Saltation threshold for pyroclasts at various bedslopes: Wind tunnel measurements
Saltation threshold for pyroclasts at various bedslopes: Wind tunnel measurements
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DOI：
10.1016/j.jvolgeores.2014.03.011
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发表时间：
2014-05-15
影响因子：
2.9
通讯作者：
Dingwell, Donald B.
中科院分区：
文献类型：
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作者：
Douillet, Guilhem Amin;Rasmussen, Keld R.;Dingwell, Donald B.
火山碎屑密度流是与爆发性火山活动有关的最具破坏性的危险之一。这种破坏性的性质不仅迫切需要而且也阻碍了对它们的物理特性进行现场测量。然而，由此产生的沉积物提供了其沉积阶段物理学的证据。稀火山岩密度流沉积物经常表现出沉积和侵蚀的重复循环，从而深入了解了地面上的湍流剪切沿着。通过在严格限制的实验室条件下校准这种流动的物理性质，可以大大提高这种实地观测的利用率。在这里，风洞测量使用火山碎屑颗粒。本文计算了火山碎屑床上方风的跃移阈值和表面粗糙度。这些结果有助于将现场观测结果与流动底部湍流剪切的定量值联系起来。研究了火山渣和浮石颗粒作为粒度（1 φ级在0.125和4 mm之间）的函数，以及床面坡度（-20度到+25度，10度步长）的影响。结果指出，占主导地位的控制密度，粒度和，与以前的假设相反，适度不同，从获得的结果为圆形珠适当利用，数据集能够建立天然矿床的粒度和剪切现存的在其侵位之间的联系。根据在现场观察到的沉积结构的类型，跃移阈值可以被用来作为一个最小或最大的剪切极限期间侵位的稀火山碎屑岩密度流存款。逆冲加积叠层可能涉及跃变阈值作为上限，而对于截断事件，它必须被克服。颗粒浓度内的流，火山碎屑密度电流的一个关键参数和数据的有效性程度的影响，进行了讨论。(C)2014作者由爱思唯尔公司出版
Pyroclastic density currents represent one of the most destructive hazards associated with explosive volcanism. This destructive nature does not only urge the need for but also prevents the obtainment of in situ measurements of their physical characteristics. The resulting deposits offer, however, evidence of the physics of their sedimentation phase. Deposits of dilute pyrodastic density currents frequently exhibit repeated cycles of deposition and erosion, yielding insights into the turbulent shearing along the ground. The utilization of such field observations can be greatly enhanced by the calibration of physical properties of such flows under well-constrained laboratory conditions. Here, wind tunnel measurements were performed using pyroclastic particles. The saltation threshold and surface roughness length were calculated for wind above a pyroclastic bed. The results serve as an aid in linking field observations to quantitative values of turbulent shear at the base of a flow. Scoria and pumice particles were investigated as a function of grain size (1 phi fractions between 0.125 and 4 mm), as well as the influence of bedslope (-20 degrees to + 25 degrees in 10 degrees steps). The results point to the dominant control of density, grain size and, contrary to previous assumptions, differ moderately from results obtained for round beads Properly utilized, the dataset enables the establishment of a link between the grain size of natural deposits and the shearing extant during their emplacement. Depending on the type of sedimentary structure observed in the field, the saltation threshold can be used as a minimal or a maximal shearing limit during emplacement of dilute pyroclastic density current deposits. Stoss-aggrading laminations likely involve the saltation threshold as an upper limit, whereas for truncation events it must have been overcome. The effect of particle concentration within the flow, a critical parameter for pyroclastic density currents and the extent of validity of the data, are discussed. (C) 2014 The Authors. Published by Elsevier B.V.