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 5.2 km 2 calcareous cliffs covering in the Lauterbrunnen Valley, Switzerland.Terrestrial laser scans were collected during nine 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 and 119.34 ± 1.07 m 3. 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 (r = 0.6, P = 0.08, 90% significance level) is observed for freeze-thaw cycles and rockfall events smaller than 1 m 3 with a 2 month 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

Seismic monitoring of small alpine rockfalls – validity, precision and limitations

“…Detachment area coordinates were obtained by georeferencing the rasterised point cloud data on referenced topographic maps and orthoimages. Given the typical rockfall volumes < 1 m 3 (Strunden et al, 2014), location uncertainty should mainly result from the georeferencing process and is quantified by the root mean square error (RMSE). All location coordinates were rounded to the full metre and transformed to the UTM coordinate system.…”

“…4b) implies a significant free-fall phase of detached rocks, followed by rock mass impacts on the cliff face or the talus slopes below, eventually grading into moderate translocation processes on the less than 250 m long depositional areas. Rockfall activity in the Lauterbrunnen Valley has been monitored by repeated TLS since 2012 (Strunden et al, 2014), yielding 122 detected rockfalls (523.72 m 3 in total) over an 18-month investigation period. These events appear to be evenly distributed throughout valley walls (15.13 events per year and km 2 ) with most frequent events being smaller than 1 m 3 .…”

“…Scans were recorded during two field campaigns on 22 September 2014 and 28 October 2014. The TLS data collection and processing approach used in this study is identical to that of previous work conducted in the same study area (for details see Strunden et al, 2014). To ensure sufficient overlap and to avoid topographic shading effects, the study area was scanned from five different positions (see Fig.…”

“…vegetation growth). Measurement uncertainty was estimated based on scan differences from stable control regions (for details see Strunden et al, 2014). Detachment area coordinates were obtained by georeferencing the rasterised point cloud data on referenced topographic maps and orthoimages.…”

“…The last of these techniques (Ring, 1963) provides high-resolution spatial data of topographic change attributable to rock detachment (e.g. Rabatel et al, 2008;Zimmer et al, 2012;Strunden et al, 2014) but is time consuming during recording and evaluation and primarily suited for monthly to annual lapse times. Over the integration time between two consecutive scans it is possible to identify spatial activity patterns, released volume ranges and magnitudefrequency relationships (Strunden et al, 2014).…”