Quantifying massive cascading sediment transport triggered by a cliff fall in a highly-active alpine basin.

In the coming decades with enhanced rainstorm activity, massive sediment redistribution in Alpine catchments will be a key hazard and challenge in Alpine communities. While several studies have collected data from massive rock slope failures, few studies have quantitatively assessed the cascading sediment redistribution in highly active alpine catchments. Recurrence intervals for cliffs falls are estimated at 80 years (Krautblatter et al., 2012 ), thus, observations of the subsequent sediment cascading are limited or inexistent despite their major role in landscape evolution and sediment fluxes. Digital aerial photogrammetry acquired by governmental agencies is becoming a relevant tool to better understand short landscape response to climate change. Repetitive yearly to bi-yearly orthophotos and DSM extracted from large format aerial surveys represent a valuable monitoring tool at regional scale because of their wide extent coverage (km) at a high spatial resolution (20 cm). 

This contribution reports the massive sediment redistribution that has been triggered by the multistage failure of >200.000 m³ from the Hochvogel dolomite peak during the summer of 2016. Seven true orthophotos and high-resolution aerial photogrammetric digital surface models (DSM) between 2010 and 2020 were 3D coregistered to a reference system for optimized volume calculation in steep terrain. Three consecutive differential DSMs (2010-2012, 2012-2014, 2014-2015) describe the catchment morphodynamics before the cliff fall, while, the subsequent differential DSMs (2015-2017, 2017-2018, 2018-2020) describe the morphodynamics one year, two years and four years after the cliff fall. Spectrograms from surrounding seismic stations expand the understanding of the cliff fall timing. We observe the decadal throughput of >200.000 m³ of sediment with massive sediments pulses that (i) respond with reaction times of 0-4 years and relaxation times beyond 10 years, (ii) with faster 0-2 years response times in the upper catchment (A&B) and >>2 years response times in the lower catchments, (iii) the inversion of sedimentary (>10²-10³ mm/a) to massive erosive regimes (>10² mm/a) within single years and vice versa and the (iv) dependency of redistribution to rainstorm frequency and intensities.

 

Krautblatter, M., Moser, M., Schrott, L., Wolf, J., Morche, D., 2012. Significance of rockfall magnitude and carbonate dissolution for rock slope erosion and geomorphic work on Alpine limestone cliffs (Reintal, German Alps). Geomorphology 167, 21–34. https://doi.org/10.1016/j.geomorph.2012.04.007