Mobility of Pyroclastic Density Currents: Integrating Field and Experimental Techniques to Understand the Controls and Consequences of Erosion

火山碎屑密度流是与爆发性火山活动有关的最危险的灾害。 这些不可预测的气流由灼热的气体、火山灰和岩石云组成，以巨大的力量和速度沿着火山喷发的斜坡向下移动。这些毁灭性气流的驱动力是它们相对于环境空气的密度（由于火山灰和岩石的混合物）。PDC将穿过景观，可能超出火山侧翼许多英里，直到电流中的灰烬和岩石脱落，电流密度降低到与环境空气相匹配。尽管PDC的普遍性及其致命的后果，他们的行为和控制的跳动距离的许多基本方面仍然知之甚少。在我们对这些水流的理解中，最重要的差距之一是PDC流动表面的侵蚀机制，以及将基质材料混合到水流中对下游流动动力学的影响。鉴于跳动距离的主要控制是电流？的密度相对于环境空气，膨胀的电流由于夹带的基板将影响和可能延长的最终跳动距离，从而增加破坏的潜力。这项工作结合了现场技术和规模化的实验室实验，以研究PDC条件和侵蚀之间的复杂关系，以及侵蚀对电流流动性的影响。该项目将探索对电流从基底侵蚀能力的三个主要参数的控制：坡度、流化程度（孔隙压力）和基质性质（颗粒尺寸、颗粒密度、可侵蚀床的厚度和基质粗糙度）。 这项工作将分三个阶段进行。第一阶段包括对1980年5月18日圣海伦山（MSH）喷发的暴露良好的PDC矿床进行实地研究，该研究以首席研究员之前对这些矿床的工作为基础。野外工作包括结构、粒度和成分研究，以确定（或推断）PDC矿床内侵蚀岩屑的来源、有利于侵蚀的基底条件（例如，坡度、表面粗糙度）以及侵蚀对侵蚀PDC下游水流动力学的影响。第二阶段涉及规模实验，以探索有利于通过底部剪切侵蚀的一般条件，而不是流化流头部的负压，这是建立在合作者Roche博士的基础工作之上的。第三阶段包括专门探索我们的解释和假设的实验，这些解释和假设是从MSH的第一阶段实地结果发展而来的，通过评估（1）地形障碍物对侵蚀和下游流动动力学的影响，（2）密度梯度增加对基底剪切应力和侵蚀的作用，（3）织物在实验室流动中的发展作为流化程度和与障碍物相互作用的函数。最终目标是更全面地了解PDC破坏潜力和跳动距离的控制，这将有助于更好地评估和减轻与未来爆炸性喷发相关的危害。

Pyroclastic density currents (PDCs) are the most dangerous hazard associated with explosive volcanism. These unpredictable currents consist of searing hot clouds of gas, ash and rock that travel down the slopes of erupting volcanoes with tremendous force and velocity. The driving force for these devastating currents is their dense nature (due to the mixture of ash and rock) relative to the ambient air. PDCs will travel across the landscape, potentially many miles beyond the volcano flanks, until the ash and rock within the current has dropped out and the current density decreases to match that of the ambient air. Despite the pervasiveness of PDCs and their deadly consequences, many fundamental aspects of their behavior and controls on runout distance remain poorly understood. One of the most important gaps in our understanding of these currents is the mechanism(s) for eroding into the surface over which a PDC flows, and the influence of mixing substrate material into the current on downstream flow dynamics. Given that the primary control on runout distance is a current?s density relative to the ambient air, bulking of the current due to entrainment of the substrate would influence and possibly extend the ultimate runout distance, thereby increasing destructive potential. This work combines field techniques and scaled laboratory experiments to examine the complex relationships between PDC conditions and erosion, and the consequence of erosion on current mobility.This project will explore the control of three main parameters on a current's ability to erode from the substrate: slope, degree of fluidization (pore pressure), and nature of the substrate (particle size, particle density, thickness of erodible bed and substrate roughness). This work will be conducted in three phases. The first phase includes field studies on the well-exposed PDC deposits from the May 18th, 1980 eruption of Mt St Helens (MSH), which builds on the previous work on these deposits of the lead investigator. Field work includes textural, granulometry and componentry studies to determine (or infer) the source of eroded lithics within the PDC deposits, the substrate conditions that favor erosion (e.g., slope, surface roughness) and the influence of erosion on downstream flow dynamics of the eroding PDCs. The second phase involves scaled experiments to explore the general conditions that favor erosion via shear at the base versus underpressure in the head of fluidized currents, which build on the fundamental work of collaborator Dr. Roche. The third phase includes experiments that specifically explore our interpretations and hypothesis developed from the phase one field results from MSH by assessing (1) the influence of topographic obstacles on erosion and downstream flow dynamics, (2) the role of an increased density gradient on basal shear stress and erosion, and (3) the development of fabric in laboratory flows as a function of degree of fluidization and interaction with obstacles. The ultimate goal is to develop a more comprehensive understanding of the controls on PDC damage potential and runout distance, which will enable better assessments and mitigation of the hazards associated with future explosive eruptions.