Grain size dependency in the occurrence of sand waves     

Hennie Henriët van der Veen  Suzanne Jacqueline Marie Hélène Hulscher  Michiel Adrianus Frederik Knaapen 《Ocean Dynamics》2006,56(3-4):228-234

Sandy shallow seas, like the North Sea, are very dynamic. Several morphological features are present on the bed, from small ripples to sand waves and large tidal sandbanks. The larger patterns induce significant depth variations that have an impact on human activities taking place in this area. Therefore, it is important to know where these large-scale features occur, what their natural behaviour is and how they interact with human activities. Here, we extend earlier research that compares the results of an idealized model of large-scale seabed patterns with data of seabed patterns in the North Sea. The idealized model is extended with a grain size dependency. The adaptations lead to more accurate predictions of the occurrence of large-scale bed forms in the North Sea. Therefore, grain size dependency and, in particular, critical shear stress are important to explain the occurrence of sand waves and sandbanks in the North Sea. Responsible Editor: Alejandro Souza  相似文献   

Comparative Rates of Western Juniper Afforestation in South‐Central Oregon and the Role of Anthropogenic Disturbance*     

Peter T. Soul  Paul A. Knapp  Henri D. Grissino‐Mayer 《The Professional geographer》2003,55(1):43-55

We examine changes in canopy cover for adult western juniper from the 1960s to 1994 in central Oregon using repeat aerial photography. We compare changes at four sites with a land‐use history of minimal anthropogenic disturbance to changes on adjacent sites that have a disturbance history more typical of central Oregon rangelands. Canopy cover increased at all sites, but afforestation on sites with domestic livestock grazing was greater. The potential driving forces common to all sites include a long fire‐free interval, early twentieth‐century favorable climatic conditions, biological inertia, and atmospheric CO₂ enrichment.  相似文献   

The terminal bulk Lorentz factor of relativistic electron–positron jets     

N. Renaud  & G. Henri 《Monthly notices of the Royal Astronomical Society》1998,300(4):1047-1056

We present a numerical simulation of the bulk Lorentz factor of a relativistic electron–positron jet driven by the Compton rocket effect from accretion disc radiation. The plasma is assumed to have a power-law distribution n _e(γ) ∝ γ^{− s} with 1 < γ < γ_max and is continuously reheated to compensate for radiation losses. We include the full Klein–Nishina (hereafter KN) cross-section, and study the role of the energy upper cut-off γ_max, spectral index s and source compactness. We determine the terminal bulk Lorentz factor in the cases of supermassive black holes, relevant to AGN, and stellar black holes, relevant to galactic microquasars. In the latter case, Klein–Nishina cross-section effects are more important and induce a terminal bulk Lorentz factor smaller than in the former case. Our result are in good agreement with bulk Lorentz factors observed in Galactic (GRS 1915+105, GRO J1655−40) and extragalactic sources. Differences in scattered radiation and acceleration mechanism efficiency in the AGN environment can be responsible for the variety of relativistic motion in those objects. We also take into account the influence of the size of the accretion disc; if the external radius is small enough, the bulk Lorentz factor can be as high as 60.  相似文献   

A new seismic tomography of Aigion area (Gulf of Corinth, Greece) from the 1991 data set   总被引：10，自引：0，他引：10  

D. Latorre  J. Virieux  T. Monfret  V. Monteiller  T. Vanorio  J.-L. Got  H. Lyon-Caen 《Geophysical Journal International》2004,159(3):1013-1031

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40Ar-39Ar ages of biotites from Corsica and arguments for Permian age of alkaline granitic intrusion     

Henri Maluski 《Contributions to Mineralogy and Petrology》1976,58(3):305-317

The Western part of Corsica is underlain by Variscan calcalkaline granites which intrude the pre-Variscan basement. These granites and their basement are also intruded by Permian hyperalkaline granites. Overprinted biotites of the basement and the calcalkaline granites show only one plateau age at about 250 m.y., except for a sample of calcalkaline granite from the Cargèse region which has a high temperature plateau age of 330 m.y. related to the emplacement and a low temperature plateau age of 230 m.y. Plateau ages around 250 m.y. due to the intrusion of hyperalkaline granites whose biotites give plateau ages between 250 and 280 m.y. reflect this important thermal event.  相似文献   

Groundwater fluctuations and footslope ferricrete soils in the humid tropical zone of southern Cameroon     

Emile Temgoua  Henri‐Bosko Djeuda Tchapnga  Emile Tanawa  Claire Guenat  Hans‐Rudolf Pfeifer 《水文研究》2005,19(16):3097-3111

