Bend theory of river meanders with spatial width variations

2011 ◽  
Vol 681 ◽  
pp. 311-339 ◽  
Author(s):  
ROSSELLA LUCHI ◽  
GUIDO ZOLEZZI ◽  
MARCO TUBINO
Keyword(s):  
Perturbation Expansion ◽  
Potential Interaction ◽  
Channel Width ◽  
Meandering Streams ◽  
Meandering River ◽  
Longitudinal Modes ◽  
Meandering Channels ◽  
Aspect Ratios ◽  
Meander Bends ◽  
Averaged Model

The present work revisits the classical, uniform-width bend theory with the aim to understand whether and how spatial width oscillations can affect the process of linear bend stability that initiates meander planform evolution. Although longitudinal oscillations of channel width are common along many meandering streams, little investigation of their properties and dynamic effects has been pursued so far. The theory therefore accounts for width variations as a geometrical forcing in a depth-averaged model of meander morphodynamics by assuming the potential interaction with the classical curvature forcing effect. A first quantification of width variations is made by referring to a freely evolving meandering river, which shows that the dimensionless amplitude of width variations is a ‘small’ parameter with comparable magnitude to that of curvature variations, thus suggesting the use of a two-parameter perturbation expansion. Moreover, it is reasonable to assume that channel width oscillates in space with a double frequency relative to curvature, which implies that one nonlinear interaction between the two forcing effects is enough to reproduce the effect of spatial width variations on the process of bend stability. Overall, width variations consistently promote the instability of shorter bends with respect to meanders with uniform width: on average, this predicted tendency is supported by analysis of field data referring to hundreds of natural meander bends. The effect on meander wavelength selection depends on the location of the widest section relative to the bend apex. Under typical formative conditions of gravel-bed rivers, with large-enough channel aspect ratios, two distinct most unstable longitudinal modes develop. Such behaviour is absent when the width is uniform, and suggests a mechanistic interpretation for the reach-scale occurrence of chute cutoffs that can be observed more frequently in wider-at-bends than in equiwidth meandering channels.

Interaction between curvature-driven width oscillations and channel curvature in evolving meander bends

2019 ◽  
Vol 876 ◽  
pp. 985-1017 ◽  
Author(s):  
F. Monegaglia ◽  
M. Tubino ◽  
G. Zolezzi
Keyword(s):  
Channel Width ◽  
Feedback Process ◽  
Remotely Sensed Data ◽  
Meandering River ◽  
Aspect Ratios ◽  
Meander Bends ◽  
Channel Curvature ◽  
Curvature Driven ◽  
Additional Equation ◽  
Insight Into

We study the morphodynamics of channel width oscillations associated with the planform development of river meander bends. With this aim we develop a novel planform evolution model, based on the framework of the classical bend theory of river meanders by Ikeda et al. (J. Fluid Mech., vol. 112, 1981), that accounts for local width changes over space and time, tied to the local hydro-morphodynamics through a two-way feedback process. We focus our attention on ‘autogenic’ width variations, which are forced by flow nonlinearities driven by channel curvature dynamics. Under the assumption of regular, sinusoidal width and curvature oscillations, we obtain a set of ordinary differential equations, formally identical to those presented by Seminara et al. (J. Fluid Mech., vol. 438, 2001, pp. 213–230), with an additional equation for the longitudinal oscillation of the channel width. The proposed approach gives insight into the interaction between autogenic width variations and curvature in meander development and between forcing and damping effects in the formation of width variations. Model outcomes suggest that autogenic width oscillations mainly determine wider-at-inflection meandering river patterns, and affect their planform development particularly at super-resonant aspect ratios, where the width oscillation reaches its maximum and reduces meander sinuosity and lateral floodplain size. The coevolution of autogenic width oscillation and curvature occurs through temporal hysteresis cycles, whereby the peak in channel curvature lags behind that of width oscillation. Width oscillation amplitudes predicted by the model are consistent with those extracted from remotely sensed data.

scholarly journals The effect of core size on the speed of compressible hollow vortex streets

2017 ◽  
Vol 836 ◽  
pp. 797-827 ◽  
Author(s):  
Darren G. Crowdy ◽  
Vikas S. Krishnamurthy
Keyword(s):  
Mach Number ◽  
Compressible Flow ◽  
Vortex Core ◽  
Perturbation Expansion ◽  
Core Size ◽  
Flow Equations ◽  
Stable Case ◽  
First Order ◽  
Aspect Ratios ◽  
Vortex Streets

