Structural and Thermal Controls on the Frequency and Magnitude of Small-size Rockfall Events (European Swiss Alps)

In steep mountain terrain rockwall erosion and sediment deposition commonly occur by rockfalls of different magnitudes and frequencies. To assess the natural hazard potential it is essential to understand and predict the causes and frequencies of rockwall failure and characterise which block sizes are deposited on a specific location. Contrary to large catastrophic instabilities, small-size rockfalls (<104 m3) have not been thoroughly investigated. Consequently, their erosional and depositional activity is poorly defined. To address these shortcomings, we investigated two adjacent rockwall-talus systems in the Swiss Alps and aim to estimate their rockfall frequency-magnitude spectrum with respect to their controls. We present an integrated approach combining: (i) geotechnical scanline surveys and thermal studies of the source rockwall with (ii) engineering and geomorphic analyses of the talus slopes. (i) Each rockwall is linked to specific rockfall-prone block volumes (0.1-100 m3), in which wedge sliding or topples dominate depending on the specific joint sets. The 2-year rock temperature data reveal further contrasting thermal regimes. Using the model by Hales and Roering (2007), we calculated cracking intensities of few cm up to 200 cm bedrock depth. (ii) Along the talus slopes, we identified typical downslope gravity sorting with mean block diameters ranging from 10 cm (apex) to 1.2 m (foot). Applying the approach by Evans and Hungr (1993), we found significant differences in the annual rockfall frequency with respect to the block magnitude and the landing position. Surprisingly - despite their similar topo-climatic, paraglacial and geological setting - each landform-complex is characterised by different frequency-magnitude relationships. We lead this back to the mechanical and thermal variability of the source rock mass. While large-size wedge sliding due to seasonal frost cracking might result in low-frequency but high-magnitude talus deposition, a more homogeneous depositional signature is linked to small-size topples released by near-surface frost action. Therefore, to improve the natural hazard assessment in steep terrain, integrative studies on rockwall-talus systems are needed, which account for the entire rockfall cascade from the instable rockwall to the talus slope.