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

至关重要的是，能够预测危险的自然岩石碎片流可能移动的距离；这是流量跳动距离。必须知道跳动距离，以便准确评估地球物理质量流（如雪板雪崩、泥石流和火山碎屑流）对人口稠密地区造成的风险。在这些流动的高固体分数区域，大的和/或低密度的颗粒通常分离到速度最大的表面，并被输送到边缘形成砾质流锋。在火山碎屑流和碎屑流中，流动的流动性源于高基底孔隙压力，这降低了运动的摩擦阻力。由于孔隙压力在粗碎屑中比在细颗粒中消散得快得多，因此岩石边缘比流动内部承受更大的运动摩擦阻力。这可能导致锋面不稳定和陡坡上的浪涌。在较浅的斜坡上，大/低密度的颗粒能够停下来，流锋面自发组织起来，使更有阻力的巨石材料积聚在两侧，形成横向堤防。大/低密度颗粒可以通过两种机制移动到一侧：(i)它们可以被后面的物质整体推开，或者（ii）它们可以被尺寸/密度分离超越并再循环，直到它们在堤防中达到稳定的位置。这两种机制在大多数流动中都很活跃。有点矛盾的是，在这些巨石周围增加的运动阻力会导致更长的跳动距离。这是因为堤防形成了一个通道，阻止了内部水流的横向扩散，有效地限制了水流向前推进。本提案旨在通过大规模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有力结合，研究分离、流动流动性和堤防形成的过程。这将大大提高我们预测潜在危险的地球物理质量流的运动和最大跳动距离的能力，并将使沉积学家更好地了解形成包括粗叶状终止和堤防在内的沉积物的母流。

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.