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

火山碎屑密度流的流动性：结合现场和实验技术来了解侵蚀的控制和后果

• 批准号: 1347385
• 负责人: Brittany Brand
• 金额: $ 26.21万
• 依托单位: Boise State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-03-15 至 2018-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1347385&HistoricalAwards=false
• 关键词: Mobility; Pyroclastic; Density; Currents; Integrating

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.

火山碎屑密度电流（PDC）是与爆炸性火山作用相关的最危险危害。 这些不可预测的电流包括灼热的气云，灰烬和岩石，它们以巨大的力量和速度沿着火山爆发的斜坡沿着山坡传播。这些毁灭性电流的驱动力是相对于环境空气的密集性（由于灰分和岩石的混合物）。 PDC将穿越整个景观，可能以外的火山侧面许多英里，直到电流内的灰烬和岩石掉落，并且电流密度降低以匹配环境空气。尽管PDC及其致命的后果存在普遍存在，但其行为的许多基本方面和在跳动距离上的控制仍然知之甚少。我们对这些电流的理解中最重要的差距之一是侵蚀PDC流向表面的机制，以及将底物材料混合到下游流动动力学上的电流中的影响。鉴于跳线距离的主要控制是相对于环境空气的电流密度，因此由于底物的夹带而导致电流的膨胀将影响并可能延长最终的跳动距离，从而增加破坏性潜力。 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). 这项工作将分为三个阶段。第一阶段包括对1980年5月18日山圣海伦斯（MS）（MSH）爆发的暴露PDC沉积物的现场研究，该研究基于先前的主要研究人员这些沉积物的作品。现场工作包括质地，颗粒测定法和组件研究，以确定（或推断）PDC沉积物中侵蚀岩石的来源，有利于侵蚀的底物条件（例如斜率，表面粗糙度）以及侵蚀对PDC侵蚀的下游流动动态的影响。第二阶段涉及缩放实验，以探索在底座上通过剪切而在流化电流的头部与不足的剪切侵蚀的一般条件，这些条件是基于合作者Roche博士的基本工作。第三阶段包括通过评估（1）地形障碍对侵蚀和下游流动动力学的影响，从MSH产生的特定探索我们的解释和假设的实验，（2）密度梯度对基础剪切应力和侵蚀的增加的作用，以及（3）在实验室流动中的织物在功能中的织物的发展，并在效率上进行了综合效果，并在实验室流动方面的作用。最终目标是对PDC损伤潜力和跳动距离的控制有更全面的了解，这将使对与未来爆炸性喷发相关的危害进行更好的评估和缓解。