1,721,112 research outputs found
Grain sorting and bar instability
Full text linkA two-dimensional model of flow and bed topography is proposed to investigate the effect of sediment heterogeneity on the development of alternate bars. Within the context of a linear stability theory the flow field, the bed topography and the grain size distribution function are perturbed leading to an integro-differential linear eigenvalue problem. It is shown that the selective transport of different grain size fractions and the resulting spatial pattern of sorting may appreciably affect the balance between stabilizing and destabilizing actions which govern bar instability. Theoretical results suggest that sediment heterogeneity leads to a damping of both growth rate and migration speed of bars, while bar wavelength is shortened with respect to the case of uniform sediment. The above findings conform, at least qualitatively, to the experimentally detected reduction of bar height, length and celerity. The observed tendency of coarser particles to pile up towards bar crests is also reproduced by theoretical results
Topographic expressions of bars in channels with variable width
No full textA three dimensional quasi-analytical model is introduced to determine the flow field and the altimetric response of movable-bed channels subject to periodic
width variations. The basic assumptions underlying the analysis are those of small amplitude of width variations and wide channel, so that non linear effects and side walt effects are neglected. The aim of the work is to determine the conditions under which the channel is planimetrically stable or unstable, i.e. it tends to damp (or enhance) a given initial (infinitesimal) perturbation
of the channel width due to bank erosion. A simple bank erosion model is adopted whereby the rate of bank retreat is related to the excess shear stress at the banks. Theoretical results suggest that the equilibrium bottom profile is mainly constituted by two components. The first component represents a purely longitudinal bottom deformation, which induces deposition at the widest section and scour at the constraint, where the cross sectionally averaged velocity attains its maximum value. The second component is mainly originated by three dimen-
sional effects and induces a transverse deformation of the bed in the form of a central bar. Its relative position with respect to the former component changes with the length of width variations: under suitable conditions the flow divergence induced by the central bar leads to a maximum velocity at the banks in wide sections, which implies that width variations tend to amplify
About The Kinetics And Mechanism Of The Reactions Of 4-(2-pyridylazo)- Resorcinol, With Zn2+, Cu2+ And Zn2++cu 2+ Equimolar Mixtures, In Aqueous Solutions
No full textThe kinetics and mechanism of the reactions between 4-(2-pyridylazo)- resorcinol and Zn2+, Cu2+ and Zn2++Cu 2+ equimolar mixtures were studied. The reactions were performed in aqueous solution (pH = 8.5, borate buffer) and monitored spectrophotometrically at 500 nm using stopped-flow technique. Spectral and kinetic data indicate that the Zn2++Cu2+ equimolar mixture behaves as an unique species and it can be attributed to the interactions of Zn2+ and of Cu2+ with water molecules in the aqueous solution. A mechanism is proposed and the rate constants are calculated.2815562Cheng, K.L., Ueno, K., Imamura, T.R., (1982) Handbook of Organic Analytical Reagents, p. 195. , Florida: CRC PressColson, S.D., Dunning Jr., T.H., The Structure of natures solvent: Water (1994) Science, 265 (5168), pp. 43-44Eigen, M., Fast elementary steps in chemical analysis (1963) Pure Appl. Chem., 6 (2), pp. 97-115Gourbatsis, S., Hadjiliadis, N., Perlepes, S.P., Garoufs, A., Butler, I.S., Structural and spectroscopic characterization of a cationic aquanitrato copper(II) complex with the tetradentate Schiff-base ligand N,N′-bis[1-(2- pyridyl)ethylidene]ethane-1,2-diamine (1998) Transition Met. Chem., 23 (5), pp. 599-604Hubbard, C.D., Pacheco, D., The kinetics of the reactions of 1-(2-Pyridylazo)-2-naphtol with the ions of nickel(II), copper(II) and zinc(II) (1977) J. Inorg. Nucl. Chem., 39 (1373-1375)James, D.W., Spectroscopic studies of ion-ion-solvent interaction in solutions containing oxyanions (1985) Progress in Inorganic Chemistry, 33, pp. 353-391. , LIPPARD, S. J. (Ed.). New York: John WileyRossi, A.V., Davanzo, C.U., Tubino, M., The structure of liquid water in aqueous solutions: A proposed model from near and mid infrared spectroscopy (1996) J. Braz. Chem. Soc., 7 (6), pp. 403-410Rossi, A.V., Rohwedder, J.J.R., Tubino, M., A low cost stopped flow system (1994) Lab. Microcomputer, 13 (2-4), pp. 67-69Rossi, A.V., Tubino, M., Kinetics and mechanism of the reaction of zincon, o-[1-(2-hydroxy-5- sulfophenyl)-3-phenyl-5-formazane] benzoic acid, with Zn2+, Cu2+ and [Zn2+ + Cu2+] equimolar mixtures (1996) J. Braz. Chem. Soc., 7 (3), pp. 161-168Shibata, S., (1972) Chelates in Analytical Chemistry, 4, p. 207. , New York: Marcel DekkerTubino, M., Um stopped flow de fácil construção (1985) Quim. Nova, 8 (4), pp. 324-328Tubino, M., Rossi, A.V., Construção de um aparelho de fluxo interrompido (1991) Quim. Nova, 14 (2), pp. 109-111Vogel, A.I., (1961) A Textbook of Quantitative Inorganic Analysis Including Elementary Instrumental Analysis, p. 358. , London: Lowe & BrydoneWelcher, F.J., (1964) Chemical Solutions, p. 185. , New York: D. Van. Nostran
Analysis of autogenic bifurcation processes resulting in river avulsion
No full textRiver bifurcations are constituent components of multi-thread fluvial systems, playing a crucial role in their morphodynamic evolution and the partitioning of water and sediment. Although many studies have been directed at exploring bifurcation dynamics, the conditions under which avulsions occur, resulting in the complete abandonment of one branch, are still not well understood. To address this knowledge gap, we develop a novel 1D numerical model based on existing nodal point relations for sediment partitioning, which allows for the simulation of the morphodynamic evolution of a free bifurcation. Model results show that when the discharge asymmetry is so high that the shoaling branch does not transport sediments (partial avulsion conditions) the dominant branch undergoes significant degradation, leading to a higher inlet step between the bifurcates and further amplifying the discharge asymmetry. The degree of asymmetry is found to increase with the length of the downstream channels to the point that when they are sufficiently long, the shoaling branch is completely abandoned (full avulsion conditions). To complement our numerical findings, we also formulate a new analytical model that is able to reproduce the essential characteristics of the partial avulsion equilibrium, which enables us to identify the key parameters that control the transition between different configurations. In summary, this research sheds light on the fundamental processes that drive avulsion through the abandonment of river bifurcations. The insights gained from this study provide a foundation for further investigations and may offer valuable information for the design of sustainable river restoration projects
Free bars in rivers
No full textIn the paper we review some recent work on the mechanics of formation and development of river bars. The emphasis is placed on the instability process which leads to the spontaneous development of bars in almost straight reaches of alluvial rivers. A three dimensional formulation of the problem is presented along with a discussion on the relevant closure relationships. Results of linear and non linear theories for free bars under bedload dominated conditions are summarised. Furthermore, account is given on the effect on bar instability induced by suspended load, grain sorting and width variations. Some as yet unpublished results are also presented
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