Large grains matter: contrasting bed stability and morphodynamics during two nearly identical experiments          下载免费PDF全文

Lucy G. MacKenzie  Brett C. Eaton 《地球表面变化过程与地形》2017,42(8):1287-1295

While the stabilizing function of large grains in step‐pool streams has long been recognized, the role they play in gravel‐bed streams is less clear. Most researchers have ignored the role of large grains in gravel‐bed streams, and have assumed that the median bed surface size controls the erodibility of alluvial boundaries. The experiments presented herein challenge this convention. Two experiments were conducted that demonstrate the significant morphodynamic implications of a slight change to the coarse tail of the bed material. The two distributions had the same range of particle sizes, and nearly identical bulk d₅₀ values (1.6 mm); however the d₉₀ of experiment GSD1 was slightly finer (3.7 mm) than that for experiment GSD2 (3.9 mm). Transport rates during GSD1 were nearly four times greater than during GSD2 (even though the dimensionless shear stress was slightly lower), and the channel developed a sinuous pattern with well‐developed riffles, pools and bars. During GSD2 the initial rectangular channel remained virtually unchanged for the duration of the experiment. The relative stability of GSD2 seems to be associated with a slightly larger proportion of stable (large) grains on the bed surface: at the beginning of GSD1, 3.5% of the bed was immobile, while almost twice as much of it (6.1%) was immobile at the beginning of GSD2. The results demonstrate that the largest grains (not the median size) exert first‐order control on channel stability. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Optical mineralogy,principles and practice by C. D. Gribble and A. J. Hall. UCL Press,London, 1992. No. of pages: 302. Price: £50.00 (hardback); £17.95 paperback     

W. S. MacKenzie 《Geological Journal》1995,30(1):80-80

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The dynamics of western North America: stress magnitudes and the relative role of gravitational potential energy, plate interaction at the boundary and basal tractions     

Lucy M. Flesch  William E. Holt  A. John Haines  Lianxing Wen  Bingming Shen-Tu 《Geophysical Journal International》2007,169(3):866-896

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I/I ratios in surface waters of the English Lake District     

M. Atarashi-Andoh  C. Schnabel  G. Cook  A.B. MacKenzie  A. Dougans  R.M. Ellam  S. Freeman  C. Maden  V. Olive  H.-A. Synal  S. Xu 《Applied Geochemistry》2007

Accelerator Mass Spectrometry (AMS) was used to measure ¹²⁹I/¹²⁷I ratios in surface sea, lake, and river water samples collected in 2004 and 2005 from the English Lake District and from SW Scotland, areas which are in relatively close proximity to the Sellafield nuclear fuel reprocessing plant in NW England. The ¹²⁹I/¹²⁷I ratios in surface water collected from the shore of the Irish Sea were in the range 2.8 × 10⁻⁶ to 8.2 × 10⁻⁶. These ratios are one order of magnitude higher than that of seawater collected from the Irish Sea in 1992, correlating with the increase in ¹²⁹I content of the Sellafield liquid effluent discharge over the last decade. The ¹²⁹I/¹²⁷I ratios in lakes in the Lake District were in the range 0.7 × 10⁻⁶ to 6.4 × 10⁻⁶ and decreased exponentially as a function of distance from Sellafield. Consideration of the relative variation of stable I concentrations and ¹²⁹I/¹²⁷I ratios suggests that Sellafield gaseous discharges may be the dominant source of ¹²⁹I to the lakes.  相似文献   

青藏高原东部上地幔各向异性及相关的壳幔耦合型式   总被引：10，自引：0，他引：10  

王椿镛  常利军  吕智勇  秦嘉政  苏伟  Paul Silver  Lucy Flesch 《中国科学D辑》2007,37(4):495-503

对国家数字地震台网和云南、四川、甘肃、青海区域数字地震台网, 以及布设在川、滇、藏地区的宽频带流动地震台网共116个台站所记录的远震SKS波形资料作偏振分析, 采用叠加分析方法求得每一个台站的SKS快波偏振方向和快慢波的时间延迟, 获得青藏高原东部及其邻近地区的上地幔各向异性图像. 将该地区全球定位系统(GPS)的观测结果与上地幔各向异性分布相结合作地壳-地幔耦合变形的分析, 研究表明青藏高原内部和高原外部的云南地区具有不同的壳幔变形特征, 在高原的东缘地区(大致位于川滇西部的26°~27°N之间)存在一个壳幔变形的横向过渡带. 过渡带以南地区的快波偏振方向从滇西南的S60°~70°E逐渐转变到滇东南的近东西向, 以北的滇西北部和川西南部, 快波偏振方向为近似的南北向. 高原内部表现为强壳幔耦合型, 高原外部则属于壳幔解耦型. 这一横向过渡带与地表的断裂走向不一致, 但在地壳和上地幔, 其地球物理场(如: 地壳厚度, 布格重力异常和构造应力方向等)都具有横向过渡的特征. 该横向过渡带邻近东喜马拉雅构造结, 在板块边界动力学上有着重要的意义.  相似文献   

