Monitoring rockfalls on alpine peaks. A trade-off between spatial extent and resolution.

The high mean rate of erosion in mountain environments is the product of events that are episodic in time and discontinuous in space. Bedrock cliffs development can be influenced by rare, large-scale failures or regular block falls. This distinction may influence the rates of sediment flux, geomorphic changes over the slopes and impose different degrees of natural hazards.

The Hochvogel summit, located at 2,592 m a.s.l at Allgäuer Alps in the German - Austrian border, is currently monitored as part of the AlpSenseRely project. The monitoring program consists of an early warning system operational from 2018 at the top of the summit (Leinauer et al., 2020, 2021). Multi-temporal, multi-scale photogrammetric monitoring aims to complement the monitoring program by quantifying geomorphological changes over the steep slopes that surround the crack. 

The multi-temporal analysis of changes over a decade of aerial imagery with bi-yearly to yearly frequency and 20 cm resolution brings attention to areas with continuous rockfall activity over the Hochvogel slopes. The estimated rockfall volume accuracy is highly influenced by the limitation of nadir aerial imagery to map complex and steep terrains. On the other hand, the pyramid-shaped summit imposes limitations to classical field slope monitoring techniques. Yearly UAV surveys have been acquired since 2017. The usage of structure-from-motion (SfM) enables the production of various high-resolution, low-cost products such as point clouds, digital surface models, and orthomosaics, which improves the quality and resolution of the rockfall mapping and volumetric calculation. Nevertheless, the limited spatial extent, combined with the steep slopes, hardly accessible and dangerous location at the Hochvogel, challenges a constant and complete slope monitoring. 

This contribution explores the capability of a multi-sensor camera system (MSKS) mounted on an Ultralight aircraft to acquire optical imagery and monitor rockfall activity at the Hochvogel. The MSKS consists of 5 optical cameras, 1 camera nadir oriented, and 4 cameras oblique oriented, to improve the data quality acquisition on steep terrain areas. The ultralight aircraft flies at a height of 450 m above the ground to acquire up to 5 cm resolution imagery over an area of 14 km². The aim of the dataset is to fill the gap between the wide areal coverage, 20 cm resolution of the aerial imagery (ultracam sensor), and high-resolution but limited to the top of the summit information of the UAV survey. The integration of a more reliable, operationally safe, fast, and lower cost aerial photogrammetric survey is highly beneficial for the mapping, monitoring, and understanding of different alpine climate-induced mass wasting processes and hazards.

 

References

• Leinauer, J., Jacobs, B. and Krautblatter, M. (2020), “Anticipating an imminent large rock slope failure at the Hochvogel (Allgäu Alps)”, Geomechanics and Tunnelling, Vol. 13 No. 6, pp. 597–603.
• Leinauer, J., Jacobs, B. and Krautblatter, M. (2021), “High alpine geotechnical real time monitoring and early warning at a large imminent rock slope failure (Hochvogel, GER/AUT)”, IOP Conference Series: Earth and Environmental Science, Vol. 833 No. 1, p. 012146.