Analogue study of particle segregation in pyroclastic density currents, with implications for the emplacement mechanisms of large ignimbrites

Abstract Analogue flume experiments were conducted to investigate the transport and sedimentation behaviour of turbulent pyroclastic density currents. The experimental currents were scaled approximately to the natural environment in three ways: (1) they were fully turbulent; (2) they had a very wide range of particle sizes and associated Rouse numbers (the ratio of particle settling velocity to effective turbulent eddy velocity in the current); and (3) they contained particles of two different densities. Two sets of surge‐type experiments were conducted in a 5 m long, water‐filled lock‐exchange flume at five different volumetric particle concentrations from 0·6% to 23%. In one set (one‐component experiments), the currents contained just dense particles; in the other set (two‐component experiments), they contained both light and dense particles in equal volume proportions. In both sets of experiments, the population of each component had a log‐normal size distribution. In the two‐component experiments, the size range of the light particle population was selected in order to be in hydrodynamic equivalence with that of the dense particles. Dense particles were normally graded, both vertically and downstream, in the deposits from both sets of experiments. The mass loading (normalized to the initial mass of the suspension) and grain size of the dense component in the deposits decreased with distance from the reservoir and were insensitive to initial total particle concentration in the currents. On the other hand, in the two‐component experiments, the light particles were extremely sensitive to concentration. They were deposited in hydrodynamic equivalence with the dense particles from dilute currents, but were segregated efficiently at concentrations higher than a few per cent. With increasing particle concentration, the large, light particles were carried progressively further down the flume because of buoyancy effects. Deposits from the high‐concentration currents exhibited reverse vertical grading of the large, light particles. Efficient segregation of the light component was observed even if the bulk density of the current was less than that of the light particles. In both sets of experiments, marked inflexions in the rate of downstream decline in mass loading and maximum grain size of the dense component can be attributed to the presence of two different particle settling regimes in the flow: (1) particles with Rouse numbers >2·5, which did not respond to the turbulence and settled rapidly; and (2) particles with Rouse numbers <2·5, which followed the turbulent eddies and settled slowly. The results are applied to the transport and sedimentation dynamics of pyroclastic density currents that generate large, widespread ignimbrites. Field data fail to reveal significant departures from aerodynamic equivalence between pumice and lithic clasts in three such ignimbrites: the particulate loads of some large ignimbrites are transported principally in turbulent suspensions of low concentration. In some ignimbrites, the well‐developed inflexions in curves of maximum lithic (ML) size vs. distance can be attributed to the existence of distinct high and low Rouse number particle settling regimes that mark the transition from an overcharged state to one in which the residual particulate load is transported more effectively by turbulence.     

抗拔桩设计方法

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大冲击功液动锤的研究及其应用

从液动锤基础理论出发，依据理论计算公，科学建立数学模型，开发计算机软件，对液动锤内部动力过程及工作参数进行计算机模拟仿真电算，寻求大幅度提高冲击功的途径和方法。     

福建省数字城市大比例尺DLG数据质量问题及检查方案研究

随着大比例尺DLG数据的广泛应用,其数据质量检查是控制数据质量的重要环节。本文针对以大比例尺地形图为数据源的DLG数据,从生产实际出发,结合其数据特点,针对现有的数据质量问题,从检查内容和检查方式上制订了数据检查方案,为数据质量检查工作给予一定的参考,提高数据质量检查的效率与准确度。     

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将地理国情普查工作中地表覆盖和地理国情要素两个重要成果之间的套合关系进行梳理,依据各项规定和生产实践经验,对重点问题进行整理和分析,明确了两者相互关系并提供一些有益的建议,为提高地理国情普查成果质量提供帮助。     

TEQC在河北CORS系统站点环境检测中的应用

介绍河北省卫星定位综合服务系统的建设情况,讨论TEQC软件对参考站周边环境的检测方法,给出了质量控制指标。并针对河北省卫星定位综合服务建设过程中存在问题的几个参考站,应用TEQC软件进行数据质量检查,对处理结果进行分析和探讨。     

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电子报批是规划管理部门实施信息化办公的一个重要方向和趋势,能够极大地提高规划管理部门办案的规范性、科学性、严密性和高效性。本文提出了中小城市实施电子报批的总体方案,并对系统建设的框架、功能、建设策略、实施模式等关键问题进行了深入地探讨。提出的策略和技术思路对中小城市实施电子报批有较好的指导意义。     

基于ArcSDE的地理数据入库技术研究

分析了GIS开发中空间数据的导入原理和技术,提出了一种基于ArcSDE的自建命令,实现了一种简单高效的空间数据入库方法。     

用时间序列模拟和预报天山乌鲁木齐河源1号冰川年径流总量

本文把1958—1984年天山乌鲁木齐河源1号冰川的融水径流总量的时间序列X(t),分解为趋势项L(t)、周期项P(t)、平稳项S(t)、随机项ε(t),使该时间序列表示为X(t)=L(t)+P(t)+S(t)+ε(t)。采用非线性回归提取L(t),用谱分析和Fourier级数提取P(t),余差用自回归方程建模,用上述项的叠加作出预报,按相关指数公式计算R=0.90,效果良好。     

Magnetic interpretation of pairs of sheets and their equivalence to dykes

Magnetic anomaly profiles over two thin sheets separated by a small distance resemble those of dykes andvice versa. Interpretation of anomalies over a pair of sheets based on the magnetic properties of dykes predicts a dyke whose centre lies midway between the positions of the sheets. The dyke, on the other hand, is magnetically equivalent to a pair of sheets, both lying at the same depth and having the same magnetization.The magnetic anomalies due to a pair of sheets can be interpreted by framing linear equations between the anomalies and their distances measured from an arbitrary reference. Application of this method to anomalies of dipping sheets with a finite depth extent is indicated.