Physical‐scale modelling of jökulhlaups (glacial outburst floods) with contrasting hydrograph shapes     

E. Lucy Rushmer 《地球表面变化过程与地形》2007,32(6):954-963

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X‐ray fluorescence measurements of the surface elemental composition of asteroid 433 Eros     

Larry R. Nittler  Richard D. Starr  Lucy Lim  Timothy J. Mccoy  Thomas H. Burbine  Robert C. Reedy  Jacob I. Trombka  Paul Gorenstein  Steven W. Squyres  William V. Boynton  Timothy P. Mcclanahan  Jesse S. Bhangoo  Pamela E. Clark  Mary Ellen Murphy  Rosemary Killen 《Meteoritics & planetary science》2001,36(12):1673-1695

Abstract— We report major element ratios determined for the S‐class asteroid 433 Eros using remote‐sensing x‐ray fluorescence spectroscopy with the near‐Earth asteroid rendezvous Shoemaker x‐ray spectrometer (XRS). Data analysis techniques and systematic errors are described in detail. Data acquired during five solar flares and during two extended “quiet Sun” periods are presented; these results sample a representative portion of the asteroid's surface. Although systematic uncertainties are potentially large, the most internally consistent and plausible interpretation of the data is that Eros has primitive Mg/Si, Al/Si, Ca/Si and Fe/Si ratios, closely similar to H or R chondrites. Global differentiation of the asteroid is ruled out. The S/Si ratio is much lower than that of chondrites, probably reflecting impact‐induced volatilization and/or photo‐ or ion‐induced sputtering of sulfur at the surface of the asteroid. An alternative explanation for the low S/Si ratio is that it reflects a limited degree of melting with loss of an FeS‐rich partial melt. Size‐sorting processes could lead to segregation of Fe‐Ni metal from silicates within the regolith of Eros; this could indicate that the Fe/Si ratios determined by the x‐ray spectrometer are not representative of the bulk Eros composition.  相似文献   

Sources of sediment to the Great Barrier Reef World Heritage Area     

McKergow LA  Prosser IP  Hughes AO  Brodie J 《Marine pollution bulletin》2005,51(1-4):200-211

To reduce sediment exports discharging to the Great Barrier Reef (GBR), it is essential to identify the sources of exported sediment. We used modelling of spatial sediment budgets (the SedNet model) to identify sources and deposition of sediment as it is transported through river networks. Catchments with high levels of land clearing, cattle grazing and cropping show the largest increases in sediment export compared with natural conditions. Hillslope erosion supplies 63% of sediment to the rivers. Gully erosion and riverbank erosion are lower sources of sediment at the GBR catchment scale, but they are important in some catchments. Overall, 70% of sediment exported from rivers comes from just 20% of the total catchment area, showing that much of the problem can be addressed in a relatively small area. This is a much more manageable problem than trying to reduce erosion across the entire GBR catchment. Areas of high contribution are all relatively close to the coast because of the high erosion and high sediment delivery potential.  相似文献   

A common sense approach for confronting coral reef decline associated with human activities     

Owen R  Mitchelmore C  Woodley C  Trapido-Rosenthal H  Galloway T  Depledge M  Readman J  Buxton L  Sarkis S  Jones R  Knap A 《Marine pollution bulletin》2005,51(5-7):481-485

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Mineralogical-petrological characterization of near-Earth asteroids     

Lucy A. McFadden  Michael J. Gaffey  Thomas B. McCord 《Icarus》1984,59(1):25-40

Studies of near-Earth asteroids are aimed at determining their dynamical and structural history. The mineralogy and petrology of 17 near-Earth asteroids are characterized using reflectance spectroscopy with ground-based telescopes as one method to address their major issues. Implications for the origin and evolution are discussed in a separate paper. Assuming the surfaces are composed of cosmically abundant materials, the presence of certain mineralogical species can be determined from diagnostic absorption features and spectral characteristics which have been studied under known laboratory conditions and understood in terms of crystal field theory. With one possible exception, the surface composition of near-Earth asteroids consists of common rock-forming minerals such as olivine, pyroxene, and phyllosilicates. Opaque components are present but cannot be mineralogically identified with existing experimental data. The spectrum of 2201 Oljato cannot be interpreted in terms of common rock-forming minerals. This spectrum was examined for cometary features because its high orbital eccentricity suggests a possible relation to comets. No common cometary features are identified in its spectrum. The predominance of mafic silicate absorption features in spectra of near-Earth asteroids compared to the majority of main-belt asteroids may be a primary compositional feature or may be the signature of relatively fresher asteroid material.  相似文献