanne, Switzerland.
EDYTEM Laboratory, CNRS, University Savoie Mont-Blanc (USMB), 73370, Le Bourget du Lac, France.
Sci Rep. 2020 Oct 15;10(1):17330. doi: 10.1038/s41598-020-74162-1.
Since the end of the Little Ice Age, the west face of the Drus (Mont Blanc massif, France) has been affected by a retrogressive erosion dynamic marked by large rockfall events. From the 1950s onwards, the rock failure frequency gradually increased until the large rockfall event (292,680 m) of June 2005, which made the Bonatti Pillar disappear. Aiming to characterize the rock failure activity following this major event, which may be related to permafrost warming, the granitic rock face was scanned each autumn between October 2005 and September 2016 using medium- and long-range terrestrial laser scanners. All the point clouds were successively compared to establish a rockfall source inventory and determine a volume-frequency relationship. Eleven years of monitoring revealed a phase of rock failure activity decay until September 2008, a destabilization phase between September 2008 and November 2011, and a new phase of rock failure activity decay from November 2011 to September 2016. The destabilization phase was marked by three major rockfall events covering a total volume of 61,494 m, resulting in the progressive collapse of a new pillar located in the northern part of the June 2005 rockfall scar. In the same way as for the Bonatti Pillar, rock failure instability propagated upward with increasing volumes. In addition to these major events, 304 rockfall sources ranging from 0.002 to 476 m were detected between 2005 and 2016. The temporal evolution of rock failure activity reveals that after a major event, the number of rockfall sources and the eroded volume both follow a rapid decrease. The rock failure activity is characterized by an exponential decay during the period following the major event and by a power-law decay for the eroded volume. The power law describing the distribution of the source volumes detected between 2005 and 2016 indicates an exponent of 0.48 and an average rock failure activity larger of more than six events larger than 1 m per year. Over the 1905-2016 period, a total of 426,611 m of rock collapsed from the Drus west face, indicating a very high rock wall retreat rate of 14.4 mm year over a surface of 266,700 m. Averaged over a time window of 1000 years, the long-term retreat rate derived from the frequency density integration of rock failure volumes is 2.9 mm year. Despite difficulty in accessing and monitoring the site, our study demonstrates that long-term surveys of high-elevation rock faces are possible and provide valuable information that helps improve our understanding of landscape evolution in mountainous settings subject to permafrost warming.
自小冰期结束以来，德吕斯山（法国勃朗峰地块）的西坡受到一种以大规模落石事件为特征的溯源侵蚀动力的影响。从20世纪50年代起，岩石崩塌频率逐渐增加，直至2005年6月发生大规模落石事件（292,680立方米），导致博纳蒂石柱消失。为了描述这一重大事件之后可能与多年冻土变暖相关的岩石崩塌活动，在2005年10月至2016年9月期间，每年秋季使用中远程地面激光扫描仪对花岗岩岩面进行扫描。依次比较所有点云，以建立落石源清单并确定体积-频率关系。十一年的监测揭示了直到2008年9月的岩石崩塌活动衰减阶段、2008年9月至2011年11月的失稳阶段以及2011年11月至2016年9月的岩石崩塌活动新的衰减阶段。失稳阶段的特征是发生了三次重大落石事件，总体积为61,494立方米，导致位于2005年6月落石疤痕北部的一根新石柱逐渐坍塌。与博纳蒂石柱的情况相同（此处原文可能有误，推测应为“与博纳蒂石柱的情况一样”），岩石崩塌失稳随着体积增加向上传播。除了这些重大事件外，在2005年至2016年期间还检测到304个落石源，体积从0.002立方米到476立方米不等。岩石崩塌活动的时间演变表明，在一次重大事件之后，落石源数量和侵蚀体积均迅速减少。岩石崩塌活动在重大事件后的时期内呈指数衰减，而侵蚀体积呈幂律衰减。描述2005年至2016年期间检测到的源体积分布的幂律表明，指数为0.48，平均每年有超过六个体积大于1立方米的岩石崩塌事件。在1905 - 2016年期间，德吕斯山西坡共有426,611立方米的岩石崩塌，表明在266,700平方米的表面上，岩壁后退速率非常高，达到每年14.4毫米。在1000年的时间窗口内平均计算，从岩石崩塌体积的频率密度积分得出的长期后退速率为每年2.9毫米。尽管该地点难以进入和监测，但我们的研究表明，对高海拔岩面进行长期调查是可行的，并提供了有价值的信息，有助于增进我们对受多年冻土变暖影响的山区景观演变的理解。
