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

泥石流是沉积物和水的混合物，是世界各地山区的主要地质灾害。泥石流灾害的准确评估在很大程度上取决于我们对泥石流如何搬运和存款沉积物，以及它们如何与河道相互作用和改变河道的了解。泥石流在流经河道网络时会造成复杂的三维侵蚀和沉积模式，对当前和未来的流动灾害产生重要影响。河岸和河床的侵蚀可能危及基础设施，并可能在事件期间显著增加流量（从而增加危害）。相反，广泛的沉积可能导致河道输送沉积物的能力下降，使后来的水流更有可能放弃河道并威胁附近地区。由于这些原因，重要的是要了解泥石流沟道中的侵蚀和沉积模式，以及它们与水流特性的关系（例如，体积、成分、速度和流动深度）。哪种类型的水流会产生最多的河岸侵蚀？哪种水流导致河床广泛沉积，从而增加了河道废弃的可能性？流量和通道变化量之间是否存在简单的关系？我们建议解决这些问题，通过测量非常高分辨率的表面地形之前和之后的个别碎屑流事件的伊尔格拉本扇，瑞士西南部。伊尔格拉本扇非常适合这种情况，因为自2000年以来，每年夏天都会发生4-6次泥石流;因此，瑞士联邦森林、雪和景观研究所（WSL）的项目合作伙伴对该扇进行了密切监测。所有穿过该扇的泥石流都被记录下来，它们的体积、速度、成分、深度和密度都被独立测量，使其成为世界上研究泥石流的独特地点。我们将使用高分辨率地面激光扫描仪对地形进行反复勘测，这些扫描仪沿伊尔格拉本扇头部附近300米的研究范围以沿着0.1米的点间距生成表面的三维表示。在发生任何泥石流之前，将在春季进行一组扫描，以确定河床和河岸的“起始”地形。在夏季一次大规模泥石流通过后，我们将重新扫描地形，以创建一个显示侵蚀和沉积区域的差异图。经过几次不同的水流事件后，这些地图将使我们能够将水流的类型和大小与其对河道的影响联系起来，还将使我们能够了解水流在穿过风扇时损失或增加了多少沉积物。我们的研究结果将补充WSL对Illgraben风扇的监测工作，并将与参与灾害评估和缓解的地方当局免费分享。

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.