The effect of weak compressibility on the speed of steadily translating staggered vortex streets of hollow vortices in isentropic subsonic flow is studied. A small-Mach-number perturbation expansion about the incompressible solutions for staggered streets of hollow vortices found recently by Crowdy & Green (Phys. Fluids, 2011, vol. 23, 126602) is carried out; the latter solutions provide a desingularization of the classical point vortex streets of von Kármán. The first-order compressible flow correction is calculated. We employ a novel scheme based on a complex variable formulation of the compressible flow equations (the Imai–Lamla method) combined with conformal mapping theory to track the vortex shape in this free boundary problem. The analysis to find the perturbed streamfunction and compressible vortex shapes is greatly facilitated by exploiting a calculus based on use of the Schottky–Klein prime function of a conformally equivalent parametric annulus. It is found that, for a vortex street of specified aspect ratio comprising vortices of specified circulation, the vortex core size is a key determinant of whether compressibility increases or decreases the steady propagation speed (relative to the incompressible street with the same parameters) and that both eventualities are possible. We focus attention on streets with aspect ratios around 0.28, which is close to the neutrally stable case for incompressible flow, and find that a critical vortex core size exists at which compressibility does not affect the speed of the street at first order in the (squared) Mach number. Streets comprising vortices with core size below the critical value speed up due to compressibility; larger vortices slow down.

scholarly journals The Spatial Distribution of Bed Sediment on Fluvial System: A Mini Review of the Aceh Meandering River

2016 ◽  
Vol 5 (2) ◽  
Author(s):  
Muhammad Irham
Keyword(s):  
Spatial Distribution ◽  
Bed Load ◽  
Sieve Analysis ◽  
Left Bank ◽  
Fluvial System ◽  
Dynamic Interactions ◽  
Meandering River ◽  
River Bed ◽  
Meander Bends ◽  
Mechanical Weathering

Dynamic interactions of hydrological and geomorphological processes in the fluvial system result in accumulated deposit on the bed because the capacity to carry sediment has been exceeded. The bed load of the Aceh fluvial system is primarily generated by mechanical weathering resulting in boulders, pebbles, and sand, which roll or bounce along the river bed forming temporary deposits as bars on the insides of meander bends, as a result of a loss of transport energy in the system. This dynamic controls the style and range of deposits in the Aceh River. This study focuses on the spatial distribution of bed-load transport of the Aceh River. Understanding the spatial distribution of deposits facilitates the reconstruction of the changes in controlling factors during accumulation of deposits. One of the methods can be done by sieve analysis of sediment, where the method illuminates the distribution of sediment changes associated with channel morphology under different flow regimes. Hence, the purpose of this mini review is to investigate how the sediment along the river meander spatially dispersed. The results demonstrate that channel deposits in the Aceh River are formed from four different type of materials: pebble deposited along upstream left bank; sand located on the upstream, downstream, and along meander belts; and silt and clay located along the cut bank of meander bends. Because of different depositional pattern, the distribution of the sediment along the river can be used as a surrogate to identify bank stability, as well as to predict critical geometry for meander bend initiation

scholarly journals A theoretical analysis of river bars stability under changing channel width

2014 ◽  
Vol 39 ◽  
pp. 27-35 ◽  
Author(s):  
S. Zen ◽  
G. Zolezzi ◽  
M. Tubino
Keyword(s):  
Growth Rate ◽  
Small Amplitude ◽  
Constant Width ◽  
Perturbation Expansion ◽  
Basic Flow ◽  
Channel Width ◽  
Nonlinear Stability Analysis ◽  
Mathematical System ◽  
Instantaneous Discharge ◽  
Two Parameters

Abstract. In this paper we propose a new theoretical model to investigate the influence of temporal changes in channel width on river bar stability. This is achieved by performing a nonlinear stability analysis, which includes temporal width variations as a small-amplitude perturbation of the basic flow. In order to quantify width variability, channel width is related with the instantaneous discharge using existing empirical formulae proposed for channels with cohesionless banks. Therefore, width can vary (increase and/or decrease) either because it adapts to the temporally varying discharge or, if discharge is constant, through a relaxation relation describing widening of an initially overnarrow channel towards the equilibrium width. Unsteadiness related with changes in channel width is found to directly affect the instantaneous bar growth rate, depending on the conditions under which the widening process occurs. The governing mathematical system is solved by means of a two-parameters (ε, δ) perturbation expansion, where ε is related to bar amplitude and δ to the temporal width variability. In general width unsteadiness is predicted to play a destabilizing role on free bar stability, namely during the peak stage of a flood event in a laterally unconfined channel and invariably for overnarrow channels fed with steady discharge. In this latter case, width unsteadiness tends to shorten the most unstable bar wavelength compared to the case with constant width, in qualitative agreement with existing experimental observations.

