The evolution and insulation effect of two recent rockfalls on the Höllentalferner glacier in the Bavarian Alps: A multi-temporal analysis of volume and morphology using LiDAR and UAV data.

In the context of ongoing glacier retreat and slope destabilisation, rockfalls in high-alpine cirque headwalls are becoming increasingly relevant and constitute an integral component of paraglacial process chains. 

On the Höllentalferner (Wetterstein Mountains, Bavarian Alps), two prominent rockfall events occurred in recent years (2016 and 2024), depositing large amounts of debris onto the glacier surface and forming distinct supraglacial debris bodies. The aim of this study is a quantitative assessment of both events focusing on (i) quantifying the rockfall volumes using multi-temporal surface reconstructions and DoD-based estimates, (ii) assessing how supraglacial debris cover modifies glacier ablation by comparing bare-ice melt with melt beneath debris (i.e., differential ablation), and (iii) characterizing the spatio-temporal evolution of debris-body morphology (extent, thickness and internal redistribution) across consecutive observation epochs.

Methodologically, a multi-temporal DEM-of-Difference (DoD) approach is applied that combines, co-registers, and differences UAV/aerial-image-based SfM photogrammetry with airborne laser scanning (ALS) datasets. For this purpose, RTK-UAV surveys (2023 and 2025) are processed photogrammetrically into point clouds, DEMs and orthomosaics. The DEMs are aligned to stable terrain before DoDs are used to quantify elevation and volume changes.

First results indicate that volume estimates strongly depend on the chosen method: for the 2016 rockfall, estimates of approx. 10,410 m³ (extrapolation from bare-ice melt) and 15,510 m³ (Topo-to-Raster interpolation) are obtained, contrasted by a detachment volume of 5,206 m³. Interpolation-based DoD analyses yield epoch-specific volumes on the order of 19,443–31,519 m³, largely driven by differential ablation between debris-covered and adjacent bare-ice areas (and associated changes in the surrounding glacier surface). For the 2024 rockfall, current estimates amount to 20,171 m³ (extrapolation) and 10,128 m³ (detachment volume). Comparing bare-ice melt with melt beneath debris for the 2016 event indicates a pronounced insulating effect: between 2016 and 2018 (extrapolated), bare ice lowered by 4.3 m, whereas the debris-covered area lowered by only 0.8 m (differential ablation −3.5 m).

The findings highlight (i) the need for a multi-method framework to robustly constrain volumes and associated uncertainties, (ii) the key role of debris-driven melt/settlement processes for interpreting DoD signals on debris-covered glacier surfaces, and (iii) the potential of rockfalls deposits to locally delay glacier melt by supplying insulating debris.