Critical specific stream power in gravel-bed rivers     

F. Petit  F. Gob  G. Houbrechts  A.A. Assani 《Geomorphology》2005,69(1-4):92-101

Experiments with marked pebbles were carried out on different sized rivers of the Belgian Ardenne (catchment areas varying from less than 1 km² to 2700 km²). Specific stream power required to cause bedload movement was evaluated and critical values were obtained. Three types of relationship between critical specific stream power (ω₀) and grain size (D) were established. The values for ω₀ in the largest river (the Ourthe) were the lowest and were close to the values obtained for mountainous rivers carrying large boulders. In medium sized rivers (catchment area between 40 and 500 km²), the critical unit stream power was higher. It is likely that it is due to the bedform's greater resistance. This resistance would use up some of the energy that can cause movement and transport of bedload. The amount of resistance of the bedform can be expressed as bedform shear stress (τ″), determined by the relationship between grain shear stress (τ′—that determines movement and transport of the bedload) and the total shear stress (τ). This ratio varies between 0.4 and 0.5 in the medium sized rivers, compared to 0.7 in the Ourthe. In headwater streams (less than 20 km²), there is greater loss of energy due to bedform resistance (τ′/τ<0.3). Critical specific stream power is higher in this third type of river than in the other two.  相似文献   

A conceptual template for integrative human–environment research     

Barry Newell  Carole L. Crumley  Nordin Hassan  Eric F. Lambin  Claudia Pahl-Wostl  Arild Underdal  Robert Wasson   《Global Environmental Change》2005,15(4):299-307

Knowledge integration, the blending of concepts from two or more disciplines to create innovative new worldviews, is a key process in attempts to increase the sustainability of human activities on Earth. In this paper, we describe a ‘conceptual template’ that can be used to catalyse this process. The template comprises (a) a list of high-level concepts that capture the essential aspects of any significant human–environment problem, plus (b) broad lists of low-level basic concepts drawn from a range of disciplines. Our high-level concepts, which we call ‘conceptual clusters’, are labelled Dynamics & System, Organisation & Scale, Controlling Models, Management & Policy, Adaptation & Learning, and History. Many of the clustered, lower-level concepts are synonyms and thus provide possible connections between disciplines—for this reason we call them ‘nexus concepts’. We suggest that a conceptual template like that presented here can provide strong support to the initial phases of integrative research programs.  相似文献   

Distribution of coliforms in the Seine River and estuary (France) studied by rapid enzymatic methods and plate counts     

I. George  M. Petit  C. Theate  P. Servais 《Estuaries and Coasts》2001,24(6):994-1002

Fecal contamination in the Seine River and its estuary was studied for the first time by rapid enzymatic methods in parallel with traditional enumerations on selective culture media. The study consisted of four sampling campaigns focused on a 450-km stretch including the Parisian area and presenting highly variable levels of fecal pollution. Enzymatic assays (based on the activity of the β-D-galactosidase and β-D-glucuronidase enzymes in total [TC] and fecal [FC] coliforms, respectively) were in good agreement with classical plate counts of TC and FC. Both methods reflected the strong impact of the wastewater discharge from the Parisian area and of the presence of a maximum turbidity zone (at the mouth of the estuary) on the abundance of fecal bacteria in the river. Downstream from the Parisian outfalls, enzymatic measurements probably detected enzymatically active but nonculturable bacteria disregarded by plate counts. Enzymatic measurements in the Seine River downstream from the Parisian area were used to estimate net disappearance rates of coliforms in the river, which were close to total mortality rates measured with a method based on bacterial DNA labeling with tritiated thymidine. The part of total mortality due to grazing by protozoa was also investigated by the thymidine method. Grazing activity was responsible for 47% to 99% of the mortality of coliforms in the river. Attachment of coliforms to suspended matter (SM) was another factor which could be important in controlling the dynamics of coliforms in the Seine River and particularly in its estuary. Results of filtration and decantation experiments suggested that a significant part of the coliforms can be linked to SM depending on the nature and concentration of the SM.  相似文献   

Impacts « multi-échelle » d'un échange (Mg,Ca)–Pb et ses conséquences sur l'augmentation de la perméabilité d'une bentonite     

Nevila Jozja  Patrick Baillif  Jean-Claude Touray  Charles-Henri Pons  Fabrice Muller  Carole Burgevin 《Comptes Rendus Geoscience》2003,335(9):729-736

