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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Turbulent boundary layer shear flows as an approximation of base surges at Campi Flegrei (Southern Italy)
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Turbulent boundary layer shear flows as an approximation of base surges at Campi Flegrei (Southern Italy)

机译：Campi Flegrei（意大利南部）的湍流边界层剪切流近似为基础涌浪

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Small-volume base-surge deposits of Astroni and Agnano-Monte Spina eruptions at Campi Flegrei consist of sets of beds formed by the passage of multiple pyroclastic density currents. Each set is made by a fining-upward sequence consisting, from base to top, of: (1) an inversely graded layer made of bombs and lapilli; (2) a finely laminated layer with wavy bedforms made of coarse ash and fine lapilli; (3) a structureless fine-ash bed. Lack of discontinuities at layer contacts allows us to hypothesize that each set of beds represents the passage of one pyroclastic density current. The sequence formed as a function of the decreasing particle transportation capability of the base surge during the waxing-sustained-waning phases of flowage. We make the simplifying assumptions that the flow, on passing from the deposition of the inversely graded layer to the deposition of the finely laminated layer, had low particle concentration, was quasi-steady and incompressible. By such assumptions, some of the equations that regulate sediment mechanics are applicable to the base surges of Campi Flegrei to reconstruct flow density and shear velocity in between the deposition of the two layers. Structural and textural features of deposits stress the importance of shear and turbulence during transportation and suggest that the turbulent boundary layer shear flow is a good approximation to such pyroclastic density currents. We used the Coles formula to reconstruct velocity as a function of flow height during the formation of the finely laminated layer of Astroni and Agnano-Monte Spina base surges. The methodology we used allows the reconstruction of flow parameters just at a particular point in time and space of base-surge evolution, and does not provide the complete picture of the flow history of pyroclastic density currents. Nevertheless, flow velocity and density data, as obtained with the proposed approach, point to dynamic pressure values that are compatible with those reported for similar currents on other volcanoes. We conclude therefore that the results obtained give approximate but helpful information on the nature of such hazardous events at Campi Flegrei.
机译：Campi Flegrei的Astroni和Agnano-Monte Spina喷发的少量基础涌动沉积物由通过多次火山碎屑流通过形成的多套床组成。每组都是由向上向上的顺序构成的，从下到上依次为：（1）由炸弹和青金石制成的反梯度层； （2）具有由灰烬和细小罗非鱼制成的波浪状床身的精细层压层； （3）无结构的细灰床。层接触处缺乏不连续性，使我们可以假设每组床代表一个火山碎屑密度流的通过。形成的序列是在流动的上蜡-保持-减弱阶段中基础喘振的颗粒输送能力降低的函数。我们作出简化的假设，即从逆梯度层的沉积到精细层压层的沉积的流动，其颗粒浓度低，是准稳定且不可压缩的。通过这样的假设，一些调节沉积物力学的方程式可应用于Campi Flegrei的基波，以重建两层沉积之间的流量密度和剪切速度。沉积物的结构和质地特征在运输过程中强调了剪切和湍流的重要性，并表明湍流边界层剪切流很好地近似于此类火山碎屑密度流。我们使用Coles公式在Astroni和Agnano-Monte Spina基础浪涌的精细层压层的形成过程中，将流速作为流动高度的函数进行重构。我们使用的方法仅允许在基础电涌演化的特定时间和空间上重建流量参数，而不能提供火成碎屑密度流的流动历史的完整图景。尽管如此，通过所提出的方法获得的流速和密度数据指向的动压值与其他火山上类似流所报告的动压值兼容。因此，我们得出的结论是，获得的结果提供了有关Campi Flegrei此类危险事件性质的近似但有用的信息。

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