Threshold of motion and settling velocities of mollusc shell debris: Influence of faunal composition     

Alissia Rieux  Pierre Weill  Dominique Mouaze  Clement Poirier  Farid Nechenache  Laurent Perez  Bernadette Tessier 《Sedimentology》2019,66(3):895-916

Bioclastic particles derived from mollusc shell debris can represent a significant fraction of sandy to gravelly sediments in temperate and cool‐water regions with high carbonate productivity. Their reworking and subsequent transport and deposition by waves and currents is highly dependent on the shape and density of the particles. In this study, the hydrodynamic behaviour of shell debris produced by eight mollusc species is investigated for several grain sizes in terms of settling velocity (measurements in a settling tube) and threshold of motion under unidirectional current (flume experiments using an acoustic profiler). Consistent interspecific differences in settling velocity and critical bed shear stress are found, related to differences in shell density, shell structure imaged by scanning electron microscopy and grain shape. Drag coefficients are proposed for each mollusc species, based on an interpolation of settling velocity data. Depending on the shell species, the critical bed shear stress values obtained for bioclastic particles fall within or slightly below empirical envelopes established for siliciclastic particles, despite very low settling velocity values. The results suggest that settling velocity, often used to describe the entrainment of sediment particles through the equivalent diameter, is not a suitable parameter to predict the initiation of motion of shell debris. The influence of the flat shape of bioclastic particles on the initiation of motion under oscillatory flows and during bedload and saltation transport is yet to be elucidated.  相似文献   

FTIR Spectroscopy Data and Carbon Isotope Characteristics of the Ophiolite‐hosted Diamonds     

Xiangzhen XU  Pierre CARTIGNY  Jingsui YANG  Yildirim DILEK  Fahui XIONG  Guolin GUO 《《地质学报》英文版》2019,93(Z2):38-38

We report new δ¹³C ‐values data and N‐content and N‐aggregation state values for microdiamonds recovered from peridotites and chromitites of the Luobusa ophiolite (Tibet) and chromitites of the Ray‐Iz ophiolite in the Polar Urals (Russia). All analyzed microdiamonds contain significant nitrogen contents (from 108 up to 589 ± 20% atomic ppm) with a consistently low aggregation state, show identical IR spectra dominated by strong absorption between 1130 cm^?1 and 1344 cm^?1, and hence characterize Type Ib diamond. Microdiamonds from the Luobusa peridotites have δ¹³C ‐PDB‐values ranging from ‐28.7‰ to ‐16.9‰, and N‐contents from 151 to 589 atomic ppm. The δ¹³C and N‐content values for diamonds from the Luobusa chromitites are ‐29‰ to ‐15.5‰ and 152 to 428 atomic ppm, respectively. Microdiamonds from the Ray‐Iz chromitites show values varying from ‐27.6 ‰ to ‐21.6 ‰ in δ¹³C and from 108 to 499 atomic ppm in N. The carbon isotopes values bear similar features with previously analyzed metamorphic diamonds from other worldwide localities, but the samples are characterized by lower N‐contents. In every respect, they are different from diamonds occurring in kimberlites and impact craters. Our samples also differ from the few synthetic diamonds; we also analyzed showing enhanced δ¹³C ‐variability and less advanced aggregation state than synthetic diamonds. Our newly obtained N‐aggregation state and N‐content data are consistent with diamond formation over a narrow and rather cold temperature range (i.e. <950°C), and in a short residence time (i.e. within several million years) at high temperatures in the deep mantle.  相似文献   

Analytical Solution for Modeling Discharge into a Tunnel Drilled in a Heterogeneous Unconfined Aquifer     

Jean‐Christophe Maréchal  Sandra Lanini  Bertrand Aunay  Pierre Perrochet 《Ground water》2014,52(4):597-605

