Meander Evolution Research Articles

Evaluating secondary flows in the evolution of sine-generated meanders

The development of meanders has been the focus of research for decades because the shifting of bank lines may potentially cause serious problems in river engineering. This study assesses the roles of secondary flows in meander evolution by testing two model parameters, A and As, which represent the effects of momentum redistribution exerted by the topography-driven and curvature-driven secondary currents, respectively. The meander-evolution model couples an analytical solution of flow field in a sine-generated channel with the Bank Erosion and Retreat Model (BERM). The contributions of this study lie in two areas. First, the authors isolated and quantified the roles of curvature-driven and topography-driven secondary flows in meander evolution. Second, the shifting processes of primary flow by secondary flows were investigated in detail by evaluating various variations of the dimensionless scour factor A.

Spatial and temporal intensification of lateral hyporheic flux in narrowing intra‐meander zones

AbstractMeander bends in alluvial rivers morphologically evolve towards meander cut‐off with narrowing intra‐meander necks, and this should steepen hydraulic gradients and intensify intra‐meander hyporheic flux. This research used dye tracking and head loss measurements in a 1 : 500 planimetrically scaled laboratory river table to quantify the spatial and temporal intensification of intra‐meander flux rates at two evolution ages. The younger meander bend, M1, had a sinuosity of 2.3, a river neck width of 0.39 cm, and 0.6% river slope, and the older meander bend, M3, had a sinuosity of 5.2, a river neck width of 0.12 cm, and 0.5% river slope. Flux into and out of the meander bend was estimated along the normalized curvilinear distance s*, with the meander neck at s* = 0.1 and s* = 0.9, the meander centroid at s* = 0.37 and s* = 0.63, and the apex at s* = 0.5. Between the meander centroid and neck, we documented a 60% spatial intensification for M1 and a 90% spatial intensification for M3. Between M1 and M3, we documented a 135% temporal intensification at the neck and a 100% intensification at the centroid. Our empirical spatial and temporal intensification rates involving the M1 and the M3 scenario were one to three times lower than theoretical rates derived from a river evolution model with equivalent M1 and M3 planimetry. Overestimation by the theoretical model was attributed to exaggerated head loss caused by the model neglecting groundwater contributions to river stage. Hyporheic exchange provides critical ecosystem services, and its spatial and temporal variation with meander evolution should be considered in river management. Copyright © 2012 John Wiley & Sons, Ltd.

A Middlebrow Dame Commander: Rose Macaulay, the ‘Intellectual Aristocracy’, and <em>The Towers of Trebizond</em>

In 1955 Noel Annan argued that a veritable ‘intellectual aristocracy’ had developed in Britain during the mid-nineteenth century and that it remained in power well into the twentieth. Tracing the accomplishments, marriages, and honours of the Darwin, Huxley, Macaulay, Stephen, Thackeray, and Strachey families, among others, Annan began his discussion with the author Rose Macaulay (1881–1956) as his exemplar of all of this acclaimed, diligent, and strategic work; his decision was based on her professional achievements, her immediate family’s contributions to academia, the arts, and politics, and her vast web of distinguished relations.1 Such a pronouncement must have come as quite a shock to Macaulay and many of her contemporaries. While her birth, education, and books on history, literary criticism, and travel earned her praise as a respectable author, the best-selling novels, popular journalism, and lively essay collections that formed most of her prolific literary output positioned her as more of a ‘middlebrow’ writer in the literary public sphere. Moreover, much of Macaulay’s career spanned the interwar ‘battle of the brows’, a period of great tensions over culture hierarchies, disparagement of popular writers by more elite authors and critics, and challenges to the cultural dominance of intellectual or highbrow authors by more popular authors and the mainstream reading public. The sudden prestige for an author predominantly associated with the middlebrow sphere, of which Annan’s essay is just one aspect, marked a significant shift in attitudes towards the middlebrow in Britain at this time. And much of this new cultural capital can be attributed to Macaulay’s successful positioning of herself as a reputable and popular public intellectual. The meandering evolution of Macaulay’s career offers an important perspective on the impact that the rise and fall of British modernism had on middlebrow women writers. As Ann Ardis has shown, there were clear indicators of middlebrow culture years before the term arose in the 1920s, yet it was not until

