1,720,980 research outputs found
Free Alternate Bars in Rivers: Key Physical Mechanisms and Simple Formation Criterion
Full text linkFree alternate bars are large-scale, downstream-migrating bedforms characterized by an alternating sequence of three-dimensional depositional fronts and scour holes that frequently develop in rivers as the result of an intrinsic instability of the erodible bed. Theoretical models based on two-dimensional shallow water and Exner equations have been successfully employed to capture the bar instability phenomenon, and to estimate bar properties such as height, wavelength and migration rate. However, the mathematical complexity of the problem hampered the understanding of the key physical mechanisms that sustain bar formation. To fill this gap, we considered a simplified version of the equations, based on neglecting the deformation of the free surface, which allows us to: (a) provide the first complete explanation of the bar formation mechanism as the result of a simple bond between variations of the water weight and flow acceleration; (b) derive a simplified, physically based formula for predicting bar formation in a river reach, depending on channel width-to-depth ratio, Shields number and relative submergence. Comparison with an unprecedented large set of laboratory experiments reveals that our simplified formula appropriately predicts alternate bar formation in a wide range of conditions. Noteworthy, the hypothesis of negligible free surface effect also implies that bar formation is fully independent of the Froude number. We show that this intriguing property is intimately related to the three-dimensional nature of river bars, which allows for a gentle lateral deviation of the flow without significant deformation of the water surface
Defining the length parameter in river bifurcation models: a theoretical approach
No full textOne-dimensional models for river bifurcations rely on a nodal point relation that determines the distribution of sediments between the downstream branches. The most widely-adopted nodal point relation describes the two-dimensional topographic effects exerted by the bifurcation by introducing two computational cells, located just upstream the bifurcation node, that laterally exchange water and sediments. The results of this approach strongly depend on a dimensionless parameter that represents the ratio between the cell length and the main channel width, whose value needs to be empirically estimated. Previous works proposed calibrating this parameter on the basis of more complete two-dimensional linear models, which directly solve the momentum and mass conservation equations. This study demonstrates that a full consistency between the one-dimensional approach and the two-dimensional models can be directly achieved by adopting different scaling for the bifurcation cell length, which results in a theoretically-defined and constant dimensionless length parameter. Comparison with experimental observations reveals that this physically-based scaling yields more accurate predictions of bifurcation stability and discharge asymmetry. This indicates that the proposed method may provide a more reliable and precise estimation of the cell length, potentially improving the performance of one-dimensional models for bifurcation processes in rivers
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
On steady alternate bars forced by a localized asymmetric drag distribution in erodible channels
Full text linkStudying the effect of different in-stream fluvial turbines siting on river morphodynamics allowed us to witness the onset of a time-Averaged, large-scale, alternate distortion of bed elevations, which could not be exclusively related to the turbine rotor blockage. The longitudinal profiles of this two-dimensional bathymetric perturbation resemble those of steady fluvial bars. In this contribution we generalize the problem addressing a spatially impulsive, asymmetric distribution of drag force in the channel cross-section. This is experimentally investigated through the deployment of differently sized grids perpendicular to the flow, and analytically explored as a finite perturbation of an open channel flow over an erodible sediment layer, as described by a coupled flow-sediment shallow water equation. The steady solutions of this fluvial morphodynamic problem, physically represented by alternate bars scaling with the channel width, highlight the importance of the resonant conditions in defining the spatial extent of the bed deformation. The equations further suggest that in very shallow flows any asymmetric obstruction may lead to an upstream propagation of the steady bars, consistent with previous studies on the effects of channel curvature. In broad terms, this study provides the preliminary framework to control the onset of river meandering through imposed finite perturbations of the cross-section. In a more applied sense, it provides a tool to predict non-local scour-deposition patterns associated with the deployment of energy converters or other flow obstructions
Quasi-Universal Length Scale of River Anabranches
Full text linkLooping patterns, where channels divide and reconnect further downstream, are widespread in natural rivers. Here, we build an extensive dataset of different gravel-bed and sand-bed rivers around the world encompassing a wide range of physiographic and sedimentological conditions. Field data show the existence of quasi-universal relations for the anabranches length when scaled with bankfull hydraulic geometry variables of the main upstream channel. The dimensionless length is found to be nearly slope-invariant, identifying a clear difference with respect to deltaic systems. This scaling relationship is explained by interpreting the dynamics of river loops as basically controlled by a two-way interaction between their constitutive elements, bifurcations and confluences. The identification of a quasi-universal length scale provides insight on the morphological evolution of multi-thread networks and constitutes a key information for the design of self-sustaining river restoration interventions
Obtención de quinua expandida con buenas características nutricionales
Full text linkFil: Redolfi, M. R. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Ciencia y Tecnología de los Alimentos; Argentina.Fil: Redolfi, M. R. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Escuela de Ingeniería Química; Argentina.Fil: Catini, F. R. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Ciencia y Tecnología de los Alimentos; Argentina.Fil: Catini, F. R. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Escuela de Ingeniería Química; Argentina.Fil: Grasso, F. V. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Ciencia y Tecnología de los Alimentos; Argentina.Fil: Calandri, E. L. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Ciencia y Tecnología de los Alimentos; Argentina.Chenopodium Quinoa, comúnmente conocido como quinua, es un grano de excepcionales
