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.

火山碎屑密度流是与火山爆发有关的最危险的危险。这些不可预测的水流由灼热的气体、火山灰和岩石云组成，它们以巨大的力量和速度沿着火山喷发的斜坡流动。这些毁灭性洋流的驱动力是它们相对于环境空气的稠密性质(由于火山灰和岩石的混合物)。PDDC将在火山侧翼以外的地方穿行，直到洋流中的火山灰和岩石脱落，洋流密度降低到与环境空气相匹配。尽管PDC的普遍存在及其致命后果，但其行为的许多基本方面以及对跳动距离的控制仍然知之甚少。在我们对这些水流的理解中，最重要的空白之一是侵蚀到PDC流表面的机制(S)，以及在水流中掺入衬底材料对下游流动动力学的影响。考虑到对跳动距离的主要控制是相对于环境空气的电流-S密度，由于衬底的卷吸而导致的电流膨胀将影响并可能延长最终的跳动距离，从而增加破坏潜力。这项工作结合了现场技术和大规模的实验室实验，研究了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。本项目将探索三个主要参数对水流从基质中侵蚀能力的控制：坡度、流态化程度(孔压)和基质的性质(颗粒大小、颗粒密度、可侵蚀床厚度和基质粗糙度)。这项工作将分三个阶段进行。第一阶段包括对1980年5月18日圣海伦斯山(MSH)喷发以来暴露良好的PDC矿床的实地研究，该研究建立在首席调查员对这些矿床的先前工作的基础上。野外工作包括结构、粒度和成分研究，以确定(或推断)PDC矿床中侵蚀岩屑的来源、有利于侵蚀的基底条件(例如坡度、表面粗糙度)以及侵蚀对PDC下游流动动力学的影响。第二阶段包括大规模实验，以探索有利于底部剪切侵蚀和流态化水流头部负压侵蚀的一般条件，这些条件建立在合作者罗奇博士的基本工作基础上。第三阶段包括实验，通过评估(1)地形障碍物对侵蚀和下游水流动力学的影响，(2)密度梯度增加对底部剪应力和侵蚀的作用，以及(3)实验室流动中作为流态化程度和与障碍物相互作用的函数的织物的发展，具体探索我们对MSH第一阶段现场结果的解释和假说。最终目标是更全面地了解对PDC破坏潜力和超限距离的控制，这将使更好地评估和减轻与未来爆炸性喷发有关的危险。