Reductionist versus holistic approaches to the study of river meandering: An ideal dialogue

We discuss some recent attempts to apply the theory of nonlinear dynamical systems to the interpretation of the long term morphodynamic evolution of meandering rivers. To make the discussion attractive for the reader, we employ the method of a Socratic dialogue between a member of the so called ’reductionist community’, who is inclined to support only theories based on physical principles and who is skeptical about fashionable new paradigms, and a member of the ’holistic community’, who supports the idea that new paradigms are needed because rivers are complex systems, whose response can only be interpreted using tools that analyze the system "as a whole". The dialogue focuses on a selection of recent contributions which assesses the progress in understanding of meander dynamics achieved by the use of the above new paradigms. The discussion suggests that some consensus has been reached on the fractal nature of meandering patterns, with the fractal dimension playing the role of a morphometric parameter. On the contrary, despite different early suggestions, recent thorough analysis has been unable to detect any clear evidence that the evolution of meanders displays the characters of either a chaotic or a self organized critical process. The dialogue is concluded with some consensus on the perspective that well founded cellular models may possibly help reconciling the reductionist and holistic viewpoints.

Influence of floodplain erosional heterogeneity on planform complexity of meandering rivers

[1] Although it is widely acknowledged that spatial heterogeneity in floodplain erosional resistance should affect the planform evolution of meandering rivers, past studies have not systematically explored the importance of this effect. Using a physically-based model of river meandering and stochastically-generated heterogeneous landscapes, we analyze in detail how spatial variability in erosional resistance influences meander evolution and the emergence of planform complexity. The simulated planforms are remarkably similar to natural freely-meandering rivers, both visually and in their spectral signatures. We quantify the dependence of meander morphology on both the scale of erosional heterogeneity and the magnitude of erosional variability in external forcing. We also show how the sensitivity of autogenic meandering processes to stochastic variability in the environment leads to different patterns of planform evolution even in landscapes with the same mean spatial heterogeneity and magnitude of variability. The results demonstrate that heterogeneity in erosional resistance has a major influence on meander evolution and should be incorporated into morphodynamic models to enhance our understanding of meandering dynamics at the landscape scale.

Spatial autoregressive structure of meander evolution revisited

Meandering rivers are intrinsically dynamic geomorphic systems that exhibit progressive change in their planforms as they migrate across their floodplains. Recent research in river meandering has focused on the development of linear mathematical models of meander migration. These models, which are based on momentum and mass conservation principles, aim to capture the basic physical mechanisms governing flow-sediment interactions in meandering rivers. A common attribute of the models is that bank erosion is represented as a linear function of near-bank velocity and near-bank velocity is expressed as a spatial convolution of channel curvature. Thus, in all of these process-based models, planform curvature, as mediated through its influence on near-bank velocity, has a strong effect on channel migration. Mathematical details of the curvature–migration relation depend on the number of curvature related convolution terms in the model; the number of such terms increases with increasing model order. Although first- through fourth-order linear models have been developed, few studies have examined rigorously the relation between planform curvature and migration in natural rivers to determine if even the simplest model-based curvature–migration relations correspond to relations derived from empirical data. This paper evaluates empirically the spatial structure of the relation between upstream planform curvature and local channel migration for bends along the Beatton River, British Columbia, Canada. In particular, we compare empirically derived curvature–migration relations to the exponential-decay form of this relation embodied in first-order linear mathematical models of meander migration. For this purpose, we use a refined empirical methodology and recently developed analytical techniques that overcome limitations of a similar analysis of the Beatton River bends conducted by Furbish (1991). The refined methodology examines the influence of upstream lags of planform curvature on migration rates. The new analytical techniques provide data on channel curvature and migration rate at a much finer spatial interval than in the analysis of Furbish (1991). The validity of the approach for revealing the exponential-decay structure of the curvature–migration relation is confirmed by applying it to a synthetic data set generated using a first-order mathematical model. In contrast to the findings of Furbish (1991), our results for the Beatton River bends show that the structure of the spatially distributed upstream curvature effect is more complex than pure exponential decay. The findings suggest that oscillatory components of the relation between curvature and migration rate are important in the planform dynamics of individual complex bends. These findings are consistent with results of recent empirical analysis of the curvature–migration relation based on discrete signal processing and with the structure of the curvature–migration relation in high-order mathematical models of meander migration.

