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排序方式: 共有423条查询结果,搜索用时 15 毫秒
181.
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184.
Michele Cappellari Eric Emsellem R. Bacon M. Bureau Roger L. Davies P. T. de Zeeuw Jesús Falcón-Barroso Davor Krajnovi Harald Kuntschner Richard M. McDermid Reynier F. Peletier Marc Sarzi Remco C. E. van den Bosch Glenn van de Ven † 《Monthly notices of the Royal Astronomical Society》2007,379(2):418-444
185.
River meandering has been extensively investigated. Two fundamental features to be explored in order to make further progress are nonlinearity and unsteadiness. Linear steady models have played an important role in the development of the subject but suffer from a number of limits. Moreover, rivers are not steady systems; rather their states respond to hydrologic forcing subject to seasonal oscillations, punctuated by the occurrence of flood events. We first derive a classification of river bends based on a systematic assessment of the various physical mechanisms affecting their morphodynamic equilibrium and their evolution in response to variations of hydrodynamic forcing. Using the database by Lagasse et al. ( 2004 ) we also show that natural meanders are typically mildly curved and long, i.e. such that both the centrifugal and the topographic secondary flows are weak, but they are almost invariably nonlinear. We then review some recent developments which allow us to treat analytically the flow and bed topography of mildly curved and long nonlinear bends subject to steady forcing, taking advantage of the fact that flow and bed topography in mildly curved long bends are slowly varying. Results show that nonlinearity has a number of consequences: most notably damping of the morphodynamic response and upstream shifting of the location of the nonlinear peak of the flow speed. Next we extend the latter model to the case of unsteady forcing. Results are found to depend crucially on the ratio between the flood duration and a morphodynamic timescale. It turns out that, in a channel subject to a repeated sequence of floods, the system reaches a dynamic equilibrium. We conclude the paper discussing how the present assessment relates to the debate on meander modelling of the late 1980s and suggesting what we see as promising lines of future developments. 相似文献
186.
This paper investigates the specific contributions of river network geomorphology, hillslope flow dynamics and channel routing to the scaling behavior of the hydrologic response as function of drainage area. Scaling relationships emerged from the observations of geomorphological and hydrological data and were reproduced in previous works through mathematical models, for both idealized self-similar networks and natural basins. Recent literature highlighted that scale invariance of hydrological quantities depends not only on the metrics of the drainage catchment but also on effective flow routing. In this study we employ a geomorphological width function scheme to test the simple scaling hypothesis adopting more realistic dynamic conditions than in previous approaches, specifically taking into account the role of hillslopes. The analysis is based on the derivation of the characteristic distributions of path lengths and travel times, inferred from DEM processing and measurements of rainfall and runoff data. The study area is located in the Tiber River region (central Italy).Results indicate that, while scaling properties clearly emerge when the hydrologic response is defined on the basis of the sole geomorphology, scale invariance is broken when less idealized flow dynamics are taken into account. Lack of scaling appears in particular as a consequence of the catchment to catchment variability of hillslope velocities. 相似文献
187.
Sediment load plays a fundamental role in natural river morphology evolution. Therefore, the correct assessment of the role
of the sediment load on natural or anthropic pools morphology downstream of river grade control structures, such as rock chute
or block ramps, is of fundamental interest for preserving the fish habitat and the river morphology. This work presents an
experimental study on the sediment load influence on rectangular expanding pools downstream of block ramps in live-bed conditions.
Several longitudinal and transversal expanding ratios have been tested. Ramp slopes were varied between 0.083 and 0.25. The
effect of the pool geometry and the sediment load on hydraulic jump downstream of block ramp as well as scour morphologies
and flow patterns have been analyzed. Equations were derived to evaluate the maximum scour hole depth, the longitudinal distance
of the section in which it occurs, and the maximum water elevations both in the pool and in the downstream contraction. 相似文献
188.
