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Lyon, France

Ginzburg I.,IRSTEA
Advances in Water Resources | Year: 2013

This paper develops a symmetrized framework for the analysis of the anisotropic advection-diffusion Lattice Boltzmann schemes. Two main approaches build the anisotropic diffusion coefficients either from the anisotropic anti-symmetric collision matrix or from the anisotropic symmetric equilibrium distribution. We combine and extend existing approaches for all commonly used velocity sets, prescribe most general equilibrium and build the diffusion and numerical-diffusion forms, then derive and compare solvability conditions, examine available anisotropy and stable velocity magnitudes in the presence of advection. Besides the deterioration of accuracy, the numerical diffusion dictates the stable velocity range. Three techniques are proposed for its elimination: (i) velocity-dependent relaxation entries; (ii) their combination with the coordinate-link equilibrium correction; and (iii) equilibrium correction for all links. Two first techniques are also available for the minimal (coordinate) velocity sets. Even then, the two-relaxation-times model with the isotropic rates often gains in effective stability and accuracy. The key point is that the symmetric collision mode does not modify the modeled diffusion tensor but it controls the effective accuracy and stability, via eigenvalue combinations of the opposite parity eigenmodes. We propose to reduce the eigenvalue spectrum by properly combining different anisotropic collision elements. The stability role of the symmetric, multiple-relaxation-times component, is further investigated with the exact von Neumann stability analysis developed in diffusion-dominant limit. © 2012 Elsevier Ltd. Source

Abadie et al., Journal of Ecology, 99, 2011, 1134 claim that 'landscape disturbance causes small-scale functional homogenization but limited taxonomic homogenization'. This statement does not seem to accurately summarize their results. Abadie et al. provide no strong arguments in favour of a cause and effect relationship between landscape disturbance and functional homogenization because their approach is correlational. Abadie et al. associate an index of mean community specialization with functional biotic homogenization (BH), and they in turn associate functional BH with ecosystem functioning. However, the community specialization index is associated with a very specific kind of 'function' - the species response - which has no clear link with ecosystem functioning. This problem is frequent in the literature on biotic homogenization. There is no clear sign in the data shown by Abadie et al. that metrics incorporating species attributes are 'much more reliable' than taxonomic diversity indices. As frequently observed in the literature on biotic homogenization within communities, their results show no sign of loser or winner species or of 'extirpation of specialist species'. Therefore, there seems to be no evidence in support of the biotic homogenization model they propose. Synthesis. The Average Community Specialization - a mean specialization index- is in itself incapable of identifying loser and winner species and has no clear link with ecosystem functioning. Methods other than mean trait approaches should be used to study either functional homogenization or the extirpation of specialist species. © 2012 The Author. Journal of Ecology © 2012 British Ecological Society. Source

This paper establishes relations between the stability and the high-order truncated corrections for modeling of the mass conservation equation with the tworelaxation- times (TRT) collision operator. First we propose a simple method to derive the truncation errors from the exact, central-difference type, recurrence equations of the TRT scheme. They also supply its equivalent three-time-level discretization form. Two different relationships of the two relaxation rates nullify the third (advection) and fourth (pure diffusion) truncation errors, for any linear equilibrium and any velocity set. However, the two relaxation times alone cannot remove the leading-order advection-diffusion error, because of the intrinsic fourth-order numerical diffusion. The truncation analysis is carefully verified for the evolution of concentration waves with the anisotropic diffusion tensors. The anisotropic equilibrium functions are presented in a simple but general form, suitable for the minimal velocity sets and the d2Q9, d3Q13, d3Q15 and d3Q19 velocity sets. All anisotropic schemes are complemented by their exact necessary von Neumann stability conditions and equivalent finite-difference stencils. The sufficient stability conditions are proposed for the most stable (OTRT) family, which enables modeling at any Peclet numbers with the same velocity amplitude. The heuristic stability analysis of the fourth-order truncated corrections extends the optimal stability to larger relationships of the two relaxation rates, in agreementwith the exact (one-dimensional) and numerical (multi-dimensional) stability analysis. A special attention is put on the choice of the equilibrium weights. By combining accuracy and stability predictions, several strategies for selecting the relaxation and free-tunable equilibrium parameters are suggested and applied to the evolution of the Gaussian hill. © 2012 Global-Science Press. Source

A large proportion of the total river length on Earth comprises rivers that are temporary in nature. However, the effects of periodical dry events have received far less attention from ecologists than those of floods and low flows. This study concomitantly examined the effects of flow intermittence on invertebrates from the streambed surface and from a depth of 30cm in the hyporheic zone. Invertebrates were collected during 3years in the Albarine River, France, before and after summer dry events from 18 sites (seven were perennial) distributed along a longitudinal flow intermittence gradient. I predicted benthic and hyporheic density and taxonomic richness to decrease, and assemblage composition to shift from desiccation-sensitive to desiccation-resistant taxa with increased dry event duration. Second, I predicted benthic and hyporheic assemblages from sites that dried for longer periods to be nested subsets of assemblages from sites that dried for shorter periods. Last, I predicted a convergence in benthic and hyporheic assemblage composition with increasing duration of dry events, resulting from increased vertical migration of benthic taxa into the hyporheic sediments to cope with dry events. Increased dry event duration in the Albarine River led to a decrease in both benthic and hyporheic density and taxonomic richness. Invertebrate assemblage composition shifted along the gradient of increasing flow intermittence, but broad taxonomic overlap between perennial and temporary reaches and nestedness patterns indicated that these shifts were because of the loss of taxa susceptible to drying rather than selection for desiccation-resistant specialists. Assemblage composition between benthic and hyporheic invertebrates diverged with increasing dry event duration, suggesting that the hyporheic zone did not act as a refuge during dry events in this river. Quantitative studies on the relationships between ecology and intermittence are still rare but are needed to predict the consequences of future changes in flow intermittence. The relationships found in this study should be tested across a wide range of temporary rivers to better evaluate the generality of these findings. © 2011 Blackwell Publishing Ltd. Source

The ability of simple equations to predict bed load transport with limited knowledge of the bed surface material was investigated. This was done using a data set consisting of 7,636 bed load transport values from the flume (1,317 data) and from 84 river reaches (6,319 field data). It was possible to collapse field and flume data by correcting the ratio between the Shields number and its critical value with a very simple hiding function proposed as a power law of the D84/D50 ratio. In so doing, a surface-based bed load transport formula was proposed. It was successfully tested on an independent data set (comprising sand and gravel bed rivers with slope in the range 0.0002-0.08), with 86% of the values predicted within a precision of 1 order of magnitude. Moreover, the formula reproduced the low transport rates well, contrary to the usual surface-based formulas also tested, and is particularly well suited for estimating low transport rates associated with near-bankfull flow discharge. This new formula is neither time consuming (no fractionwise calculation) nor data consuming (the required parameters are the flow discharge, the active width, the slope, and the surface grain diameters D50 and D84). Copyright 2010 by the American Geophysical Union. Source

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