Predicting transient inflow rates into a tunnel is an important issue faced by hydrogeologists. Most existing analytical solutions overestimate the initial discharge due to the assumption that drilling was instantaneous over the entire tunnel length. In addition, they assume a homogeneous system. An alternative model was recently developed for tunnels intersecting heterogeneous formations, but its application was reduced to the case of confined flow to deep tunnels in weakly diffusive aquifers. In this paper, we adapt existing analytical solutions for drainage systems to the specific case of a tunnel progressively drilled in a highly diffusive heterogeneous unconfined aquifer. The case of a tunnel overlying an impervious layer is analytically solved by applying the superposition principle, while the case of a tunnel constructed some distance above an impervious layer is solved by discretizing the tunnel length into subsectors. Both models can simulate transient discharge into a tunnel drilled at various speeds through a heterogeneous unconfined aquifer, and allow the prediction of discharge rates in shallow tunnels located in highly diffusive aquifers. We successfully applied this approach to a tunnel in heterogeneous volcanic rock.  相似文献   

Initial versus tangent stiffness‐based Rayleigh damping in inelastic time history seismic analyses          下载免费PDF全文

Pierre Jehel  Pierre Léger  Adnan Ibrahimbegovic 《地震工程与结构动力学》2014,43(3):467-484

In the inelastic time history analyses of structures in seismic motion, part of the seismic energy that is imparted to the structure is absorbed by the inelastic structural model, and Rayleigh damping is commonly used in practice as an additional energy dissipation source. It has been acknowledged that Rayleigh damping models lack physical consistency and that, in turn, it must be carefully used to avoid encountering unintended consequences as the appearance of artificial damping. There are concerns raised by the mass proportional part of Rayleigh damping, but they are not considered in this paper. As far as the stiffness proportional part of Rayleigh damping is concerned, either the initial structural stiffness or the updated tangent stiffness can be used. The objective of this paper is to provide a comprehensive comparison of these two types of Rayleigh damping models so that a practitioner (i) can objectively choose the type of Rayleigh damping model that best fits her/his needs and (ii) is provided with useful analytical tools to design Rayleigh damping model with good control on the damping ratios throughout inelastic analysis. To that end, a review of the literature dedicated to Rayleigh damping within these last two decades is first presented; then, practical tools to control the modal damping ratios throughout the time history analysis are developed; a simple example is finally used to illustrate the differences resulting from the use of either initial or tangent stiffness‐based Rayleigh damping model. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Long-Term Power Transmission Failures in Southeastern Brazil and the Geophysical Environment     

Magda A. S. Duro  Pierre Kaufmann  Fernando C. P. Bertoni  Emilio C. N. Rodrigues  José Pissolato Filho 《Surveys in Geophysics》2012,33(5):973-989

High-voltage transmission networks represent a large electrical circuit just above the ground subjected to a number of transient overcharges of various kinds, some of which may lead to failures. Some failures might be related to anomalies of the geophysical environment. We have analyzed one unprecedented long series of transmission grid failures (9?years) on high-voltage networks located in S?o Paulo state, southeastern Brazil, from 1998 to 2006, which includes an important fraction of the past solar activity cycle 23. Ninety-five distinct failure causes were given by the power line operator to explain the transmission grid shut downs. Most failures were attributed to atmospheric discharges, corresponding to 1,957 failures out of a total of 4,572 for the whole period at 138?kV, and 170 out of 763 at 440?kV, respectively. They correspond to less than one ten thousandth of the actual number of atmospheric discharges recorded in the same area, demonstrating the grid’s high resilience to breakdowns due to lightning. A clear concentration of failures in the region’s thunderstorm season has been found. A significant 67 and 77?% reduction in the number of failure rates per year has been found for the 138 and 440?kV grids, respectively, for the period studied, in good correspondence with the decay in the sunspot numbers. No obvious correlation was found between power failures and the planetary index of geomagnetic activity or major geomagnetic storms in the period, either on short or on long time scales. Assuming that the dependence of the electrosphere/ionosphere-ground coupling on the external geophysical environment plays a major role in explaining the reduction in power failures as the solar cycle wanes, it is suggested that the increase in atmosphere conductivity caused by the larger cosmic ray flux then reduces the threshold voltage required to produce lightning strokes, so reducing their effectiveness in disrupting high-voltage power lines.  相似文献   

