Rapid assessment of sediment dynamics on an active debris-flow fan

Debris flows are mixtures of sediment and water that form a primary geologic hazard in mountainous areas worldwide. Accurate assessment of debris-flow hazard depends very much on our knowledge of how debris flows transport and deposit sediment, and how they interact with, and change, their channels. Debris flows cause complex three-dimensional patterns of erosion and deposition as they move through a channel network, with important consequences for present and future flow hazards. Erosion of banks and beds can endanger infrastructure, and can significantly increase the flow volume (and thus the hazard) during an event. Conversely, widespread deposition can lead to a decrease in the capacity of the channel to transport sediment, making it more likely that later flows will abandon the channel and threaten nearby areas. For these reasons, it is important to understand both the patterns of erosion and deposition in debris-flow channels, and how they relate to the characteristics of the flows (e.g., volume, composition, velocity, and flow depth). Which types of flows produce the most bank erosion? Which flows lead to widespread deposition on the bed, thus increasing the likelihood of channel abandonment? Are there simple relationships between flow volume and the amount of channel change? We propose to address these questions by measuring very high-resolution surface topography before and after individual debris-flow events on the Illgraben fan, southwestern Switzerland. The Illgraben fan is well-suited to this because it has had 4-6 debris flows per summer since 2000; as a consequence, it is closely monitored by project partners at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL). All debris flows that cross the fan are recorded and their volume, velocity, composition, depth, and density are independently measured, making this a unique locality worldwide for studying debris flows. We will make repeated surveys of the topography using high-resolution, ground-based laser scanners, which produce three-dimensional representations of the surface with point spacings of 0.1 m along a 300 m study reach near the head of the Illgraben fan. A set of scans in spring, before the occurrence of any debris flows, will be used to define the 'starting' topography of the channel bed and banks. After passage of a major debris flow during the summer, we will re-scan the topography to create a difference map showing areas of erosion and deposition. After several different flow events, these maps will allow us to relate the type and size of flow to its effect on the channel, and will also enable us to understand how much sediment the flows lose or gain as they move across the fan. Our results will complement the monitoring efforts of the WSL on the Illgraben fan, and will be freely shared with local authorities involved in hazard assessment and mitigation.