scholarly journals PENGARUH PERGERAKAN MEANDER TERHADAP KESEIMBANGAN ALUR SUNGAI

2019 ◽  
Vol 1 (2) ◽  
pp. 45
Author(s):  
Siti Murniningsih
Keyword(s):  
Sediment Load ◽  
Low Impact Development ◽  
Channel Cross Section ◽  
Fluvial System ◽  
Outer Banks ◽  
Meandering River ◽  
Sediment Loads ◽  
Meander Bends ◽  
Minimum Velocity ◽  
Sedimentation Processes

<span><em>Most of the river in Indonesia has a meander area especially located at the lower reach. Since the pattern of </em><span><em>the community behaviour in Indonesia, people live in surrounding or along the river while the river plains </em><span><em>and delta consist of alluvial soils, thus, meander migration usually occurs as a response to natural or manmade disturbances of the fluvial system. Meander River in urban area usually encourage sediment transport </em><span><em>problem such as riverbed aggradations at the area surrounding inside of the bank then will decrease the </em><span><em>river capacity in retaining flood while simultaneously eroding on the outer banks of meander bends. </em><span><em>Regarding velocity distribution at the channel cross section, the minimum velocity is occurring on the inside </em><span><em>of the meander, therefore some of the sediment loads is deposited. Related with the phenomena, protection </em><span><em>against sedimentation and erosion along meandering river extremely needed. Previously, river adjustment </em><span><em>within meander area are continually being made, therefore, eventually the gradient of a stream is altered to </em><span><em>accommodate the volume of water and the velocity necessary to transport the sediment load. In this paper,</em><span><em>understanding of various phenomenon’s in the meander area due to the sedimentation processes are </em><span><em>described and propose the new concept of protection with low impact development (LID) approach.</em></span></span></span></span></span></span></span></span></span></span></span><br /><br class="Apple-interchange-newline" /></span>

Capturing Radial Mixing in Axial Compressors With Computational Fluid Dynamics

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Lorenzo Cozzi ◽  
Filippo Rubechini ◽  
Matteo Giovannini ◽  
Michele Marconcini ◽  
Andrea Arnone ◽  
...  
Keyword(s):  
Strain Field ◽  
Axial Compressor ◽  
Potential Interaction ◽  
Tip Clearance ◽  
Navier Stokes ◽  
Streamwise Vorticity ◽  
Radial Transport ◽  
Axial Compressors ◽  
Aspect Ratios ◽  
Clearance Flow

The current industrial standard for numerical simulations of axial compressors is the steady Reynolds-averaged Navier–Stokes (RANS) approach. Besides the well-known limitations of mixing planes, namely their inherent inability to capture the potential interaction and the wakes from the upstream blades, there is another flow feature which is lost, and which is a major accountable for the radial mixing: the transport of streamwise vorticity. Streamwise vorticity is generated for various reasons, mainly associated with secondary and tip-clearance flows. A strong link exists between the strain field associated with the vortices and the mixing augmentation: the strain field increases both the area available for mixing and the local gradients in fluid properties, which provide the driving potential for the mixing. In the rear compressor stages, due to high clearances and low aspect ratios, only accounting for the development of secondary and clearance flow structures, it is possible to properly predict the spanwise mixing. In this work, the results of steady and unsteady simulations on a heavy-duty axial compressor are compared with experimental data. Adopting an unsteady framework, the enhanced mixing in the rear stages is properly captured, in remarkable agreement with experimental distributions. On the contrary, steady analyses strongly underestimate the radial transport. It is inferred that the streamwise vorticity associated with clearance flows is a major driver of radial mixing, and restraining it by pitch-averaging the flow at mixing planes is the reason why the steady approach cannot predict the radial transport in the rear part of the compressor.

