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1.
The evolution of meander bends and formation of cutoffs, including a series of cutoffs developed simultaneously in a number of bends, have been investigated by many researchers. However, relatively little is known about factors that lead to the development of multiple cutoffs that are formed subsequently at one location. The present study aims to determine the influence of meander bend development on multiple chute cutoff formation in a single bend. The research is based on the sedimentary record of meander migration and cutoffs preserved in a lowland river floodplain (the lower Obra River, Poland). Analysis of changes in meander geometry was conducted to describe the influence of their migration on cutoff formation and in other rivers where multiple cutoffs occurred. The results showed that multiple cutoffs in the lower Obra River have occurred during the last 3000 years, owing to the interaction of upstream and downstream controls: migration of meander bends in opposing directions accompanied by an increase of flood frequency and sediment supply. The flow and sediment supply has been further altered since the nineteenth century due to anthropogenic impacts: an artificial cutoff of the downstream bend and elevation of channel levées. Similar mechanisms driving the formation of multiple cutoff have been found in other river courses, despite significantly higher energy of the compared rivers. Moreover, development of a confined‐shape bend (caused by artificial barrier or autogenic bend behaviour) may also favour the formation of multiple cutoffs. However, counter migration of meanders enhanced by increased flood frequency and sediment supply are primary triggers for such events. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

2.
Recent field and modeling investigations have examined the fluvial dynamics of confluent meander bends where a straight tributary channel enters a meandering river at the apex of a bend with a 90° junction angle. Past work on confluences with asymmetrical and symmetrical planforms has shown that the angle of tributary entry has a strong influence on mutual deflection of confluent flows and the spatial extent of confluence hydrodynamic and morphodynamic features. This paper examines three‐dimensional flow structure and bed morphology for incoming flows with high and low momentum‐flux ratios at two large, natural confluent meander bends that have different tributary entry angles. At the high‐angle (90°) confluent meander bend, mutual deflection of converging flows abruptly turns fluid from the lateral tributary into the downstream channel and flow in the main river is deflected away from the outer bank of the bend by a bar that extends downstream of the junction corner along the inner bank of the tributary. Two counter‐rotating helical cells inherited from upstream flow curvature flank the mixing interface, which overlies a central pool. A large influx of sediment to the confluence from a meander cutoff immediately upstream has produced substantial morphologic change during large, tributary‐dominant discharge events, resulting in displacement of the pool inward and substantial erosion of the point bar in the main channel. In contrast, flow deflection is less pronounced at the low‐angle (36°) confluent meander bend, where the converging flows are nearly parallel to one another upon entering the confluence. A large helical cell imparted from upstream flow curvature in the main river occupies most of the downstream channel for prevailing low momentum‐flux ratio conditions and a weak counter‐rotating cell forms during infrequent tributary‐dominant flow events. Bed morphology remains relatively stable and does not exhibit extensive scour that often occurs at confluences with concordant beds. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

3.
The distal reach of the Lower Jingjiang River(LJR)in the middle of the Yangtze River consists of five adjacent bends,among which the Qigongling Bend is a U-shaped meander with a mean sinuosity of 2.2 and the narrowest neck 525 m in width.This bend is slowly approaching neck cutoff owing to progressive bank erosion.An abnormal phenomenon has occurred in this bend since the Three Gorges Reservoir(TGR)began to operate in 2003 which is erosion in the inner bank zone and deposition in the outer bank zone.This problem has not been fully understood because of the interplay of changes in water-sediment,bank erosion,and artificial bank revetment.In this study,aerial and remote sensing images,hydrological data,channel topography,and an existing bank erosion model are used to reveal channel morphodynamics of this bend and the trend of the potential neck cutoff induced by bank erosion.The study results show that the clear water released from the TGR has provided by forcefully eroded the point bar of inner bank but failed to scour the outer bank due to the protection of bank revetment since the 1990 s.Thus far,the outer bank zone near the bend apex has increasingly widened in conjunction with the formation of 2 emerging sand bars.Consequently,the thalweg of the main channel has laterally shifted toward the inner bank by roughly 800 m.More severely,the rate of bank retreat on the upstream side of the bend neck was about 4.5 m/yr in 2010-2019,but the downstream side of this neck was experienced slight deposition.Bank erosion could be accelerated by progressively increasing erosion and eventually trigger the occurrence of neck cutoff in the next few decades,thereby significantly altering the quasi-equilibrium regime of channel morphodynamics in the LJR.  相似文献   