A nonlinear model for river meandering

We develop a nonlinear asymptotic theory of flow and bed topography in meandering channels able to describe finite amplitude perturbations of bottom topography and account for arbitrary, yet slow, variations of channel curvature. This approach then allows us to formulate a nonlinear bend instability theory, which predicts several characteristic features of the actual meandering process and extends results obtained by classical linear bend theories. In particular, in agreement with previous weakly nonlinear findings and consistently with field observations, the bend growth rate is found to peak at some value of the meander wave number, reminiscent of the resonant value of linear stability theory. Moreover, a feature typical of nonlinear waves arises: the selected wave number depends on the amplitude of the initial perturbation (for given values of the relevant dimensionless parameters), and in particular, larger wavelengths are associated with larger amplitudes. Meanders are found to migrate preferentially downstream, though upstream migration is found to be possible for relatively large values of the aspect ratio of the channel, a finding in agreement with the picture provided by linear theory. Meanders are found to slow down as their amplitude increases, again a feature typical of nonlinear waves, driven in the present case by flow rather than geometric nonlinearities. The model is substantiated by comparing predictions with field observations obtained for a test case. The potential use of the present approach to investigate a number of as yet unexplored aspects of meander evolution (e.g., chute cutoff) is finally discussed.

Intra‐meander hyporheic flow in alluvial rivers

Several geomorphological fluvial features are able to induce hyporheic exchange between the rivers and the alluvial sediments. However, while the small‐scale exchange induced by bed forms has been thoroughly investigated, the role of the larger features remains poorly understood. Here, we focus on the hyporheic flows driven by the channel sinuosity in the intrameander zone. A physically based model is adopted to simulate the morphodynamic evolution of three different meandering rivers, from the first stages of the meander evolution until the incipient meander cutoff. For each stage, the sinuosity‐driven intrameander hyporheic flow is computed. In this way, the hyporheic flow field, the fluxes exchanged with the river, and the residence times are described during the whole meander evolution. The main result concerns the existence of a remarkable zonation induced by the flow field in the intrameander zone. The more the meander evolves, the more the zonation becomes pronounced, and the probability distribution of the residence times shows a bimodal shape with an intermediate power law behavior. Some general rules governing the typical timescales of the intrameander hyporheic flow and the mean exchanged flux are also deduced.

Examining the physical meaning of the bank erosion coefficient used in meander migration modeling

Widely used models of meander evolution relate migration rate to vertically averaged near-bank velocity through the use of a coefficient of bank erosion ( E). In applications to floodplain management problems, E is typically determined through calibration to historical planform changes, and thus its physical meaning remains unclear. This study attempts to clarify the extent to which E depends on measurable physical characteristics of the channel boundary materials using data from the Sacramento River, California, USA. Bend-average values of E were calculated from measured long-term migration rates and computed near-bank velocities. In the field, unvegetated bank material resistance to fluvial shear ( k) was measured for four cohesive and noncohesive bank types using a jet-test device. At a small set of bends for which both E and k were obtained, we discovered that variability in k explains much of the variability in E. The form of this relationship suggests that when modeling long-term meander migration of large rivers, E depends largely on bank material properties. This finding opens up the possibility that E may be estimated directly from field data, enabling prediction of meander migration rates for systems where historical data are unavailable or controlling conditions have changed. Another implication is that vegetation plays a limited role in affecting long-term meander migration rates of large rivers like the Sacramento River. These hypotheses require further testing with data sets from other large rivers.

