An integrated approach to investigate climate-driven rockfall occurrence in high alpine slopes: the Bessanese glacial basin, Western Italian Alps

Abstract

Rockfalls are one of the most common instability processes in high mountains. They represent a relevant issue, both for the risks they represent for (infra) structures and frequentation, and for their potential role as terrestrial indicators of climate change. This study aims to contribute to the growing topic of the relationship between climate change and slope instability at the basin scale. The selected study area is the Bessanese glacial basin (Western Italian Alps) which, since 2016, has been specifically equipped, monitored and investigated for this purpose. In order to provide a broader context for the interpretation of the recent rockfall events and associated climate conditions, a cross-temporal and integrated approach has been adopted. For this purpose, geomorphological investigations (last 100 years), local climate (last 30 years) and near-surface rock/air temperatures analyses, have been carried out. First research outcomes show that rockfalls occurred in two different geomorphological positions: on rock slopes in permafrost condition, facing from NW to NE and/or along the glacier margins, on rock slopes uncovered by the ice in the last decades. Seasonal thaw of the active layer and/or glacier debutressing can be deemed responsible for slope failure preparation. With regard to timing, almost all dated rock falls occurred in summer. For the July events, initiation may have been caused by a combination of rapid snow melt and enhanced seasonal thaw of the active layer due to anomalous high temperatures, and rainfall. August events are, instead, associated with a significant positive temperature anomaly on the quarterly scale, and they can be ascribed to the rapid and/or in depth thaw of the permafrost active layer. According to our findings, we can expect that in the Bessanese glacierized basin, as in similar high mountain areas, climate change will cause an increase of slope instability in the future. To fasten knowledge deepening, we highlight the need for a growth of a network of high elevation experimental sites at the basin scale, and the definition of shared methodological and measurement standards, that would allow a more rapid and effective comparison of data.

Rock temperature variability in high-altitude rockfall-prone areas

“…Even if ΔT between air and rock decreases with increasing rock depth (see Figure 4), it never becomes zero. These differences have been reported in some papers that compared air temperature with NSRT, such as in Haberkorn et al (2016), Viani et al (2020). Our study confirms that there is a low linear correlation between daily maximum air temperature and daily maximum NSRT (zero-curtain days excluded, see point v).…”

“…In this figure, particular evident is the zero-curtain period. The zero-curtain state is characterized by a constant temperature close to 0 °C due to the snow cover (thermal insulation) and due to the latent heat released in the melting snow pack (Hasler et al 2011;Kellerer-Pirklbauer 2017;Viani et al 2020). This figure also highlights the limited number of diurnal freeze-thaw cycles in the rock: in fact, when the rock is out of the zero-curtain, air temperature tends to remain above 0 °C.…”

“…7.1433 °E, Figure 1). The main features of this experimental site are described in detail in Viani et al (2020). The lithotype of the TMS is an outcrop of calc-schists.…”

“…are few. Despite these difficulties, during the last 20 years several studies on rock temperature have been carried out in the Alps, with interesting results (Gruber et al 2004;Hasler et al 2011;Moor et al 2011;Magnin et al 2017;Weber et al 2017;Viani et al 2020). These studies highlight the importance of local studies, carried out on high-elevation experimental sites.…”

Evolution of temperature indices in the periglacial environment of the European Alps in the period 1990–2019

“…moraines or talus) to instability, because ground ice melting decreases the strength of those materials (Ravanel et al 2017;Paranunzio et al 2019). At the same time, seasonal freeze-thaw cycles occur at higher altitudes and cause thermal stress on rock masses which can become unstable and collapse (Viani et al 2020).…”