Segregation and levee formation in geophysical mass flows and their feedback on runout distance

至关重要的是，能够预测危险的天然岩石碎石流可能传播的距离；这就是流量溢出距离。必须知道超限距离，以便准确评估雪板雪崩、泥石流和火山碎屑流等地球物理物质流在人口稠密地区构成的风险。在这些流动的高固相分数区域，大颗粒和/或低密度颗粒通常偏析到速度最大的表面，并被输送到边缘形成巨石状流动前锋。在火山碎屑流和泥石流中，水流的流动性是由于高的基础孔压降低了运动的摩擦阻力造成的。由于孔隙压力在粗碎屑中比在细粒材料中消散得更快，因此巨石边缘的运动摩擦阻力比流动内部的摩擦阻力大得多。这可能会导致锋面不稳定和陡峭斜坡上的涌浪。在浅斜坡上，大/低密度颗粒能够停留在那里，流动锋会自发地组织起来，使更具阻力的巨石物质聚集在两侧，形成侧向堤坝。大/低密度颗粒可以通过两种机制移动到一边：-(I)它们可以被后面的材料集体推到一边，或者(Ii)它们可以通过尺寸/密度分离被溢出并重新循环，直到它们达到堤坝中的稳定位置。这两种机制在大多数流中都是活跃的。有些自相矛盾的是，在这些巨石周围增加运动阻力可能会导致更长的跳动距离。这是因为堤坝形成了一条渠道，阻止了内部水流的横向扩散，实际上限制了水流向前推进。这项建议旨在利用大型水槽试验和现场试验、小规模实验室试验以及理论和计算模型的强大组合，研究分离、水流流动和堤防形成的过程。这将显著提高我们预测潜在危险地球物理物质流的运动和最大径流距离的能力，并将使沉积学家更好地了解形成沉积物的母流，包括粗大的分叶末端和堤坝。

It is crucially important to be able to predict the distance to which a hazardous natural flow of rocky debris might travel; this is the flow runout distance. Runout distance has to be known for accurate assessment of the risks posed in populated areas by geophysical mass flows, such as snow slab avalanches, debris-flows and pyroclastic flows. In the high solids fraction regions of these flows the large and/or low-density particles commonly segregate to the surface, where the velocity is greatest, and are transported to the margins to form bouldery flow fronts. In pyroclastic and debris-flows the flow mobility results from high basal pore pressures that reduce the frictional resistance to motion. Since the pore pressure is dissipated much more rapidly amidst the coarse clasts than in the finer grained material, the bouldery margins experience much greater frictional resistance to motion than the flow interior. This can lead to frontal instabilities and surge waves on steep slopes. On shallow slopes, where the large/low density particles are able to come to rest, the flow front spontaneously organizes itself so that the more resistive bouldery material accumulates at the sides to form lateral levees. There are two mechanisms by which the large/low density particles can move to the side:- (i) they can be shoved en masse out of the way by the material behind or (ii) they can be over-run and recirculated by size/density segregation until they reach a stable position in the levee. Both mechanisms are active in most flows. Somewhat paradoxically, an increased resistance to motion in these bouldery perimeters can lead to much longer runout distances. This is because the levees form a channel that resists lateral spreading of the interior flow, in effect constraining the flow to push forward. This proposal aims to study the processes of segregation, flow mobility and levee formation, using a powerful combination of large-scale flume tests and field experiments, small-scale laboratory experiments and theoretical and computational modelling. This will significantly improve our ability to predict the motion and maximum runout distance of potentially hazardous geophysical mass flows, and will give sedimentologists an improved understanding of the parent flows that form deposits including coarse lobate terminations and levees.