Meanders of the Jialing River in China: Morphology and formation

As a fundamental category of river system, comparisons in diverse regions and types with quantitative parameters on meander morphology will help to distinguish different characteristics and approach formations and evolutions. It also can provide scientific basis of geomorphology for their protections and exploitation of the meander resources. Based on the images of Google Earth, meanders’ parameters are proposed and then measured and calculated. The result suggests that incised meanders in the Jialing River are among the most complex, irregular and sinuous channels. Special river gradient, geological structures, strata and lithology, backwater effect of landform are key factors to shape meanders of the Jialing River. The evolution of meander in the Jialing River obeys the principle of minimum-maximum energy dissipation. Easy acquisitions of remote sensing data make regional and global comparisons possible, and then the differences of formations can be explored. The morphology can probably provide an evidence of antecedent river and demonstrate the channel incisions and crustal uplifts. Meander core in Chinese is named after the isolated hills in the Jialing River. The incised meander is a kind of resource and needs to be protected.

Significance of the riparian vegetation dynamics on meandering river morphodynamics

A river and its surrounding riparian vegetation are two dynamical systems that interact through several hydrological, geomorphological, and ecological processes. This work focuses on the role played by vegetation on meandering river morphodynamics: River planform evolution forces the riparian vegetation dynamics, which, in turn, affect the mechanical characteristics of the river banks and influence the meandering dynamics of the river itself. It follows that despite the fact that a traditional engineering approach considers vegetation as a static element the study of river morphodynamics should be coupled with the riparian vegetation evolution. To this end, a fluid dynamic model of meandering rivers is here coupled with a process‐based model for the riparian biomass dynamics. The feedback of vegetation on river morphology is provided by a relation that links the biomass density to the bank erodibility. The numerical results highlight (1) the remarkable effects of the vegetation dynamics on meander evolution and (2) the role of the temporal scales of vegetation growth and decay in relation to typical morphodynamic scales. In particular, the differences with respect to the constant erodibility case can be of the order of tens or hundreds of meters (10–20% of the meander wavelength), and peculiar meander shapes that do not show the usual marked upstream skewness emerge.

Meander-Bend Evolution, Alluvial Architecture, and the Role of Cohesion in Sinuous River Channels: A Flume Study

Abstract Realistic physical models of meandering rivers have proven extremely difficult to produce, particularly in comparison to the formation of braided rivers in the laboratory. Here we address the question of why such realistic model meanders are so difficult to reproduce, through the most realistic physical modeling of meandering channels yet achieved. This paper demonstrates that cohesion is a key variable in the development and maintenance of single-thread channels. In particular, cohesion must be sufficient to force the planform away from a braided state but low enough for active migration to continue and for the avoidance of a gradual reduction and eventual cessation of planform movement (ossification). The enhanced realism of the experiments also enables the processes of meander evolution, and critically the resultant alluvial architecture, to be examined in a physical model for the first time. Planform history can be linked to deposits, and this process–product linkage enables the depositional development of the experimental deposits to be compared against, and to test, existing models of bedding geometries within point bars. Here we document three mechanisms for bend cutoff, provide new process explanations for certain modes of bend evolution in coarse-grained meandering rivers, examine the geometries and spatial distribution of alluvial architecture, and demonstrate that existing models of point-bar geometry successfully reproduce the larger-scale aspects of point-bar accretion in rivers dominated by episodic unit-bar accretion.

Simulating meander evolution of the Geul River (the Netherlands) using a topographic steering model

AbstractActive meandering rivers are capable of reworking and removing large quantities of valuable land. Therefore, understanding the characteristics of meandering rivers and predicting future meander behaviour can be of great value for local authorities. In this study, we apply a topographic steering meander model to the Geul River (southern Netherlands), using field data to calibrate the model. The present channel characteristics of the Geul River were mapped in the field. Cut‐banks were classified as erosive, unstable or stable. The model outcomes were compared with these field data.Several model runs were carried out, using different sets of parameter values. After studying the results and using the field data, we introduced the concept of a variable channel width in the simulation model. In reality, the river has different channel widths varying from 8 to more than 15 m. These widths are a linear function of local curvature. The model runs using a variable channel width show that the model is capable of predicting locations of lateral migration in conformity with observed active lateral migration and erosive banks. With both models, the sediment reworking time of the floodplain can be calculated. Floodplain reworking times of 200–300 years were calculated. In combination with the lateral migration rate, this reworking time is an important element in catchment sediment budget calculations. Copyright © 2007 John Wiley & Sons, Ltd.