características nutricionales, pero que actualmente no es consumida en gran cantidad en nuestro
país.http://www.inv.icta.efn.uncor.edu/archivos/Libro_resumenes_Jornada_ICTA.pdfFil: Redolfi, M. R. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Ciencia y Tecnología de los Alimentos; Argentina.Fil: Redolfi, M. R. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Escuela de Ingeniería Química; Argentina.Fil: Catini, F. R. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Ciencia y Tecnología de los Alimentos; Argentina.Fil: Catini, F. R. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Escuela de Ingeniería Química; Argentina.Fil: Grasso, F. V. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Ciencia y Tecnología de los Alimentos; Argentina.Fil: Calandri, E. L. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Ciencia y Tecnología de los Alimentos; Argentina.Otras Ingeniería Químic
A knowledge-based approach to hierarchical classification: A voting metaphor
No full textThe paper proposes a new approach to hierarchical classification based on condition-action rules that represent expert knowledge in a given domain. The approach adopts a voting metaphor: each rule is regarded as a voter that expresses a preference for a given category to be assigned to an item to be classified; the category that receives more votes wins. Novel performance measures of hierarchical classifiers are also introduced that aim at overcoming the limitations of the current concepts of precision and recall. The proposed approach can be applied to any hierarchical classification task, for which expert knowledge is available. The viability of the approach and its performance are shown through a real-size application concerning the e-mail dispatching task inside a large public administration. The results obtained demonstrate that the proposed knowledge-based approach to hierarchical classification can reach a performance level comparable to that of human experts, if not even better
Morphometric properties of alternate bars and water discharge: A laboratory investigation
Full text linkThe formation of alternate bars in straightened river reaches represents a fundamental process of river morphodynamics that has received great attention in the last decades. It is well-established that migrating alternate bars arise from an autogenic instability mechanism occurring when the channel width-to-depth ratio is sufficiently large. While several empirical and theoretical relations are available for predicting how bar height and length depend on the key dimensionless parameters, there is a lack of direct, quantitative information about the dependence of bar properties on flow discharge. We performed a series of experiments in a long, mobile-bed flume with fixed and straight banks at different discharges. The self-formed bed topography was surveyed, different metrics were analyzed to obtain quantitative information about bar height and shape, and results were interpreted in the light of existing theoretical models. The analysis reveals that the shape of alternate bars highly depends on their formative discharge, with remarkable variations in the harmonic composition and a strong decreasing trend of the skewness of the bed elevation. Similarly, the height of alternate bars clearly decreases with the water discharge, in quantitative agreement with theoretical predictions. However, the disappearance of bars when discharge exceeds a critical threshold is not as sharp as expected due to the formation of so-called "diagonal bars". This work provides basic information for modeling and interpreting short-term morphological variations during individual flood events and long-term trajectories due to alterations of the hydrological regime
Modeling Steady Grain Sorting in River Bifurcations
No full textA striking feature of rivers is their ability to sort the sediments composing them. One of the chief causes for grain sorting consists in the selective transport of different sizes moving downstream. This process can be substantially influenced by the presence of lateral topographic variations, as those produced by channel bifurcations, in which water and sediment are diverted between two smaller anabranches. In particular, field and laboratory observations have shown that the asymmetric flux distribution commonly observed in actual bifurcations is associated with a coarsening of the most-carrying branch. Here, equilibrium sorting in river bifurcations is addressed through a quasi-2D model. Differently from previous works, a fully physically based description of the processes tied to a heterogeneous sediment mixture is introduced. The main mechanisms consist in the lateral pull of sediment due to a sloping bed upstream the bifurcation, and the variation of transport capacity in downstream branches. The model indicates that grain sorting tends to reduce the degree of flux asymmetry between branches for increasing heterogeneity of the mixture. Moreover, the uneven discharge distribution is associated with a different bed surface composition, with bed coarsening of the dominating channel and fining of the other branch. The reduced sediment transport asymmetry and bed surface fining enhance the transport of fine material in the less-carrying branch, thus contributing to keep it morphologically active. Finally, the model predicts an overall fining of bed surface material with respect to the feeder channel
Does a braided river aggrade or degrade in response to changes in water and sediment fluxes?
No full textBraided rivers undergo continuous morphological evolution in response to variations in water and sediment fluxes associated with a plethora of natural and anthropogenic disturbances, which has to date remained poorly understood. Here, a one-dimensional hydro-sediment-morphodynamic model is proposed for braided rivers. It is built upon an extended synthetic channel geometric model which encodes the essential, three-dimensional morphological information into a one-dimensional synthetic channel. It is first applied to the Rees River (New Zealand) to investigate overall morphological evolution (aggradation/degradation) during a single flood event. It is shown that the channel tends to degrade with increasing peak discharge and decreasing sediment supply, while the effect of baseflow discharge on aggradation/degradation is negligible unless the baseflow discharge exceeds a critical value. Then the proposed model is applied to resolve long-term morphological evolution in the Tuotuo River (China) reach (up to the year 2050), the source region of the Yangtze River, in response to projected water and sediment fluxes. Sustained degradation in the river reach is observed. The present study yields a viable physics-based method to predict overall aggradation or degradation of braided rivers under unsteady water and sediment fluxes over both short and long time scales
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