Spatial and temporal variations in rockfall determined from TLS measurements in a deglaciated valley, Switzerland

Abstract

Steep U-shaped valleys produced by glaciation are commonly eroded by rockfall. In this study we investigate modern and long-term (power law predicted) rates of rock wall retreat in a deglaciated valley. Our emphasis is on the 5.2km2 calcareous cliffs covering in the Lauterbrunnen Valley, Switzerland. Terrestrial laser scans were collected during nine field campaigns over 18months 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 and 119.34±1.07m3. 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 0-6months. The highest correlation factor (r=0.6, P=0.08, 90% significance level) is observed for freeze-thaw cycles and rockfall events smaller than 1m3 with a 2month delay 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. A Monte Carlo analysis was applied to the frequency-magnitude relationship to evaluate the sensitivity of results to their stochastic nature. An average power law exponent of 1.71±0.09 was calculated using linear regression. The calculated exponents are similar to other studies in calcareous rock settings conducted over different observation durations. Finally, we determine an average total eroded volume of 2.0-0.01+0.6 103m3/yr and a corresponding long-term (power law predicted) cliff retreat rate of 0.39-0.02±0.12mm/yr. Key Points 122 rockfall events were identified from repeat TLS scans Rockfall events are correlated with the freeze-thaw history Long-term cliff retreat rates are 0.39+0.12/-0.02mm/yr