Debris flows and river dynamics – a case study of two Alpine catchments in western Switzerland

The Diablerets Massif, in the Swiss Prealps, acts as a topographical barrier for northward and westward winds, contributing to the weather conditions and numerous hazards present in the area (avalanches, floods, landslides, etc.). Two main catchments emerge from this massif. The Dar River catchment originates at the Sex Rouge Glacier, comes down through a glacial circus, a series of cascades, and makes a sharp left turn before bordering the massif with a southwest direction. It joins the Grande Eau River catchment, which stems from the aggregation of torrents in the Creux du Champ, a steep-walled, complex-glacial circus followed by a U-shaped valley. After the confluence of both rivers, the Grande Eau crosses the village of Les Diablerets, a major tourist destination in the area. The erosion and sedimentation dynamics in both catchments are similar, with an increasing production of available sediment in the upper part of the catchments in recent years, due to glacial melt and permafrost degradation. As well as a gradual unearthing of the bedrock in the Dar river due to the erosion of moraine deposits after the first cascade, and increased sedimentation after the second cascade. In the Grande Eau, the enlargement of erosion areas in the top part of the circus has caused numerous debris flow events along the tributaries. These large volumes of transported sediments produce a very dynamic environment, with lateral erosion and a series of small landslides on both sides of the river. The sediment excess is managed through extraction, carried out by the municipality, before the main river crosses the town, but remains a major problem for the inhabitants. Large flooding events such as the June 2005 event are relatively rare, but small debris flows caused by very localized storm cells have become increasingly common throughout the valley, as well as high discharge events. Expected changes in climate, as depicted by the official Swiss climate change hydrological scenarios (HydroCH2018), include wetter regimes in winter and spring, and drier summers. This is already visible through recent events, such as the November 14th 2023 event, characterized by a high river discharge, and the debris flow of December 13th 2023, near Aigremont. The dynamics in both catchments were studied using historical aerial images, topographical data, LiDAR scans, wildlife cameras and meteorological data. Changes in the morphology of the riverbed, caused by major natural events in the last 50 years, have been established. Available sediment and erosion rates, as well as erosion and accumulation zones, have been determined for the Dar catchment and will be calculated for the upper part of the Grande Eau catchment. The aforementioned events as well as other particularities in the daily and seasonal dynamics of both catchments related to their source areas have also been analyzed, using the time-lapses for the wildlife cameras and precipitation data. Using the identified source zones for debris flow hazard and the river dynamics we expect to model potential large events in these catchments, comparing single and multi-phase event scenarios, including damming and outburst.