The article addresses the structural effects of solutions of lead nitrate on a suspended or compacted bentonite. A permeability increase is observed on compacted clay. Investigating the composition of output solution, using X-Rays Diffusion at Small Angles and Scanning Electron Microscopy, this permeability increase is explained from structural variations at nanometric (reduction of particle size) and micrometric scales (microfissuration of aggregates). To cite this article: N. Jozja et al., C. R. Geoscience 335 (2003).  相似文献   

Debiased Orbital and Absolute Magnitude Distribution of the Near-Earth Objects     

William F. Bottke Jr.Alessandro Morbidelli  Robert JedickeJean-Marc Petit  Harold F. LevisonPatrick Michel  Travis S. Metcalfe 《Icarus》2002,156(2):399-433

The orbital and absolute magnitude distribution of the near-Earth objects (NEOs) is difficult to compute, partly because only a modest fraction of the entire NEO population has been discovered so far, but also because the known NEOs are biased by complicated observational selection effects. To circumvent these problems, we created a model NEO population which was fit to known NEOs discovered or accidentally rediscovered by Spacewatch. Our method was to numerically integrate thousands of test particles from five source regions that we believe provide most NEOs to the inner Solar System. Four of these source regions are in or adjacent to the main asteroid belt, while the fifth one is associated with the transneptunian disk. The nearly isotropic comets, which include the Halley-type comets and the long-period comets, were not included in our model. Test bodies from our source regions that passed into the NEO region (perihelia q<1.3 AU and aphelia Q≥0.983 AU) were tracked until they were eliminated by striking the Sun or a planet or were ejected out of the inner Solar System. These integrations were used to create five residence time probability distributions in semimajor axis, eccentricity, and inclination space (one for each source). These distributions show where NEOs from a given source are statistically most likely to be located. Combining these five residence time probability distributions with an NEO absolute magnitude distribution computed from previous work and a probability function representing the observational biases associated with the Spacewatch NEO survey, we produced an NEO model population that could be fit to 138 NEOs discovered or accidentally rediscovered by Spacewatch. By testing a range of possible source combinations, a best-fit NEO model was computed which (i) provided the debiased orbital and absolute magnitude distributions for the NEO population and (ii) indicated the relative importance of each NEO source region.Our best-fit model is consistent with 960±120 NEOs having H<18 and a<7.4 AU. Approximately 44% (as of December 2000) have been found so far. The limits on this estimate are conditional, since our model does not include nearly isotropic comets. Nearly isotropic comets are generally restricted to a Tisserand parameter (with respect to Jupiter) of T<2, such that few are believed to have a<7.4 AU. Our computed NEO orbital distribution, which is valid for bodies as faint as H<22, indicates that the Amor, Apollo, and Aten populations contain 32±1%, 62±1%, and 6±1% of the NEO population, respectively. We estimate that the population of objects completely inside Earth's orbit (IEOs) arising from our source regions is 2% the size of the NEO population. This value does not include the putative Vulcanoid population located inside Mercury's orbit. Overall, our model predicts that ∼61% of the NEO population comes from the inner main belt (a<2.5 AU), ∼24% comes from the central main belt (2.5<a<2.8 AU), ∼8% comes from the outer main belt (a>2.8 AU), and ∼6% comes from the Jupiter-family comet region (2<T?3). The steady-state population in each NEO source region, as well as the influx rates needed to replenish each region, were calculated as a by-product of our method. The population of extinct comets in the Jupiter-family comet region was also computed.  相似文献   

Symplectic Integrators: Rotations and Roundoff Errors     

Jean-Marc Petit 《Celestial Mechanics and Dynamical Astronomy》1998,70(1):1-21

We investigate the numerical implementation of a symplectic integrator combined with a rotation (as in the case of an elongated rotating primary). We show that a straightforward implementation of the rotation as a matrix multiplication destroys the conservative property of the global integrator, due to roundoff errors. According to Blank et al. (1997), there exists a KAM-like theorem for twist maps, where the angle of rotation is a function of the radius. This theorem proves the existence of invariant tori which confine the orbit and prevent shifts in radius. We replace the rotation by a twist map or a combination of shears that display the same kind of behaviour and show that we are able not only to recover the conservative properties of the rotation, but also make it more efficient in term of computing time. Next we test the shear combination together with symplectic integrator of order 2, 4, and 6 on a Keplerian orbit. The resulting integrator is conservative down to the roundoff errors. No linear drift of the energy remains, only a divergence as the square root of the number of iterations is to be seen, as in a random walk. We finally test the three symplectic integrators on a real case problem of the orbit of a satellite around an elongated irregular fast rotating primary. We compare these integrators to the well-known general purpose, self-adaptative Bulirsch–Stoer integrator. The sixth order symplectic integrator is more accurate and faster than the Bulirsch–Stoer integrator. The second- and fourth- order integrators are faster, but of interest only when extreme speed is mandatory. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

