Monitoring current impacts of climate change on slope stability in the Ormonts valley, western Switzerland

The Upper Ormonts Valley (Ormonts-Dessus), located in western Switzerland, corresponds to the catchment area of the Grande Eau River and is located on the border between the Pre-Alps and the Alps. The valley has a general east-west orientation and is bounded by the Pic Chaussy – La Para massif to the north, the Diablerets massif to the east and southeast, and the Chamossaire – Col de la Croix massif to the south. Historically, it has been exposed to many natural hazards such as avalanches, floods, landslides, rockfalls and debris flows. The southern slope of the Pic Chaussy – La Para massif, facing the valley, is subject to avalanches as well as rockfalls, debris flows and shallow landslides. This slope has been monitored using temperature sensors near the summit, combined with data from a SLF weather station (Swiss National Institute for Snow and Avalanche Research), and annual lidar scans from the opposite side of the valley. In the Diablerets massif, two tributaries of the Grande Eau River, the Dar (10 km²) and the "upper" Grande Eau (12 km²), were also studied. After the confluence of the two alpine streams, the Grande Eau flows through the village of Les Diablerets, a major tourist destination in the area. Here, floods and high bedload events have occurred, and riverbank erosion is common. The Dar glacial cirque is an area of high sediment production due to permafrost thaw, while landslides are common in the lower part of the Dar catchment. Both tributaries have been monitored using time-lapse wildlife cameras and annual lidar scans. The Dar catchment has been studied more extensively using DoD’s, drone orthomosaics, lidar scans and sediment budget estimates. A drone lidar scan is planned for this spring. Despite  the short observation period (2023-2024), some drivers of change have been identified. Mild winters and wet springs such as that of 2023/2024 resulted in exceptional precipitations at mid-elevations, as well as large daily temperature variations at high elevations. Wet conditions such as these favored shallow landslides, strong riverbank erosion and a few high discharge events in the Grande Eau River. Changes in rockfall frequency have not yet been observed. And the effects of a stronger winter like 2024/2025 remain to be seen.