Self-organization and run-out behaviour of geophysical mass flows

至关重要的是，必须预测地球物理物质流的超限行为，从而预测其影响范围和最大破坏力，并制定有效战略，以改善世界各地“危险”人口的安全。地球物理物质流包括广泛的自然灾害，包括雪崩、泥石流、火山碎屑流和泥石流。它们都是湿颗粒流或干颗粒流的例子，在这种情况下，“大”颗粒向速度最大的表面分离，并优先向流动前沿输送。在这里，它们可能会泛滥，因种族隔离而崛起，并被重新循环以产生巨石般的水流前缘。它们往往比颗粒较细的内部对运动的阻力更大，要么是因为颗粒更粗糙，要么是因为在碎屑和火山碎屑流中，它们分散了赋予流动性的内部孔隙压力。这些分离-迁移率反馈效应可能会导致破坏性的高质量浓度涌浪锋的发展，并可能导致扩散流自发地发展波瓣和下泄通道，从而很容易地将质量转移到长距离(超限)。这种自组织对危险评估和风险缓解具有重要意义，因为大浪涌可能具有极强的破坏性，而堤坝的导流作用可以显著改变影响区域。在我们以前的研究中，我们发展了一个深度平均的分离理论，允许分离-迁移率反馈效应很容易地纳入现有的地球物理质量流模型中。数值模拟表明，它们捕捉到了莱维德手指的形态，以及复杂的非线性粗化、分裂和合并行为，但也有一个意想不到的问题，表明与耗散有关的一些重要物理问题在模型中缺失。我们的目标是确定所涉及的物理耗散机制。将与美国地质调查局(USGS)的合作伙伴一起进行小规模模拟实验和大型水槽试验，以研究可对消散性质产生重要见解的关键水流，例如：(1)大颗粒再循环单体的大小；(2)巨石流锋的演变；(3)横浪的开始和粗化动力学；(4)堤墙之间的速度分布。我们还将前往圣海伦斯山的浮石平原，这是一个几乎是独一无二的天然实验室，拥有丰富的过程和条件信息，这些过程和条件导致了强烈的涌流和扩散流。这些沉积物现在已经被河流横切，这将允许对详细的横断面进行检查和采样，以确定1980年6月、7月和8月喷发的不同阶段沉积的浮石碎屑的大小和密度。我们的理论、计算、大小实验和现场工作的多方面方法是极其强大的，将为控制物理条件和过程提供关键的线索，并导致我们在理解这些复杂的非线性流动方面取得重大进展。

It is vitally important to anticipate the run-out behaviour of geophysical mass flows and thus anticipate their impact area and peak destructive power, to develop effective strategies to improve the safety of "at risk" populations throughout the world. Geophysical mass flows encompass a wide range of natural hazards including snow avalanches, debris-flows, pyroclastic flows and lahars. They are all examples of either wet or dry granular flows in which "large" particles segregate towards the surface, where the velocity is greatest, and are preferentially transported towards flow fronts. Here they may be over-run, rise up by segregation, and be recirculated to produce bouldery flow fronts. These tend to be more resistive to motion than the finer grained interior, either because the grains are rougher or because in debris- and pyroclastic flows they dissipate the internal pore pressures that confer mobility. These segregation-mobility feedback effects can lead to the development of damaging high-mass-concentration surge fronts and can cause spreading flows to spontaneously develop lobes and leveed channels that transfer the mass readily for long distances (run-out). Such self-organization has important implications for hazard assessment and risk mitigation, because large surges can be highly destructive and the channelizing effect of levees can significantly alter an impact area. In our previous research we developed a depth-averaged theory for segregation that allowed segregation-mobility feedback effects to be incorporated easily into existing geophysical mass flows models. Numerical simulations showed that these captured the morphology of leveed fingers, as well as complex nonlinear coarsening, splitting and merging behaviour, but there was also an unexpected problem indicating that some important physics, related to dissipation, is missing in the model. We aim to identify the physical dissipation mechanisms involved. Small-scale analogue experiments and large-scale flume tests with our United States Geological Survey (USGS) partners will be used to study key flows that yield important insights into the nature of the dissipation, e.g. (i) the size of large particle recirculation cells (ii) the evolution of bouldery flow fronts (iii) the inception and coarsening dynamics of roll-waves and (iv) the velocity profile between levee walls. We will also go to the Pumice Plain of Mount St Helens, which is a virtually unique natural laboratory rich with information on the processes and conditions that led to both strongly leveed flows as well as spreading flows. These deposits have now been cross-cut by streams, which will allow detailed transects to be examined and sampled to establish the size and density of pumice clasts that were deposited by the various phases of June, July and August 1980 eruptions. Our multi-fronted approach of theory, computation, large- and small-scale experiments and field work is extremely powerful and will shed critical light on the controlling physical conditions and processes, and lead to major advances in our understanding of these complex nonlinear flows.