Structural and thermal controls of rockfall frequency and magnitude within rockwall–talus systems (Swiss Alps)

Abstract

Both from a systemic and natural hazard perspective, it is essential to understand the causes and frequency of rockfalls in mountain terrain and to predict the block sizes deposited at specific locations. Commonly, rockfalls are studied either retrospectively, using talus slopes, or directly by rockwall surveys. Nevertheless, our understanding of rockfall activity, particularly at the lower magnitude spectrum, is still incomplete. Moreover, the explanatory framework is rarely addressed explicitly. In this study, we investigate two rockwall-talus systems in the Swiss Alps to estimate the rockfall frequency-magnitude pattern and their key controls. We present a holistic approach that integrates deductive geotechnical and thermal investigations of the source rockwalls with abductive talus-based explanations of rockfall volume and frequency.The rockwalls' three-dimensional (3D) joint pattern indicates that 75% of the blocks may be released as debris fall (< 14 m 3 ) and boulder falls (14-61 m 3 ), which is mirrored in the corresponding talus material. Using two-year records of near-surface rockwall temperatures as input for a 1D heat conduction model underlines the destabilizing role of seasonal ice segregation. Deepest frost cracking of 300 cm may occur on the north-northeast (NNE)-exposed, snow-rich rockwall, with peaks at the outermost surface. The synthesis of all data suggests that infrequent, large planar slides (approximately every 250 years) overlain by smaller, more frequent wedge and toppling failures (approximately every 17-50 years) as well as high-frequency flake-like clasts (3-6 events/year) characterize the rockfall frequency-magnitude pattern at Hungerli Peak.Here, we argue that small-size rockfalls need more scientific attention, particularly in discontinuous permafrost zones. Our study emphasizes that future frequency-magnitude research should ideally incorporate site-specific structural and thermal properties, rather than just focusing on climatic or meteorological triggers. We discuss how holistic rockwall-talus approaches, as proposed here, could help to increase our process understanding of rockfalls in mountain environments.

Linking rock weathering, rockwall instability and rockfall supply on talus slopes in glaciated hanging valleys (Swiss Alps)

“…In the first monitoring period (2014–15), MARTs of north‐exposed rockwalls (Figure ) were between 2.8°C in the Brandji Valley (Br1 in Figure a) and 0.67°C in the Pipji Valley (Pr2 in Figure c). MARTs of the north‐exposed rockwalls in the Hungerli Valley were 1.39°C (Hr1) and 2.48°C (Hr2) as presented in Messenzehl and Dikau (Hr1 and Hr2 correspond to R1b and R2a). One year later (2015–16), MARTs dropped slightly to a minimum of 0.14°C (Pr2 in Figure c), indicating permafrost boundary conditions.…”

“…After this winter cooling, NSRTs rapidly increased followed by daily frost cycles in early spring (−5 to 7°C). In contrast, constant NSRTs have been recorded for several winter months (~116 d) at the Hungerli Peak, at Br3 (Figure a) and to some extent at Pr1 (Figure b). Stagnating winter temperatures between 0 and − 9°C recorded at those sites and the lack of temperature oscillations reveal an insulating rockwall snow cover on and close to the MTL drilling position that locally prevents progressive winter cooling and heat loss during spring.…”

“… Field sites of rockfall depositional landforms (upper photographs) and results of particle size distribution along field transect surveys (lower graphs shown as boxplots) for the (a‐b, c‐d) Brändji Valley, (e‐f) Hungerli Valley and (g‐h) Pipji Valley. Results of longitudinal block size distribution of Ht1 and Ht2 (Hungerli Valley) are presented in Messenzehl and Dikau . Positions of Pt1, Pt3 and Ht4 are illustrated in Figure [Colour figure can be viewed at wileyonlinelibrary.com]…”

“… Near‐surface rockw temperature (NSRT) records of north‐exposed monitoring sites in the (a) Brändji and (b, c) Pipji Valley over the period August 2014 to August 2016. Temperature records of Hrt1 and Hr2 are illustrated in Messenzehl and Dikau in Figure 9 (corresponding to R1b and R2a) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…In all three valleys, the most active sediment transfer occur via rockfalls from unstable rockwalls to talus slopes that are either buffered at their lower foot by glacial moraine deposits or connected to rock glaciers of different activity states. Recent field studies from Hungerli Peak reveal that the local rockfall frequency‐magnitude pattern is dominated by boulder falls (14–61 m 3 ) and debris falls (< 14 m 3 ) with average annual to decadal return periods as well as seasonal surficial flaking (~10 −3 m 3 ) . Additionally, Messenzehl et al demonstrated in regional‐scale modeling of vegetation‐free talus slopes that rockfall activity is highest on north‐facing, metamorphic rockwalls with anaclinal structural conditions (in‐dipping orientation of schistosity and main joint sets) in the middle and upper valleys.…”