4.
River bends occasionally meander to the point of cutoff, whereby a river shortcuts itself and isolates a portion of its course. This fundamental process fingerprints a river's long-term planform geometry, its stratigraphic record, and biogeochemical fluxes in the floodplain. Although meander cutoffs are common in fast-migrating channels, timelapse imagery of the Earth surface typically does not offer a long enough baseline for statistically robust analyses of these processes. We seek to bridge this gap by quantifying cutoff kinematics along the Humboldt River (Nevada) – a stream that, from 1994 to 2019, hosted an exceptionally high number of cutoffs (specifically, 174 of the chute type and 53 of the neck type). A coincidence between major floods and cutoff incidence is first suggestive of hydrographic modulation. Moreover, not just higher sinuosity but also upstream planform skewness is associated with higher cutoff incidence and channel widening for a sub-population of chute cutoffs. We propose a conceptual model to explain our results in terms of channel-flow structure and then examine the distances between adjacent cutoffs to understand the mechanisms governing their clustering. We find that both local and nonlocal perturbations together trigger the clustering of new cutoffs, over distances capped by the backwater length and over yearly to decadal timescales. Our research suggests that planform geometry and backwater controls might sway the occurrence of cutoff clusters – both local and nonlocal – thereby offering new testable hypotheses to explore the evolution of meandering-river landscapes that have significant implications for river engineering and stratigraphic modelling. © 2020 John Wiley & Sons, Ltd.  相似文献   

5.
The morphological evolution of the entrances and exits of abandoned river channels governs their hydrological connectivity. The study focusses on flow and sediment dynamics in the exit of a cutoff meander where the downstream entrance is still connected to the main channel, but the upstream entrance is closed. Two similar field and laboratory cases were investigated using innovative velocimetry techniques (acoustic Doppler profiling, image analysis). Laboratory experiments were conducted with a mobile‐bed physical model of the Morava River (Slovakia). Field measurements were performed in the exit of the Port‐Galland cutoff meander, Ain River (France). Both cases yielded consistent and complementary results from which a generic scheme for flow patterns and morphological evolution was derived. A simple analogy with flows in rectangular side cavities was used to explain the recirculating flow patterns which developed in the exit. A decelerating inflow deposits bedload in the downstream part of the cavity, while the upstream part is eroded by an accelerating outflow, leading to the retreat of the upstream bank. In the field, strong secondary currents were observed, especially in the inflow, which may enhance the scouring of the downstream corner of the cavity. Also, fine sediment deposits constituted a silt layer in a transitional zone, located between the mouth of the abandoned channel and the oxbow‐lake within the cutoff meander. Attempts at morphological prediction should consider not only the flow and sediment conditions in the cavity, but also the dynamics of the main channel. Copyright © 2010 John Wiley & Sons, Ltd  相似文献   

6.
Two reaches of Aguapeí River, a left‐bank tributary of the Paraná River in western São Paulo state, Brazil, were studied with the objective of assessing the role of bend curvature on channel migration in this wet‐tropical system and examining if land‐use changes or ENSO (El Niño Southern Oscillation) driven climate anomalies over nearly half a century have changed migration behaviour and planform geometry. Meander‐bend migration rates and morphometric parameters including meander‐bend curvature, sinuosity, meander wavelength and channel width, were measured and the frequency of bend cutoffs was analysed in order to determine the rate of change of channel adjustment over a 48 year period to 2010. Results show that maximum average channel migration rates occur in bends with curvatures of about 2–3 rc/w, similar to other previously studied temperate and subarctic freely meandering rivers although not as pronounced and with a tendency to favour tighter curvature. From 1962 to 2010 the Aguapeí River has undergone a significant reduction in sinuosity, a shift from tightly curving to more open bends, an overall decline in channel migration rates, an associated decrease in the frequency of neck‐cutoffs and an overall increase in channel width. As the majority of the drainage basin (96%) was already deforested in 1962, channel form and process changes were, unlike an interpretation for an adjacent river system, not attributed to altered land‐use but rather to a sharp ENSO‐driven increase in the magnitude of peak flow‐discharges of some 32% since 1972. In summary, this research revealed that recent climate and associated flow regime changes are having a pronounced effect on river channel behaviour in the Aguapeí River investigated here. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

