Erosional power in the Swiss Alps: characterization of slope failure in the Illgraben

Abstract

Landslides and rockfalls are key geomorphic processes in mountain basins. Their quantification and characterization are critical for understanding the processes of slope failure and their contributions to erosion and landscape evolution. We used digital photogrammetry to produce a multi-temporal record of erosion (1963–2005) of a rock slope at the head of the Illgraben, a very active catchment prone to debris flows in Switzerland. Slope failures affect 70% of the study slope and erode the slope at an average rate of 0.39 ± 0.03 m yr¯¹. The analysis of individual slope failures yielded an inventory of ~2500 failures ranging over 6 orders of magnitude in volume, despite the small slope area and short study period. The slope failures form a characteristic magnitude–frequency distribution with a rollover and a power-law tail between ~200 m³ and 1.6 × 106 m³ with an exponent of 1.65. Slope failure volume scales with area as a power law with an exponent of 1.1. Both values are low for studies of bedrock landslides and rockfall and result from the highly fractured and weathered state of the quartzitic bedrock. Our data suggest that the magnitude–frequency distribution is the result of two separate slope failure processes. Type (1) failures are frequent, small slides and slumps within the weathered layer of highly fractured rock and loose sediment, and make up the rollover. Type (2) failures are less frequent and larger rockslides and rockfalls within the internal bedded and fractured slope along pre-determined potential failure surfaces, and make up the power-law tail. Rockslides and rockfalls of high magnitude and relatively low frequency make up 99% of the total failure volume and are thus responsible for the high erosion rate. They are also significant in the context of landscape evolution as they occur on slopes above 45° and limit the relief of the slope. Copyright © 2012 John Wiley & Sons, Ltd

Quantitative reconstruction of late Holocene surface evolution on an alpine debris-flow fan

“…Neither inheritance mechanism can be conclusively ruled out, although we note that at present, sediment delivered to the main channel by small debris flows and rock fall in the upper Illgraben catchment (Fig. 1) is efficiently and rapidly evacuated to the fan by larger debris flows (Berger et al, 2010;Bennett et al, 2012Bennett et al, , 2013. We therefore regard inheritance arising from long residence times of boulders in the catchment as likely to be minor, if present-day rates of catchment activity are indicative of past conditions.…”

“…Sediment is supplied to the Illgraben catchment-fan system by frequent landslides and debris flows within the catchment, as documented by Schlunegger et al (2009) andBennett et al (2012). This material accumulates within the trunk channel of the Illgraben and is re-entrained in debris flows that reach the fan (Bennett et al, 2013) and in most cases the Rhone River (Schlunegger et al, 2009;Berger et al, 2011;Schürch et al, 2011).…”

Lithological control on the landscape form of the upper Rhône Basin, Central Swiss Alps

“…For example, climate and denudation are coupled in such a way that increased precipitation yields higher river discharges, which in turn tend to enhance rates of fluvial channel incision (see, e.g., Willett, 1999;Willett et al, 2006). Rainfall intensity, paired with the total amount of precipitation, plays an important role in erosional processes by driving hillslope erosion (see, e.g., Wischmeier, 1959) and by contributing to the triggering of mass wasting events that are responsible for mobilizing large amounts of sediment (see, e.g., Bennett et al, 2012). Glacial carving was found to be even more efficient than fluvial erosion, particularly where glaciers have relatively high sliding rates and high basal shear stresses and where subglacial water pressure gradients are large (see, e.g., Hallett et al, 1996;Montgomery, 2002;Norton et al, 2010a, b;Spotila et al, 2004;Shuster et al, 2005;Valla et al, 2011;Jansen et al, 2014;Dürst Stucki et al, 2012).…”

Field Measurements of Forces in Debris Flows at the Illgraben: Implications for Channel-Bed Erosion

“…Sediment produced by large bedrock landslides [e.g., Caduff et al, 2014;Bennett et al, 2012] and rockfall supplies sediment to gullies and the main channel, which have been identified as initiation zones for debris-flows [e.g., Berger et al, 2011b].…”