This paper discusses the relationship between the differentiation of ferruginous accumulations and the variable water saturation of footslope soil patterns. An analysis of the slope morphology of a typical hill in the forest zone of southern Cameroon and a seasonal survey of the levels of groundwaters, springs and rivers were considered in relation to the petrology of different soil patterns. The study site is a tabular hillock whose slopes present a progressive development from steep to gentle slopes. The variable residence time of water within the soil, creating an alternation of reducing and oxidizing conditions, affects soil chemistry, structure and lateral extension of the soil patterns. The ferruginous soil patterns, being formed on the footslopes, gradually increase in extent with decreasing slope angle and the relative rise of the groundwater level. The steep footslopes, where groundwater has a shorter residence time, show a soft mottled clay pattern, restricted to the bottom part of the slope. The moderate footslopes exhibit a deep permanent and a temporary perched groundwater table. The latter, with its regular capillary fringe, contributes to more reducing conditions within isolated domains in the soil patterns, and thus to the alternation with oxidizing conditions, generating a continuous hard soil pattern (massive carapace). The more gently dipping footslopes exhibit groundwater levels near the surface and also a significant amplitude of groundwater fluctuation. Iron, previously accumulated in moderate footslope patterns, is reduced, remobilized, and leached. The soil patterns formed develop into a variegated carapace, more extended along the slope, containing less iron, but nevertheless more hardened, due to the important fluctuations of the groundwater table. These patterns are limited to the zone of groundwater fluctuation and deteriorate as the water fluctuation zone recedes. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Truly isotropic biaxial hysteresis with arbitrary knee sharpness          下载免费PDF全文

P. Scott Harvey Jr.  Henri P. Gavin 《地震工程与结构动力学》2014,43(13):2051-2057

The inelastic responses of framed structures and seismic isolation systems undergoing biaxial (horizontal) shaking are coupled. In existing models for biaxial coupling of hysteretic behavior, the smoothness (or ‘knee') of the transition from elastic to yielding behavior cannot be modified without affecting the shape of the yield surface. This paper presents a model for coupled biaxial hysteretic behavior in which the knee from pre‐yield to post‐yield can be adjusted while maintaining an isotropic yield surface. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

In Memoriam: David le Sueur     

Henri EZ Tonnang  Richard YM Kangalawe  Pius Z Yanda 《Global Change & Human Health》2002,3(1):74-74

Biography

In Memoriam: David le Sueur  相似文献   

In situ Measurement of H2O and CH4 with Telecommunication Laser Diodes in the Lower Stratosphere: Dehydration and Indication of a Tropical Air Intrusion at Mid-Latitudes     

Georges Durry  Alain Hauchecorne  Joelle Ovarlez  Henri Ovarlez  Ivan Pouchet  Virginie Zeninari  Bertrand Parvitte 《Journal of Atmospheric Chemistry》2002,43(3):175-194

Telecommunication laser diodes emitting near 1.39 m and 1.65 m in combination with direct-differential absorption spectroscopy are efficient tools to monitor in situ stratospheric H₂O andCH₄ with a good precision error (a few percents), a high temporal resolution (ranging from 10 ms to 1 s), a large dynamic range in the concentration measurements (four orders of magnitude) and a high selectivity in the analyte species. To illustrate the capability of laser probing technique, we report balloonborne H₂Oand CH₄ simultaneous measurements obtained on October 2001 atmidlatitudes (43° N). The H₂O vertical profile achieved with the lasersensor in the lower stratosphere is compared with the H₂O data yielded by a balloonborne frost-point hygrometer. The total hydrogen mixing ratio in the lower stratosphere, 2[CH₄] + [H₂O], appears to beconstant at 7.5 ± 0.1 ppmv. Nevertheless, an unexpected largedehydration of 0.5 ppmv was detected by both the laser sensor and thehygrometer between 16 km and 23 km. We suspect the occurrence of a tropicalair intrusion into mid-latitudes. We support this interpretation using a high-resolution advection model for potential vorticity.  相似文献   

Diel variability of glycolate in the eastern tropical Atlantic Ocean     

Christophe Leboulanger  Louise Oriol  Henri Jupin  Chantal Desolas-gros 《Deep Sea Research Part I: Oceanographic Research Papers》1997,44(12):2131-2139

The concentration of dissolved glycolate in sea water was measured by high performance liquid chromatography in the eastern tropical Atlantic Ocean during the Eumeli 4 oceanographic cruise in June 1992. Diurnal concentrations of glycolate reached 74 kg 1⁻¹ 1⁻¹ in mesotrophic waters and 17 μg 1⁻¹ 1 in oligotrophic waters. At midday total dissolved glycolate exceeded 1 g of carbon per m² of ocean, decreasing strongly during the night (less than 0.1 g of carbon per m²). At the three stations studied, glycolate carbon difference between night and day in the water column was of the same order as the daily primary production estimated by incorporation of ¹⁴C0₂. Disappearance of this compound at night suggested a rapid consumption by heterotrophic organisms. These data suggest that glycolate excretion by phytoplankton may be important, and possibly influenced by ambient nutrient concentrations. Further, with glycolate representing up to 50% of daily productivity, our estimates of the total biological reduction of C0₂ need to be re-examined.  相似文献