7.
Sedimentary architecture of abandoned channel fills   总被引：1，自引：0，他引：1  
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8.
Field observations on small rivers of the Amazon basin are less common due to their remote location and difficult accessibility. Here we show, through remote sensing analysis and field works, the planform evolution and riverbed topography of a small river located in the upper foreland Amazon basin, the Ichilo River. By tracking planform changes over 30 years, we identified the factors that control meander migration rates in the Ichilo River: cutoffs, climate and human interventions. The data suggest that neck cutoffs are the main controls in the Ichilo River, with an annual density of 0.022 cutoffs/km. In addition, climate controls have been identified in the form of high-precipitation events that may have promoted cutoffs, an increase in meander migration rate and channel widening. The width distribution of the Ichilo River is well represented by general extreme value and inverse Gaussian distributions. The spatiotemporal variability of meandering migration rates in the Ichilo River is analysed in two locations where neck cutoffs are expected. Analysing the distance across the neck in these two points, we predict the occurrence of a new cutoff. The combined methodology of bathymetric surveys and structure from motion photogrammetry shows us the Ichilo riverbed topography and banks at high resolution, where two scour holes were identified. Finally, we discuss the impact of planform changes of the Ichilo River on communities that are established along its riverbanks.  相似文献   

9.
Meander 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.  相似文献   

10.
A sustained dynamic inflow perturbation and bar–floodplain conversion are considered crucial to dynamic meandering. Past experiments, one-dimensional modelling and linear theory have demonstrated that the initiation and persistence of dynamic meandering require a periodic transverse motion of the inflow. However, it remains unknown whether the period of the inflow perturbation affects self-formed meander dynamics. Here, we numerically study the effect of the inflow perturbation period on the development and meander dynamics of a chute-cutoff-dominated river, which requires two-dimensional modelling with vegetation forming floodplain on bars. We extended the morphodynamic model Nays2D with growth and mortality rules of vegetation to allow for meandering. We tested the effect of a transversely migrating inflow boundary by varying the perturbation period between runs over an order of magnitude around typical modelled meander periods. Following the cutoff cascade after initial meander formation from a straight channel, all runs with sufficient vegetation show series of growing meanders terminated by chute cutoffs. This generates an intricate channel belt topography with point bar complexes truncated by chutes, oxbow lakes, and scroll-bar-related vegetation age patterns. The sinuosity, braiding index and meander period, which emerge from the inherent biomorphological feedback loops, are unrelated to the inflow perturbation period, although the spin-up to dynamic equilibrium takes a longer time and distance for weak and absent inflow perturbations. This explains why, in previous experimental studies, dynamic meandering was only accomplished with a sustained upstream perturbation in flumes that were short relative to the meander wavelength. Our modelling of self-formed meander patterns is evidence that scroll-bar-dominated and chute-cutoff-dominated meanders develop from downstream convecting instabilities. This insight extends to many more fluvial, estuarine and coastal systems in morphological models and experiments, which require sustained dynamic perturbations to form complex patterns and develop natural dynamics. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.  相似文献   

