A 6-year lidar survey reveals enhanced rockwall retreat and modified rockfall magnitudes/frequencies in deglaciating cirques

Abstract

Abstract. Cirque erosion contributes significantly to mountain denudation and is a key element of glaciated mountain topography. Despite long-standing efforts, rates of rockwall retreat and the proportional contributions of low-, mid- and high-magnitude rockfalls have remained poorly constrained. Here, a unique, terrestrial-lidar-derived rockfall inventory (2011–2017) of two glaciated cirques in the Hohe Tauern range, Central Alps, Austria, is analysed. The mean cirque wall retreat rate of 1.9 mm a−1 ranks in the top range of reported values and is mainly driven by enhanced rockfall from the lowermost, freshly deglaciated rockwall sections. Retreat rates are significantly elevated over decades subsequent to glacier downwasting. Elongated cirque morphology and recorded cirque wall retreat rates indicate headward erosion is clearly outpacing lateral erosion, most likely due to the cataclinal backwalls, which are prone to large dip-slope failures. The rockfall magnitude–frequency distribution – the first such distribution derived for deglaciating cirques – follows a distinct negative power law over 4 orders of magnitude. Magnitude–frequency distributions in glacier-proximal and glacier-distal rockwall sections differ significantly due to an increased occurrence of large rockfalls in recently deglaciated areas. In this paper, the second of two companion pieces, we show how recent climate warming shapes glacial landforms, controls spatiotemporal rockfall variation in glacial environments and indicates a transient signal with decadal-scale exhaustion of rockfall activity immediately following deglaciation crucial for future hazard assessments.

Current glacier recession causes significant rockfall increase: the immediate paraglacial response of deglaciating cirque walls

“…Steep joint sets (J1, J2) facilitate large detachments in W-and SE-oriented terrain, while strike and dip of the mica-schist cleavage promote frequent dip-slope failures in N-facing rockwall sections. Particularly the latter mode of failure may represent a key mechanism of cirque expansion, as pronounced northsouth elongated cirque morphologies at the Kitzsteinhorn indicate effective cleavage-driven headwall sapping over long timescales (Hartmeyer et al, 2020a). Further analysis reveals considerably increased rockfall activity in the immediate proximity (10-20 vertical metres) of the current glacier surface, which emerged from the ice only very recently.…”

“…Dashed lines represent the regression lines of both distributions. 18.7 103.3 6.8 41.9 59.9 3.5 falls below 1 m 3 represent 80 % of the total number but account for only 3.7 % of the overall rockfall volume; see companion study by Hartmeyer et al (2020a) for detailed discussion of magnitude-frequency distributions. Frontal views of the monitored rockwalls with indicated rockfall source areas are provided in Fig.…”

“…12). To test the statistical robustness of the discovered differences between glacierproximal (< 10 m above glacier) and glacier-distal (> 10 m above glacier) rockfall activity, the goodness of fit was analysed using a bootstrapping approach (full details are given in Hartmeyer et al, 2020a). In the analysis, 20 % of the rockfalls were randomly removed and the dataset was resampled 100 000 times to assess the sensitivity of the power-law exponent b, which represents a frequently used variable to characterize spatiotemporal rockfall variation (e.g.…”

“…Data quality is analysed, spatial patterns of rockfall and rockfall failure depth are presented and causes of the observed rockfall patterns discussed. Magnitude-frequency relationships and rockwall retreat rates derived from these data are discussed in a companion study (Hartmeyer et al, 2020a).…”

Evolution of debris cover on glaciers of the Eastern Alps, Austria, between 1996 and 2015

“…Slope failure in bedrock walls is attributed to disturbed stress regimes due to glacier oversteepening, modified ground thermal conditions and increased frost weathering at the deglaciated rockwalls (Ballantyne, 2002; Hales & Roering, 2007; Wegmann et al, 1998). Increased rockfall rates and examples of individual larger slope failures have been reported for several sites throughout the Alps (Fischer et al, 2011; Hartmeyer et al, 2020; Rabatel et al, 2008; Ravanel et al, 2013). On a regional scale, data on rockfall rates or slope failure activity in glacier surroundings are lacking, even though national and regional agencies in Austria collect information on slope failure events (Herrera et al, 2018).…”