Use of time-lapse photogrammetry to capture substantial accumulation rates on an on-glacier avalanche deposit

Avalanches are critical contributors to the mass balance and spatial accumulation patterns of mountain glaciers. While gravitational snow redistribution models predict high localized accumulation, these predictions lack field validation due to the difficulty of monitoring highly dynamic avalanche cones. Here, we present two years of high-resolution monitoring of a large avalanche cone in the accumulation area of Argentière Glacier (French Alps). To capture these dynamics, we employed a multi-sensor approach: Uncrewed Aerial Vehicle (UAV) surveys and a time-lapse photogrammetry array consisting of 7 low-cost cameras deployed ~1 km away from the cone. The distance of the sensors from the surveyed area, its geometry (>30°), its surface characteristics (smooth snow surface) and the absence of fixed stable terrain due to the surrounding headwalls being episodically covered in snow made this environment particularly challenging for the photogrammetry methods applied. Point clouds and Digital Elevations Models were produced at a two-week resolution using Structure-from-Motion photogrammetry in Agisoft Metashape v1.8.3. with the alignment being constrained with Pseudo Ground Control Points. We could further co-register all point clouds to a September UAV acquisition with the Iterative Closest Point algorithm from the open-source project Py4dgeo, using automatically-derived stable ground from the RGB information of the images.

Methodological validation shows that while side-looking time-lapse photogrammetry captures the overall trend, it tends to underestimate elevation changes compared to UAV data, with biases up to 1.8 m and standard deviations of 2–6 m. Winter-time acquisitions with low light conditions over smooth snow surfaces also lead to reduced correlation over the cone. Despite these uncertainties, our results reveal extreme spatial variability in accumulation. The top of the cone is the most active zone, exhibiting elevation changes of ~30 m annually and a strong accumulation of 60 m w.e. between March 2023 and 2025 when accounting for the ice flow—roughly 15 times the annual mass balance recorded by the GLACIOCLIM program in the nearby accumulation area not affected by avalanche deposits. We identify a topographical threshold for snow storage: the upper cone fills early in the season until reaching a critical slope of ~35°, after which subsequent avalanches bypass the apex to deposit mass at the cone’s base. From May onwards, mass redistribution is further modulated by the development of surface channels. Our findings demonstrate that time-lapse photogrammetry is a viable tool for monitoring dynamic glacier surfaces and provide rare empirical evidence of the dominant role avalanches play in glacier mass budgets.