11.
Compound meander bends with multiple lobes of maximum curvature are common in actively evolving lowland rivers. Interaction among spatial patterns of mean flow, turbulence, bed morphology, bank failures and channel migration in compound bends is poorly understood. In this paper, acoustic Doppler current profiler (ADCP) measurements of the three‐dimensional (3D) flow velocities in a compound bend are examined to evaluate the influence of channel curvature and hydrologic variability on the structure of flow within the bend. Flow structure at various flow stages is related to changes in bed morphology over the study timeframe. Increases in local curvature within the upstream lobe of the bend reduce outer bank velocities at morphologically significant flows, creating a region that protects the bank from high momentum flow and high bed shear stresses. The dimensionless radius of curvature in the upstream lobe is one‐third less than that of the downstream lobe, with average bank erosion rates less than half of the erosion rates for the downstream lobe. Higher bank erosion rates within the downstream lobe correspond to the shift in a core of high velocity and bed shear stresses toward the outer bank as flow moves through the two lobes. These erosion patterns provide a mechanism for continued migration of the downstream lobe in the near future. Bed material size distributions within the bend correspond to spatial patterns of bed shear stress magnitudes, indicating that bed material sorting within the bend is governed by bed shear stress. Results suggest that patterns of flow, sediment entrainment, and planform evolution in compound meander bends are more complex than in simple meander bends. Moreover, interactions among local influences on the flow, such as woody debris, local topographic steering, and locally high curvature, tend to cause compound bends to evolve toward increasing planform complexity over time rather than stable configurations. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

12.
This study reports the results of a large woody debris (LWD) removal experiment in a meander bend along a low‐energy stream in the Midwestern United States. The LWD obstacle was located in the center of the channel at the bend exit and consisted of a mature tree with an intact soil‐covered root wad and a large accumulation of logs, branches and pieces of lumber on top of and adjacent to the main tree. The results indicate that the LWD obstruction influenced 3D flow structure in this bend at all flow stages. The main effect of LWD is to dramatically decelerate flow throughout the majority of the bend, while locally accelerating flow where it passes through the narrow chute at the downstream end of the LWD obstruction. Results from the LWD removal experiment indicate that patterns of three‐dimensional flow structure in meander bends are sensitive to complete removal of LWD. After the removal of LWD from the bend, both downstream and secondary velocities increased and, though still weak, secondary flow intensified. Large, relatively stable, obstructions that span a significant portion of the channel may act as natural dams, effectively ponding water upstream of the LWD, thereby producing substantial convective deceleration of the flow. This research is the first to document three‐dimensional flow structure before and after a controlled removal of LWD from a meander bend. Studies of the type reported here represent a first step toward determining the ensemble of process interactions between LWD and bend dynamics. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

13.
This study assesses hydrodynamic and morphodynamic model sensitivity and functionality in a curved channel. The sensitivity of a depth‐averaged model to user‐defined parameters (grain size, roughness, transverse bed slope effect, transport relations and secondary flow) is tested. According to the sensitivity analysis, grain size, transverse bed slope effect and sediment transport relations are critical to simulated meander bend morphodynamics. The parametrization of grain size has the most remarkable effect: field‐based grain size parametrization is necessary in a successful morphodynamic reconstruction of a meander bend. The roughness parametrization method affects the distribution of flow velocities and therefore also morphodynamics. The combined effect of various parameters needs further research. Two‐dimensional (2D) and three‐dimensional (3D) reconstructions of a natural meander bend during a flood event are assessed against field measurements of acoustic Doppler current profiler and multi‐temporal mobile laser scanning data. The depth‐averaged velocities are simulated satisfactorily (differences from acoustic Doppler current profiler velocities 5–14%) in both 2D and 3D simulations, but the advantage of the 3D hydrodynamic model is unquestionable because of its ability to model vertical and near‐bed flows. The measured and modelled near‐bed flow, however, differed notably from each other's, the reason of which was left open for future research. It was challenging to model flow direction beyond the apex. The 3D flow features, which also affected the distribution of the bed shear stress, seem not to have much effect on the predicted morphodynamics: the 2D and 3D morphodynamic reconstructions over the point bar resembled each other closely. Although common features between the modelled and measured morphological changes were also found, some specific changes that occurred were not evident in the simulation results. Our results show that short‐term, sub‐bend scale morphodynamic processes of a natural meander bend are challenging to model, which implies that they are affected by factors that have been neglected in the simulations. The modelling of short‐term morphodynamics in natural curved channel is a challenge that requires further study. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

