An unnatural disaster—the 2021–2024 landslide at Nordic Waste, Denmark

Abstract The 2021–2024 Nordic Waste Landslide, located near the village of Ølst in East Jutland, Denmark, was a significant geohazard event, occurring within a former clay pit that had been repurposed as a landfill for polluted soil. This study provides a first analysis of the landslide’s development, characteristics, and causative factors. The slow-to-moderate-moving landslide gained public attention in December 2023 when it protruded beyond the landfill area, threatening to reach Ølst and dispense pollutants to large downstream watercourses. We analyzed the landslide’s evolution and causative factors utilizing aerial imagery, digital elevation models (DEMs), satellite data, and field observation. The landslide’s evolution can be categorized into two distinct phases driven by two modes of soil deposition. In Phase 1, spanning 2021 to spring 2023, the landslide developed due to gradual vertical soil aggradation on the gently sloping surface of the former clay pit. In Phase 2, from spring 2023 to January 2024, the landslide developed rapidly due to substantial soil dumping on the western slope of the clay pit, forming two earthflows that moved east, forcing acceleration in most of the Phase 1 landslide. The Phase 2 landslide encompassed approximately 1.2 million cubic meters of soil, accounting for over half of the total soil deposited at Nordic Waste between 2015 and 2023. After the practice of dumping soil at the edge of the clay pit ended the landslide slowed down, eventually stopping by late January 2024. This was while the water table was at a record high and still increasing, pointing to soil deposition, and not elevated water table, as the main preconditioning factor. Runout modelling indicated the landslide was best replicated using a Voellmy friction angle of atan(μ) = 2.9°. However, even in a worst-case modelling scenario, assuming an unrealistically low atan(μ) of 1.7°, the landslide did not reach inhabited areas in Ølst. From a landslide point of view, our findings emphasize the need for land-use planning and regulatory frameworks of landfills to take slope instability into account. Furthermore, they highlight the need for increased public awareness, and for educating decision-makers and oversight authorities into the risks associated with landslides.