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

泥石流是泥沙和水的混合物，在世界各地的山区形成了主要的地质灾害。对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输送和沉积泥沙，以及它们如何与泥石流河道相互作用和改变的知识。泥石流在河网中移动时，造成复杂的三维侵蚀和沉积模式，对目前和未来的水流危害造成重大后果。堤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(从而增加危险)。相反，大范围的淤积可能会导致河道输送泥沙的能力下降，使后来的水流更有可能放弃河道，威胁到附近地区。出于这些原因，必须了解泥石流河道中的侵蚀和沉积模式，以及它们与水流特征(例如，体积、成分、速度和水流深度)之间的关系。哪种类型的水流产生的岸滩侵蚀最多？哪些水流会导致河床上的广泛淤积，从而增加河道放弃的可能性？流量和渠道变化量之间是否存在简单的关系？我们建议通过测量瑞士西南部伊尔格拉本扇个别泥石流事件前后的极高分辨率地表地形来解决这些问题。Illgraben风扇非常适合这样做，因为自2000年以来，它每年夏天都会有4-6次泥石流；因此，它受到瑞士联邦森林、雪和景观研究所(WSL)的项目合作伙伴的密切监测。所有穿过风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，使这里成为世界上研究泥石流的独特地点。我们将使用高分辨率的地面激光扫描仪对地形进行重复测量，该扫描仪将沿着伊尔格拉本风扇头部附近300米的研究范围产生点间距为0.1米的表面三维表示。在泥石流发生之前，在春季进行一组扫描，将用来确定河床和河堤的开始地形。在夏季经过一场主要的泥石流后，我们将重新扫描地形，以创建一张显示侵蚀和沉积区域的不同地图。在几次不同的水流事件之后，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获得了多少泥沙。我们的结果将补充WSL对Illgraben风扇的监测工作，并将免费与参与危险评估和缓解的地方当局分享。

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.