14.
The Andean Cordillera and piedmont significantly influence river system and dynamics, being the source of many of the important rivers of the Amazon basin. The Beni River, whose upper sub‐catchments drain the Andean and sub‐Andean ranges, is a major tributary of the Madeira River. This study examines the river in the south‐western Amazonian lowlands of Bolivia, where it develops mobile meanders. Channel migration, meander‐bend morphology and ox‐bow lakes are analysed at different temporal and spatial scales. The first part of this study was undertaken with the aim to link the erosion–deposition processes in the active channel with hydrological events. The quantification of annual erosion and deposition areas shows high inter‐annual and spatial variability. In this study, we investigate the conditions of sediment exportation in the river in relation to three hydrological parameters (flood intensity, date of discharge peak and duration of the bank‐full stage level). The second part of this study, focusing on the abandoned meanders, analyses the cutoff processes and the post‐abandonment evolution during 1967–2001. This approach shows the influence of the active channel behaviour on the sediment diffusion and sequestration of the abandoned meanders and allows us to build a first model of the contemporary floodplain evolution. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

15.
Rivers may dramatically change course on a fluvial plain. Such an avulsion temporarily leads to two active channels connected at a bifurcation. Here we study the effect of dynamic meandering at the bifurcation and the effect of channel width adjustment to changing discharge in both downstream branches on the evolution of a bifurcation and coexisting channels. As an example, we reconstructed the last major avulsion at the Rhine delta apex. We combined historical and geological data to reconstruct a slowly developing avulsion process spanning 2000 years and involving channel width adjustment and meandering at the bifurcation. Based on earlier idealised models, we developed a one‐dimensional model for long‐term morphodynamic prediction of upstream channel and bifurcates connected at the bifurcation node. The model predicts flow and sediment partitioning at the node, including the effect of migrating meanders at the bifurcation and channel width adjustment. Bifurcate channel width adaptation to changing discharge partitioning dramatically slows the pacing of bifurcation evolution because the sediment balance for width adjustment and bed evolution are coupled. The model further shows that meandering at the bifurcation modulates channel abandonment or enlargement periodically. This explains hitherto unrecognised reactivation signals in the sedimentary record of the studied bifurcation meander belts, newly identified in our geological reconstruction. Historical maps show that bifurcation migration due to meander bend dynamics increases the bifurcation angle, which increases the rate of closure of one bifurcate. The combination of model and reconstruction identifies the relevant timescales for bifurcation evolution and avulsion duration. These are the time required to fill one downstream channel over one backwater length, the time to translate one meander wavelength downstream and, for strong river banks, the adaptation timescale to adjust channel width. The findings have relevance for all avulsions where channel width can adjust to changing discharge and where meandering occurs. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

16.
Although alluvial cutoffs record accurately the geometry, bedforms, and bed material of the channel when last active, few attempts have been made to use cutoffs in studies of channel changes. A detailed record of historical channel changes on the lower Hunter River in southeastern Australia has shown that this channel responds to naturally alternating periods of high and low flood activity, called flood- and drought-dominated regimes respectively. Sinuosity decreased from 3·84 in 1870, to 2·66 in 1893 and to 1·38 in 1970 through the development of eight cutoffs. The channel also aggraded with medium sand burying the former bed material of mixed mud, coarse sand, and gravel. Channel straightening was a response to increased flood frequencies during the flood-dominated regimes of the late 19th and 20th centuries, combined with localized river engineering works and increased sand load. Detailed stratigraphic studies were carried out on three neck cutoffs and one chute cutoff which were abandoned in 1890, 1950, 1952, and 1956. A comparison of former and present bed elevations and bed material size showed similar trends to those determined by the historical record, confirming the reliability of cutoffs as indicators of former channel conditions. The sedimentary infills of the cutoffs are not uniformly fine grained as recorded previously in the literature. Relatively thin, fine-grained fills were deposited during the drought-dominated regime of the first half of this century but thick, coarser-grained fills were deposited after 1949 during the flood-dominated regime. All fills fine upwards. Cutoff infills provide a record of changing flood activity and sediment loads.  相似文献   