The immediate environmental effects of tephra emission   总被引：1，自引：0，他引：1  

Paul Martin Ayris  Pierre Delmelle 《Bulletin of Volcanology》2012,74(9):1905-1936

The Earth’s history is punctuated by large explosive eruptions that eject large quantities of magma and silicate rock fragments into the atmosphere. These tephra particles can sometimes be dispersed across millions of square kilometres or even entire continents. The interaction of tephra with or in receiving environments may induce an array of physical, chemical and biological effects. The consequences for affected systems and any dependent communities may be chronic and localised in the event of frequent, small eruptions, while larger and rarer events may have acute, regional-scale impacts. It is, therefore, necessary to document the range of possible impacts that tephra may induce in receiving environments and any resulting effects in interconnected systems. We collate results from many studies to offer a detailed multidisciplinary and interdisciplinary review of the immediate post-eruptive effects of tephra emission into the atmosphere, onto vegetation, soil or ice/snow surfaces and in aquatic systems. We further consider the repercussions that may be induced in the weeks to years afterwards. In the atmosphere, tephra can influence cloud properties and air chemistry by acting as ice nuclei (IN) or by offering sites for heterogeneous reactions, respectively. Tephra on vegetation causes physical damage, and sustained coverage may elicit longer-term physiological responses. Tephra deposits on soils may alter their capacity to exchange gas, water and heat with the atmosphere or may have a specific chemical effect, such as nutrient input or acidification, on sensitive soils. Tephra deposition onto snow or ice may affect ablation rates. Rivers and lakes may experience turbidity increases and changes in their morphology as a result of fallout and prolonged (months or years) erosion from the tephra-covered catchment. In the first weeks after deposition, tephra leaching may affect river chemistry. The abundance and speciation of phytoplankton populations in lakes may be altered by tephra-induced changes in water chemistry or sediment–water nutrient cycling. In the oceans, tephra deposition may fertilise Fe-limited waters, with potential impacts on the global carbon cycle. Embracing the full complexity of environmental effects caused by tephra fall demands a renewed investigative effort drawing on interdisciplinary field and laboratory studies, combined with consideration of the interconnectivity of induced impacts within and between different receiving environments.  相似文献   

Influence of pit removal methods on river network position   总被引：1，自引：1，他引：0  

Laura Poggio  Pierre Soille 《水文研究》2012,26(13):1984-1990

Digital elevation models often contain depressions that result in areas described as having no drainage, referred to as sinks or pits. These depressions disrupt the drainage surface, which disrupt routing of flow over the surface. Most of the attributes that can be extracted from a digital elevation model rely on flow‐routing algorithms to calculate the upslope contributing area. There is little information on the influence of the various algorithms on the position and on the connectivity of the extracted networks. The aim of this study was to assess the effects of pit removal methods, data sources and flow‐routing algorithms on the position of river networks. The results show that all factors and methods have an impact on the position of the extracted networks. The pit removal method combining filling and carving extracted river networks closer to the reference, as well the elevation models with higher resolution. Single‐flow direction methods provided more accurate positioning of river network, in this test area where the drainage is generally well defined. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Oligotrophication from wetland epuration alters the riverine trophic network and carrying capacity for fish     

Christiane Hudon  Antonella Cattaneo  Anne-Marie Tourville Poirier  Philippe Brodeur  Pierre Dumont  Yves Mailhot  Jean-Pierre Amyot  Simon-Pierre Despatie  Yves de Lafontaine 《Aquatic Sciences - Research Across Boundaries》2012,74(3):495-511

