196,315 research outputs found
Free Alternate Bars in Rivers: Key Physical Mechanisms and Simple Formation Criterion
No full textFree 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
The Impact of Climate Change on River Alternate Bars
No full textClimate change is expected to alter the distribution of flow discharge in rivers worldwide. We study the impact of climate-driven flow changes on the shape of riverbed, and specifically on alternate bars, large deposits of gravel/sand that often form in rivers. We consider the illustrative example of the Alpine Rhine River, showing two nearby reaches with similar hydro-morphological characteristics, but different channel width. Hydrological projections are obtained from literature, while the evolution of alternate bars is predicted through a novel, semi-analytical model. Results show a remarkably different behavior of the two reaches: the upstream one, being wide enough for a full development of alternate bars, is resistant to flow alterations; the downstream reach, whose width is close to threshold conditions, is highly susceptible to future changes, showing a strong tendency to increase bar prominence. These findings reflect a general tendency of near-threshold geomorphic systems to be vulnerable to anthropic stressors
Laboratory data - physical modelling of gravel bed rivers under unsteady flow conditions
No full textSediment transport and topography data from laboratory experiments under unsteady flow conditions.
Data supporting the manuscript:
Redolfi, M., Bertoldi, W., Tubino, M., & Welber, M. (2018). Bed load variability and morphology of gravel bed rivers
subject to unsteady flow: A laboratory investigation. Water Resources Research, 54, 842–862. https://doi.org/
10.1002/2017WR021143
Details about the physical model and the experimental procedure can be found in the paper.For further information please contact the corresponding author at [email protected]
On steady alternate bars forced by a localized asymmetric drag distribution in erodible channels
No full textStudying 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
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
Matlab code for simulating the evolution of river alternate bars under different climate change scenarios
No full textMatlab code for simulating the evolution of river alternate bars under different climate change scenarios
->as detailed in the manuscript: Redolfi, M., Carlin, M. & Tubino, M. The impact of climate change on river alternate bars. Geophysical Research Letters.
*Originally developed by Mattia Carlin; revised and reorganized by Marco Redolfi
*The code is developed using Matlab R2022a under Linux Ubuntu and MS Windows
*Please note that this is a research code, not fully tested!
*Simply run:
->main.m to perform simulations for the two reaches in all scenarios
->plotting_basic.m to visualize basic results for individual simulations
->change_factors_HYDROCH.m to visualize yearly variation of change factors
*For further questions please contact [email protected]
Bed Load Variability and Morphology of Gravel Bed Rivers Subject to Unsteady Flow: A Laboratory Investigation
No full textQuasi-Universal Length Scale of River Anabranches
No full textLooping 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
Literacy skills in L2 Italian children with a migrant background: a qualitative analysis of spelling errors
No full textL2 children experience more difficulties in literacy acquisition than their monolingual peers, and their fragilities could be misdiagnosed as learning disabilities. In this light, an accurate identification of the L2 literacy profile is of utmost importance for improving clinical and educational practices. The few available studies on L2 Italian indicate a wider bilingual gap in spelling than in reading, but research on L2 learners’ spelling deficits is still limited. In the present study, we assessed the literacy profile of 25 L2 Italian children and 15 Italian monolingual children by administering standardized word and nonword reading and spelling tasks. We especially focused on spelling skills and error types, which were classified along various dimensions. Results showed that while monolingual children outperformed L2 children in word reading, the two groups did not differ in nonword reading. Spelling was more severely impaired in L2 children, who were less accurate than monolinguals with both words and non-words and had a below-average performance with words. The analysis of errors also revealed significant group differences, with L2 children falling behind monolinguals in the application of orthographic rules and consonant doubling, while no group difference was found in the phonological plausibility of the errors. Our findings hence contribute to a better comprehension of the L2 learner’s literacy profile, with implications for clinical and educational settings
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