Spatial and Temporal Variations in Rockfall Determined from Repeat Lidar Measurements in a Deglaciated Valley, Switzerland

Abstract:

Steep U-shaped valleys produced by glaciation are eroded by rockfall and pose a risk to humans. Our knowledge of the postglacial erosion of these valleys is limited. In this study, calcareous cliffs covering an area of 5.2 km² are investigated in the deglaciated Lauterbrunnen Valley, Switzerland. Terrestrial light detection and ranging (LiDAR) scans were collected during 9 field campaigns over 18 months to provide a continuous coverage of most of the valley walls. Results indicate a total of 122 rockfalls with volumes ranging between 0.06±0.01 to 119.34±1.07 m³. These events suggest average cliff retreat rates of 0.04 and 0.08 mm/yr for the east and west valley walls, respectively. Different size groups of rockfall events were correlated with environmental factors (e.g., freeze-thaw cycles (temperature), precipitation and seismicity) using a linear regression with variable lag times of 0-6 months. The highest correlation factor (0.6) is observed for freeze-thaw cycles and rockfall events smaller than 1 m³ with a two month lag time between temperature extremes and rockfall. Frequency-magnitude relationships for rockfall events were calculated to predict less frequent larger rockfall events that did not occur during the observation period. Power law exponents of 0.72, 1.62 and 0.67±0.07 were calculated using linear regression and maximum likelihood approaches, respectively. Calculated exponents are similar to other calcareous rock studies in different settings conducted over different observation durations. In comparison to these studies, we find exponents are more influenced by lithology rather than geomorphic setting (e.g., sea cliff, non-glaciated catchments, deglaciated valley).