Submerged aquatic vegetation (SAV) constitutes a major component of fish habitat, providing support for epiphytes and invertebrates as well as shelter from predators. The effects of wetland epuration from a mesotrophic to a nearly oligotrophic state were examined over a 15?km long reach of the St. Lawrence River under the direct influence of major farmland tributaries. We hypothesized that the nutrient-enriched zone would support a higher biomass of SAV, epiphytes, macroinvertebrates and fish than the nitrogen-deficient epurated zone located downstream of the wetland. Predictions included that the enriched habitat would support a richer fish assemblage, with higher biomass and growth of juvenile yellow perch than found in the epurated zone. Results supported these hypotheses, demonstrating the chain of effects of nutrient reduction on the biomass of SAV (fourfold drop), invertebrate prey (ninefold), small (threefold) and large (1.5-fold) fish between the two zones. In addition to the reduction in SAV biomass, the replacement of filamentous chlorophytes by benthic mats of filamentous cyanobacteria in the epurated zone resulted in a less complex 3-D habitat structure and a low invertebrate availability for fish. Oligotrophication by wetland epuration exerted negative effects on fish habitat quality, food quantity and availability, with an impairment of juvenile perch growth and recruitment. A generalized model of the changes in habitat carrying capacity occurring with epuration (oligotrophication) or eutrophication is presented, with examples of other aquatic systems in which strong linkages between trophic status, SAV, invertebrates and fish productivity were also demonstrated.  相似文献   

Hydrogeological characterization of groundwater storage and drainage in an alpine karst aquifer (the Kanin massif,Julian Alps)   总被引：1，自引：0，他引：1       下载免费PDF全文

Janez Turk  Arnauld Malard  Pierre‐Yves Jeannin  Metka Petrič  Franci Gabrovšek  Nataša Ravbar  Jonathan Vouillamoz  Tadej Slabe  Valentin Sordet 《水文研究》2015,29(8):1986-1998

The Kanin massif is an important trans‐boundary aquifer, which stretches between Slovenia and Italy. The groundwater is only partially exploited, mainly for water supply, but the aquifer exhibits great potential for future exploitation. Since no consistent regional overview of the hydrogeological functioning of the Kanin massif was available, the decision was made to perform a study of this area, using a pragmatic approach based on 3D geological and hydrogeological modelling. The so‐called KARSYS approach was applied, with the aim of characterizing the groundwater reserves within this karst massif and of locating the main drainage axes that carry groundwater from the recharge areas to the respective springs. Delineation of the catchment areas of the corresponding springs was carried out, and some new explanations were obtained, especially with regard to the Mo?nica spring, which is located in Slovenia and forms a potential source of drinking water. It was found that this spring's catchment area extends as far as the Italian ski resort of Sella Nevea. The conceptual model also provides a possible explanation about the underground drainage towards the Boka spring and waterfall, which has been a challenge for decades. This new explanation is based on the existence of a perched groundwater body that feeds the Boka spring via a system of conduits. Despite some limitations, the results, which consist of a visualization of the underground drainage and groundwater storage within the Kanin massif, can be used as a basis for planning the sustainable management of karst waters in the studied area. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Sediment load calculations from point measurements in sand-bed rivers     

Seema C.SHAH-FAIRBANK  Pierre Y JULIEN 《国际泥沙研究》2015,(1):1-12

Point velocity and suspended sediment concentration measurements are used to calculate the total sediment discharge in sand-bed rivers.Calculations with the Series Expansion of the Modified Einstein Point Procedure(SEMEPP) depend on grain diameter d_s and settling velocity ω,flow depth h,shear velocity u*,and sampling depth h_p.This procedure extends the applicability of the Modified Einstein Procedure(MEP) by using point sediment concentration and velocity measurements.This procedure is tested using the laboratory data from Coleman,and field measurements from the Enoree,Middle Rio Grande and Mississippi Rivers.Based on 801 point measurements over 124 verticals at flow depths ranging from 0.17 m to 33.5 m and sediment concentrations less than 0.1 kg L~(-1),the accuracy of the calculations depends on u*/ω and h_p/d_s.Point measurement techniques like SEMEPP are well-suited when u*/ω 5 where at least 60%of the total sediment load is measured when 90%of the flow depth is sampled.The determination of sediment discharge from point measurements is most accurate in deep rivers when h_p/d_s 10.000.and u*/ω10.Point measurements are not well-suited for shallow rivers and laboratory flumes where h 0.5 m and when u*/ω 2.  相似文献