Joints in the Mesozoic sediments around the Bristol Channel Basin   总被引：1，自引：0，他引：1  

K. D. Rawnsley  D. C. P. Peacock  T. Rives  J. -P. Petit 《Journal of Structural Geology》1998,20(12):1641-1661

Analysis has been carried out at four locations on the edges of the Bristol Channel Basin to illustrate the later phases of deformation of a sedimentary basin, and to illustrate the control on joint patterns of subtle changes in the stress system. The characteristics of the joints are described and influences on joints are determined, including the roles of faults, folds and beds. There is a low coefficient of correlation between joint spacing and bed thickness, except in very thin limestone beds, which have a high density of joints. The lengths and spacings of earlier joint phases are usually greater than those of later phases. Later joints normally abut against earlier joints.The joints abut the latest faults but are not displaced by them, so the joints post-date the main Alpine contraction. The joints formed in five main phases during reduction of the Alpine stresses. Phase 1 joints are sub-parallel to the regional compression direction (160–180°). Phase 2 joints are perturbed by faults, often curving towards points of stress concentrations along the faults. Phase 3 joints are sub-parallel to the earlier E–W-striking fold axes. Phase 4 joints are cross-joints, and phase 5 joints form polygonal patterns within joint-bound blocks. Phases 2 and 3 do not occur in the absence of faults and folds, and correspond with a reduction in horizontal compression and an increase in the importance of local factors. Phases 4 and 5 occur at all locations.  相似文献   

Several location methods for underwater shots in the Gulf of Genoa (Western Mediterranean): Structural implications     

N. Bethoux  F. Petit  J.P. Rehault  B. Massinon  J.P. Montagner 《Tectonophysics》1986,128(3-4)

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Hydrotalcite-like solid solutions with variable SO 4 2− and CO 3 2− contents at 50° C     

Carole Le Bail  Jean-Hugues Thomassin  Jean-Claude Touray 《Physics and Chemistry of Minerals》1987,14(4):377-382

Hydrotalcite-like solid solutions have been synthesized by coprecipitation in basic solutions with variable SO ₄ ^2? /CO ₃ ^2? ratios. Chemical determination of CO ₃ ^2? in the interlayer was impossible because of the presence of minor hydromagnesite. SO ₄ ^2? was determined both by chemical analysis and X-ray photoelectron spectroscopy (XPS), the two methods giving similar results. A Raman spectrometry gave additional data on the SO ₄ ^2? /CO ₃ ^2? ratio. Then, the stoichiometry of the anionic interlayers, X _s , X _c , and X _OH were determined, and the influence of X _s on the c′ parameter (increasing from c′=7.97 Å to c′=8.63 Å between X _s =0 and X _s =1) was characterized. In addition, a partitioning curve of SO ₄ ^2? and CO ₃ ^2? between aqueous solutions and hydrotalcite-like compounds was established. Its general shape strongly suggests a miscibility gap between a sulfate-rich end and a carbonate-rich solid solution (maximum SO ₄ ^2? /CO ₃ ^2? about 0.2). This result explains why most of the hydrotalcites synthesized during experimental alteration of basaltic glasses by sea-water (a sulfate-rich solution) are CO ₃ ^2? -rich solid solutions.  相似文献   

The racemization rate constant for protein-bound aspartic acid in woodrat middens     

Michael G. Petit 《Quaternary Research》1974,4(3):340-345

The racemization rate constant for aspartic acid has been determined from the d/l isomeric ratio in four strata of radiocarbon dated woodrat midden in Arizona. Two different methods of stereospecifically deaminating l-aspartic acid prior to the assay are compared. It is found that pure l-amino acid oxidase pretreatment of the dl aspartic acid mixture requires one less step than treatment with crude, dialyzed venom (Crotalus viridis) but that the two methods give the same results. Application of the theory of amino acid racemization dating is discussed in the context of the steric properties ofthe protein environment in which the racemization actually occurs.  相似文献