Modelling free-forming meander evolution in a laboratory channel using three-dimensional computational fluid dynamics

The present study investigated the use of computational fluid dynamics (CFD) in predicting the formation, development, and migration of free-forming meander bends. The three-dimensional CFD model computed water flow and sediment transport in alluvial channels and predicted vertical and horizontal bed changes. Different algorithms and parameters were tested to provide an insight into the application range of CFD when modelling free-forming meander formation. The computational domain was discretized by an unstructured grid. A control volume method was used for the discretization of the Navier–Stokes equations for the flow calculation and of the convection–diffusion equation for the sediment transport calculation. Turbulence was modelled by the k– ε turbulence model. The simulation was started from an initially straight grid, with neither sediment feed nor any perturbation at the inflow boundary. The model computed the river bed evolution over a real time period of 3 d. Results were compared with laboratory experiments and showed that the CFD model can predict many of the characteristics of the alluvial meander formation and migration. However, some limitations and uncertainties exist that have to be clarified in future investigations.

Flow structure in sinuous submarine channels: Velocity and turbulence structure of an experimental submarine channel

Submarine channels have long been considered analogous to meandering fluvial channels due to their similar planform geometry, and this has given rise to strong analogies in terms of the fluid dynamics of these channel types. However, there is a paucity of direct measurements detailing the structure of velocity or sediment concentration, and nothing is known about the structure of secondary flow, within submarine channels. Within fluvial channels, secondary circulation has been shown to be an important control of erosion, deposition, lateral sediment transport and overall bend stability. This paper presents a series of experiments in which saline gravity currents flowed through a submerged, fixed-form, sinuous channel model. Ultrasonic Doppler velocity profiling provided high-resolution, three-dimensional visualisation of flow fields, thereby allowing the first quantitative visualisation of velocity profiles and secondary flows of a saline gravity current within a sinuous submarine channel. The same model was also used to measure the structure of flow within a subaerial channel, thereby providing a detailed comparison between a submarine and fluvial channel. The data reveal that secondary flow cells in submarine channels are best developed at bend apexes and that the basal component of the flow cell moves from the inside to the outside of the bend, in a reversed direction to that expected from models of fluvial bend flow. These results are of prime importance when assessing the validity of comparisons between submarine and fluvial channels, and the role of secondary flow cells in the migration and evolution of meander bends. This work demonstrates that the fundamental flow processes controlling the evolution and migration of submarine and fluvial channels may be very different, and suggest that the form and evolution of submarine channels may also show different properties to their fluvial counterparts.

Simulating sine-generated meandering channel evolution with an analytical model

An analytical model to simulate the evolution of a meandering channel was developed based on the analytical solution of flow field in a sine-generated meandering channel and application of the bank erosion model by Duan et al. The flow-field solution was derived from two-dimensional, depthaveraged, steady-flowequations in channel-fitted, curvilinear orthogonal coordinates where the transverse bed slopewas treated as a variable increasing with channel sinuosity. The flow-field solution indicated that the location where the shift of maximum velocity zone from the convex to concave banks in sine-generated channels varies with the transverse bed slope and the strength of secondary current. The rate of bank erosion was calculated as a gradient function of the longitudinal sediment transport rate and strength of the secondary flow rather than being proportional to the magnitude of excessive near-bank velocity or shear stress. This model replicated the evolution of meandering channels from mildly to highly sinuous ones. Simulation results showed the migration of meandering channel is a combination of downstream translation, lateral extension, expansion, and downstream and upstream rotation. Low-sinuous free meanders migrate rapidly toward downstream. As the sinuosity increases, downstream translation diminishes, and meandering loops expand laterally with its head rotating toward downstream and then upstream. The simulated results indicated the gradient of the longitudinal sediment transport rate is essential in modeling meandering evolution.