17.
Tian Zhou  Ted Endreny 《水文研究》2012,26(22):3378-3392
River restoration projects have installed j‐hook deflectors along the outer bank of meander bends to reduce hydraulic erosion, and in this study we use a computational fluid dynamics (CFD) model to document how these deflectors initiate changes in meander hydrodynamics. We validated the CFD with streamwise and cross‐channel bankfull velocities from a 193° meander bend flume (inlet at 0°) with a fixed point bar and pool equilibrium bed but no j‐hooks, and then used the CFD to simulate changes to flow initiated by bank‐attached boulder j‐hooks (1st attached at 70°, then a 2nd at 160°). At bankfull and half bankfull flow the j‐hooks flattened transverse water surface slopes, formed backwater pools upstream of the boulders, and steepened longitudinal water slopes across the boulders and in the conveyance region off the mid‐channel boulder tip. Streamwise velocity and mass transport jets upstream of the j‐hooks were stilled, mid‐channel jets were initiated in the conveyance region, eddies with a cross‐channel axis formed below boulders, and eddies with a vertical axis were shed into wake zones downstream of the point bar and outer bank boulders. At half bankfull depth conveyance region flow cut toward the outer bank downstream of the j‐hook boulders and the secondary circulation cells were reshaped. At bankfull depth the j‐hook at 160° was needed to redirect bank‐impinging flow sent by the upstream j‐hook. The hooked boulder tip of both j‐hooks funneled surface flow into mid‐channel plunging jets, which reversed the secondary circulation cells and initiated 1 to 3 counter rotating cells through the entire meander. The main outer bank collision zone centered at 50° without the j‐hook was moved by the j‐hook to within and just beyond the 70° j‐hook boulder region, which displaced other mass transport zones downstream. J‐hooks re‐organized water surface slopes, streamwise and cross‐channel velocities, and mass transport patterns, to move shear stress from the outer bank and into the conveyance and mid‐channel zones at bankfull flow. At half bankfull flows a patch of high shear re‐attached to the outer bank below the downstream j‐hook. J‐hook geometry and placement within natural meanders can be analyzed with CFD models to help restoration teams reach design goals and understand hydraulic impacts. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

18.
The geomorphic evolution of the Jordan River in recent decades indicates that interaction between incision and high-magnitude floods controls sinuosity changes under increasing mouth gradients during base-level fall. The evolution of the river was analyzed based on digital elevation models, remotely sensed imagery, hydrometric data, and a hydraulic model. The response varies along the river. Near the river mouth, where incision rate is high and a deep channel forms, overbank flooding is less likely. There, large floods exert high shear stress within the confined channel, increasing sinuosity. Upstream, near the migrating knickzone channel gradients also increase, incision is more moderate and floods continue to overtop the banks, favoring meander chute cutoffs. The resulting channel has a downstream well-confined meandering segment and an upstream low-sinuosity segment. These new insights regarding spatial differences along an incising channel can improve interpretations of the evolution of ancient planforms and floodplains that responded to base-level decline. © 2018 John Wiley & Sons, Ltd.  相似文献   

19.
We propose a bio-morphodynamic model at bend cross-sectional scale for the lateral migration of river meander bends, where the two banks can migrate separately as a result of the mutual interaction between river flow, sediments and riparian vegetation, particularly at the interface between the permanently wet channel and the advancing floodplain. The model combines a non-linear analytical model for the morphodynamic evolution of the channel bed, a quasi-1D model to account for flow unsteadiness, and an ecological model describing riparian vegetation dynamics. Simplified closures are included to estimate the feedbacks among vegetation, hydrodynamics and sediment transport, which affect the morphology of the river-floodplain system. Model tests reveal the fundamental role of riparian plants in generating bio-morphological patterns at the advancing floodplain margin. Importantly, they provide insight into the biophysical controls of the ‘bar push’ mechanism and into its role in the lateral migration of meander bends and in the temporal variations of the active channel width.  相似文献   

20.
Over a period of several decades, gullies have been observed in various stages of forming, growing and completing the cutoff of meander necks in Powder River. During one episode of overbank flow, water flowing over the down-stream bank of the neck forms a headcut. The headcut migrates up-valley, forming a gully in its wake, until it has traversed the entire neck, cutting off the meander. The river then follows the course of the gully, which is subsequently enlarged as the river develops its new channel. The complete process usually requires several episodes of high water: in only one of the five cases described herein was a meander cutoff initiated and completed during a single large flood. © 1998 John Wiley & Sons, Ltd.  相似文献   

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