Michele Cercato 《Geophysical Prospecting》2011,59(2):210-226
Practical applications of surface wave inversion demand reliable inverted shear‐wave profiles and a rigorous assessment of the uncertainty associated to the inverted parameters. As a matter of fact, the surface wave inverse problem is severely affected by solution non‐uniqueness: the degree of non‐uniqueness is closely related to the complexity of the observed dispersion pattern and to the experimental inaccuracies in dispersion measurements. Moreover, inversion pitfalls may be connected to specific problems such as inadequate model parametrization and incorrect identification of the surface wave modes. Consequently, it is essential to tune the inversion problem to the specific dataset under examination to avoid unnecessary computations and possible misinterpretations. In the heuristic inversion algorithm presented in this paper, different types of model constraints can be easily introduced to bias constructively the solution towards realistic estimates of the 1D shear‐wave profile. This approach merges the advantages of global inversion, like the extended exploration of the parameter space and a theoretically rigorous assessment of the uncertainties on the inverted parameters, with the practical approach of Lagrange multipliers, which is often used in deterministic inversion, which helps inversion to converge towards models with desired properties (e.g., ‘smooth’ or ‘minimum norm' models). In addition, two different forward kernels can be alternatively selected for direct‐problem computations: either the conventional modal inversion or, instead, the direct minimization of the secular function, which allows the interpreter to avoid mode identification. A rigorous uncertainty assessment of the model parameters is performed by posterior covariance analysis on the accepted solutions and the modal superposition associated to the inverted models is investigated by full‐waveform modelling. This way, the interpreter has several tools to address the more probable sources of inversion pitfalls within the framework of a rigorous and well‐tested global inversion algorithm. The effectiveness and the versatility of this approach, as well as the impact of the interpreter's choices on the final solution and on its posterior uncertainty, are illustrated using both synthetic and real data. In the latter case, the inverted shear velocity profiles are blind compared with borehole data. 相似文献
189.
Etienne Balan Simon Delattre Damien Roche Loïc Segalen Guillaume Morin Maxime Guillaumet Marc Blanchard Michele Lazzeri Christian Brouder Ekhard K. H. Salje 《Physics and Chemistry of Minerals》2011,38(2):111-122
The crystallinity of natural and synthetic apatite samples is often determined from the broadening of ν
4 PO4 infrared absorption bands. However, various physical mechanisms contribute to the observed linewidth. In the present study,
the factors determining the linewidth in the powder spectrum of synthetic fluorapatite and hydroxyapatite samples are investigated.
The temperature dependence of the infrared spectrum (10–270 K) is used to assess the respective contributions of homogeneous
broadening, related to the decay of phonons through anharmonic coupling, and heterogeneous broadening related to elastic strain
and macroscopic electrostatic effects. This latter contribution is dominant in the investigated samples and depends on the
shape of powder particles. It is discussed under the light of the theoretical modeling of the low-frequency dielectric properties
of apatite based on first-principles density functional theory calculations. The linewidth of the weak ν
1 PO4 absorption band provides a reliable information on microscopic sources of broadening, i.e., apatite crystallinity. In comparison,
the other more intense PO4 bands are more sensitive to long-range electrostatic effects. 相似文献
190.
A method of estimating bulk potential permeability in fractured-rock aquifers using field-derived fracture data and type curves 总被引:1,自引:0,他引:1
Alex K. Manda Stephen B. Mabee David F. Boutt Michele L. Cooke 《Hydrogeology Journal》2013,21(2):357-369
A method is devised for estimating the potential permeability of fracture networks from attributes of fractures observed in outcrop. The technique, which is intended as a complement to traditional approaches, is based on type curves that represent various combinations of fracture lengths, fracture orientations and proportions (i.e., intensities) of fractures that participate in flow. Numerical models are used to derive the type curves. To account for variations in fracture aperture, a permeability ratio (R) defined as the permeability of a fracture network in a domain divided by the permeability of a single fracture with identical fracture apertures, is used as a dependent variable to derive the type curves. The technique works by determining the point on the type curve that represents the fracture characteristics collected in the field. To test the performance of the technique, permeabilities that were derived from fractured-rock aquifers of eastern Massachusetts (USA) are compared to permeabilities predicted by the technique. Results indicate that permeabilities estimated from type curves are within an order of magnitude of permeabilities derived from field tests. First-order estimates of fracture-network permeability can, therefore, be easily and quickly acquired with this technique before more robust and expensive methods are utilized in the field. 相似文献