Cutoffs galore!: occurrence and causes of multiple cutoffs on a meandering river

The creation of cutoffs and of oxbow lakes is a well-known phenomenon of meandering rivers, but views on the extent to which they are inherent in meander behaviour have varied. Assumptions of meander behaviour have shifted from those of stability and equilibrium to recognition of gradual evolution and increased complexity of form. Alternative explanations of cutoff occurrence are discussed here in relation to a remarkable set of cutoffs that occurred in one reach of the River Bollin, UK, for which long-term historical evidence of meander evolution existed and which has been monitored for change and processes over the last 20 years. The cutoffs occurred during the high floods of winter 2000–2001. A series of hypotheses is examined, including the occurrence of floods and effects of hydrological changes. Although the flood events actually caused the cutoffs, the long-term pattern accords with ideas of chaotic behaviour and sinuosity of a river reaching a critical state at which clustering of meander cutoffs takes place. It is suggested that the occurrence of the cutoffs can be explained as inherent in meander behaviour.

The effect of bidirectional flow on tidal channel planforms

AbstractSalt marsh tidal channels are highly sinuous. For this project, field surveys and aerial photographs were used to characterize the planform of tidal channels at China Camp Marsh in the San Francisco Bay, California. To model the planform evolution, we assume that the topographic curvature of the channel centreline is a key element driving meander migration. Extraction of curvature data from a planimetric survey, however, presents certain problems because simple calculations based on equally distanced points on the channel axis produce numerical noise that pollutes the final curvature data. We found that a spline interpolation and a polynomial fit to the survey data provided us with a robust means of calculating channel curvature. The curvature calculations, combined with data from numerous cross‐sections along the tidal channel, were used to parameterize a computer model. With this model, based on recent theoretical work, the relationship between planform shape and meander migration as well as the consequences of bidirectional flow on planform evolution have been investigated. Bank failure in vegetated salt marsh channels is characterized by slump blocks that persist in the channel for several years. It is therefore possible to identify reaches of active bank erosion and test model predictions. Our results suggest that the geometry and evolution of meanders at China Camp Marsh, California, reflect the ebb‐dominated regime. Copyright © 2004 John Wiley & Sons, Ltd.

Channel evolution of the pre‐channelized Danube River in Bratislava, Slovakia (1712–1886)

AbstractChannel adjustments of the Danube River near Bratislava, Slovakia, are reconstructed from historical river maps for 1712–1886, the period preceding mid‐course channelization. The study reach comprised an anastomosing–meandering planform, characteristic of the upper area of a large alluvial fan. The key mechanisms of channel change were avulsions through channel switching into chute channels, meander development through progression and cutoffs, and abandonment of secondary channels. The gravel‐bed Danube River was characterized by rapid rates of lateral erosion (maxima from 7·5 to 37 m a−1), and extensive areas of point bars and gravel bars. Within three to seven years, new bars were stabilized by Salici–Populetum woodlands which subsequently developed into more stable vegetated islands. Human interventions were relatively common and frequent in the past, though increasing fluvial activity during the 18th century was responsible for their limited effectiveness. Uncontrollable lateral erosion at Petržalka village, terminated only by a switch in position of the Danube River upstream from Bratislava (1766–1774), appears to be closely related to increasing ice‐floods and other high magnitude flood events in the latter half of the 18th century. These events were coincident with the peak of the Little Ice Age. The effect of floods was probably amplified by human works which resulted in stabilization of secondary channels, simplification of the river pattern, and concentration and widening of the main channel. A further sign of channel readjustment to new conditions was the evolution of new large meanders, a tendency for channel switching and production of new anastomosing–meandering parallel channels. Copyright © 2002 John Wiley & Sons, Ltd.

Real‐time forecasting of biological and physical dynamics at the Iceland‐Faeroes Front in June 2001

Real‐time physical and biological forecasting was carried out at sea in June 2001 at the Iceland‐Faeroes Front using a 3D coupled physical and biological model. Data collected during three repeated mesoscale surveys were used for model initialisation, data assimilation and forecast verification. The forecast successfully predicted the evolution of an anticyclonic meander and associated deep mixed layer leading to severe light limitation of phytoplankton production.