Coevolution of width and sinuosity in meandering rivers

2014 ◽  
Vol 760 ◽  
pp. 127-174 ◽  
Author(s):  
Esther C. Eke ◽  
M. J. Czapiga ◽  
E. Viparelli ◽  
Y. Shimizu ◽  
J. Imran ◽  
...  
Keyword(s):  
Erosion Rate ◽  
Bank Erosion ◽  
Channel Width ◽  
Channel Migration ◽  
Meandering Rivers ◽  
Local Curvature ◽  
Meandering River ◽  
Bed Elevation ◽  
Dependent Rate ◽  
Variation Patterns

AbstractThis research implements a recently proposed framework for meander migration, in order to explore the coevolution of planform and channel width in a freely meandering river. In the model described here, width evolution is coupled to channel migration through two submodels, one describing bank erosion and the other describing bank deposition. Bank erosion is modelled as erosion of purely non-cohesive bank material damped by natural armouring due to basal slump blocks, and bank deposition is modelled in terms of a flow-dependent rate of vegetal encroachment. While these two submodels are specified independently, the two banks interact through the medium of the intervening channel; the morphodynamics of which is described by a fully nonlinear depth-averaged morphodynamics model. Since both banks are allowed to migrate independently, channel width is free to vary locally as a result of differential bank migration. Through a series of numerical runs, we demonstrate coevolution of local curvature, width and streamwise slope as the channel migrates over time. The correlation between the local curvature, width and bed elevation is characterized, and the nature of this relationship is explored by varying the governing parameters. The results show that, by varying a parameter representing the ratio between a reference bank erosion rate and a reference bank deposition rate, the model is able to reproduce the broad range of river width–curvature correlations observed in nature. This research represents a step towards providing general metrics for predicting width variation patterns in river systems.

The case of the braided river that meandered: Bar assemblages as a mechanism for meandering along the pervasively braided Missouri River, USA

2020 ◽  
Author(s):  
John M. Holbrook ◽  
Sarah D. Allen
Keyword(s):  
Missouri River ◽  
Braided River ◽  
Stream Power ◽  
Meandering Rivers ◽  
Meandering River ◽  
Braided Rivers ◽  
Single Thread ◽  
Point Bars ◽  
Meander Bends ◽  
Exclusive Processes

This paper offers a mechanism for meandering in an otherwise braided river and then discusses its general implications for river processes and fluvial deposits. Braided rivers manage to meander without the paired point bars and single-thread channels that are instrumental in developing bends in other meandering rivers. The driving processes for meandering in these braided systems remain enigmatic. The unchannelized and prechannelized Missouri River is an example of a braided meandering river, and it provides an opportunity to gain insight into these processes. This study utilized historical maps, sequential air photos, and surficial geologic maps both to define the processes by which this braided river meanders, and to characterize the deposits produced by these processes. These data show that the Missouri River meanders by building point assemblages instead of point bars. Repeated accretion of midchannel and lateral bars to a common point on the bank forces development of a meander bend around a point assemblage comprising multiple amalgamated compound bars. This differs from single-thread systems, which expand and translate bends around a single compound point bar. Alternating development of point assemblages forces meandering over successions of meander bends. Braided meander loops grow by expansion and translation like single-thread rivers, but they also may contract to produce counterpoint assemblages. Contraction appears to be the more common means of loop abandonment compared to loop cutoff for the braided Missouri River. This differs from single-thread meandering rivers, where contraction is limited, and loop cutoff is consistently the dominant abandonment process. Deposits of the braided meandering Missouri River differ from deposits of single-thread rivers in the rarity of both meander scrolls and single-thread channel fills. Instead, point and counterpoint assemblages comprise fusiform bar elements bound by small filled remnants of anabranch channels. These assemblages are commonly bound by meander cutbank scars. Cutbank scars associated with contraction, however, tend to be composite rather than discrete erosional surfaces, and they do not tend to bind river-scale abandoned channel fills. The braided meandering Missouri River also differs from wandering rivers because wandering rivers meander by building compound bars instead of assemblages, are more gravelly, have less pervasive and much less mobile midchannel bars, and appear to reflect a transitional intermediate pattern instead of a stable hybrid pattern. Braiding and meandering both expend stream power, and both are mechanisms for achieving channel equilibrium. The Missouri River exhibits both of these processes in tandem; thus, meandering and braiding are not mutually exclusive processes. Braided meandering rivers like the Missouri River are less common than either straight-braided or single-thread-meandering rivers, but they are not unique. The long-held distinction of braided versus meandering patterns for rivers thus may be practical but is not definitive.

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