Three‐dimensional flow structure and channel change in an asymmetrical compound meander loop,Embarras River,Illinois

The planform dynamics of meandering rivers produce a complex array of meander forms, including elongated meander loops. Thus far, few studies have examined in detail the flow structure within meander loops and the relation of flow structure to patterns of planform change. This field‐based investigation examines relations between three‐dimensional fluid motion and channel change within an elongated, asymmetrical meander loop containing multiple pool–riffle structures. The downstream velocity field is characterized by a high‐velocity core that shifts slightly outward as flow moves through individual lobes of the loop. For some of the measured flows this core becomes submerged below the water surface downstream of the lobe apexes. Vectors of cross‐stream/vertical velocities indicate that skew‐induced helical motion develops within the pools near lobe apexes and decays over riffles where channel curvature is less pronounced. Maximum rates of bank retreat generally occur near lobe apexes where impingement of the flow on the outer channel bank is greatest. However, maximum rates and loci of bank retreat differ for upstream and downstream lobes of the loop, leading to increasing asymmetry of loop geometry over time—a finding consistent with experimental investigations of loop evolution. Copyright © 2003 John Wiley & Sons, Ltd.     

Evolution of a bifurcation in a meandering river with adjustable channel widths,Rhine delta apex,The Netherlands

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.     

Rates,distributions and mechanisms of change in meander morphology over decadal timescales,River Dane,UK

This paper analyses types and rates of change in river meander morphology and the links between mechanisms of change and emergent behaviour of planform morphology. It uses evidence of four dates of aerial photography combined with annual field mapping and ground photography to examine the morphological changes and mechanisms of change in a series of bends on an active meandering river, the River Dane in NW England, over a 25 year period. This unique data set allows insight into the spatial and temporal variability of bank line movement and component processes. Bank lines were mapped photogrametrically from air photos of 1984, 1996, 2001 and 2007 and the digitised courses compared in ArcGIS to produce calculations of erosional and depositional areas and rates. Most bends exhibit morphological change that largely follows the autogenic sequence, identified in qualitative models of meander development, from low sinuosity curves through simple symmetric and asymmetric bends to compound forms with lobe development in the apex region. Rates of erosion and bankline movement increase through this sequence until the compound phase. Relationships of amounts of movement to various curvature measures of bend morphology are complex. Several new loops, distinct from compound bend behaviour, have developed during the study period in formerly straight sections. Mechanisms of morphological change are illustrated for four types of bends: new, rapid growth bend; sharp‐angled bend with mid‐channel bar development; symmetric migrating bend; and simple to compound bend development. The changes take place in phases that are not simply related to discharge but to inherent sequences and feedbacks in development of bars and bend morphology and timescales for these are identified. Overall, emergent behaviour of systematic planform change, moderated by channel confinement and boundary features, is produced from spatially and temporally varied channel processes. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical calculation for bed variation in compound-meandering channel using depth integrated model without assumption of shallow water flow

In a compound meandering channel, patterns of flow structures and bed variations change with increasing water depth owing to complex momentum exchange between high-velocity flow in a main channel and low-velocity flows in flood plains. We have developed a new quasi-three-dimensional model without the shallow water assumption, i.e., hydrostatic pressure distribution; our method is known as the general bottom velocity computation (BVC) method. In this method, a set of depth-integrated equations, including depth-integrated momentum and vorticity equations, are prepared for evaluating bottom velocity and vertical velocity distributions. The objective of this study is to develop a bed variation calculation method for both single and compound meandering channels by using the BVC method coupled with a sediment transport model. This paper shows that the BVC method can reproduce the pattern change of bed variation in a compound meandering channel flow with increasing relative depth. The variation in sediment transport rate due to overbank flow is explained by experimental and computational results.     

Upstream perturbation and floodplain formation effects on chute-cutoff-dominated meandering river pattern and dynamics

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.     

Tidal channel meander formation by depositional rather than erosional processes: examples from the prograding Skagit River Delta (Washington,USA)

Channel meander dynamics in fluvial systems and many tidal systems result from erosion of concave banks coupled with sediment deposition on convex bars. However, geographic information system (GIS) analysis of historical aerial photographs of the Skagit Delta marshes provides examples of an alternative meander forming process in a rapidly prograding river delta: deposition‐dominated tidal channel meander formation through a developmental sequence beginning with sandbar formation at the confluence of a blind tidal channel and delta distributary, proceeding to sandbar colonization and stabilization by marsh vegetation to form a marsh island opposite the blind tidal channel outlet, followed by narrowing of the gap between the island and mainland marsh, closure of one half of the gap to join the marsh island to the mainland, and formation of an approximately right‐angle blind tidal channel meander bend in the remaining half of the gap. Topographic signatures analogous to fluvial meander scroll bars accompany these planform changes. Parallel sequences of marsh ridges and swales indicate locations of historical distributary shoreline levees adjacent to filled former island/mainland gaps. Additionally, the location of marsh islands within delta distributaries is not random; islands are disproportionately associated with blind tidal channel/distributary confluences. Furthermore, blind tidal channel outlet width is positively correlated with the size of the marsh island that forms at the outlet, and the time until island fusion with mainland marsh. These observations suggest confluence hydrodynamics favor sandbar/marsh island development. The transition from confluence sandbar to tidal channel meander can take as little as 10 years, but more typically occurs over several decades. This depositional blind tidal channel meander formation process is part of a larger scale systemic depositional process of delta progradation that includes distributary elongation, gradient reduction, flow‐switching, shoaling, and narrowing. Copyright © 2010 John Wiley & Sons, Ltd.     

Modulation of the flow structure by progressive bedforms in the Kinoshita meandering channel

An in‐house fully three‐dimensional general‐purpose finite element model is applied to solve the hydrodynamic structure in a periodic Kinoshita‐generated meandering channel. The numerical model solves the incompressible Reynolds‐averaged Navier–Stokes equations for mass and momentum, while solving the k ? ε equations for turbulence. The free surface is described by the rigid‐lid approximation (using measured water surface data) for flat (smooth‐bed) and self‐formed (rough‐bed) conditions. The model results are compared against experimental measurements in the ‘Kinoshita channel’, where three‐dimensional flow velocities and turbulence parameters were measured. This validation was carried out for the upstream‐valley meander bend orientation under smooth (flat bed) conditions. After validation, several simulations were carried out to predict the hydrodynamics in conditions where either it was not possible to perform measurements (e.g. applicability of the laboratory acoustic instruments) and to extrapolate the model to other planform configurations. For the flat smooth‐bed case, a symmetric (no skewness) planform configuration was modeled and compared to the upstream‐skewed case. For the self‐formed rough‐bed case, prediction of the hydrodynamics during the progression of bedforms was performed. It appears that the presence of bedforms on a bend has the following effects: (i) the natural secondary flow of the bend is disrupted by the presence of the bedforms, thus depending on the location of the dune, secondary flows might differ completely from the traditional orientation; (ii) an increment on both the bed and bank shear stresses is induced, having as much as 50% more fluvial erosion, and thus a potential increment on the migration rate of the bend. Implications on sediment transport and bend morphodynamics are also discussed in the paper. Copyright © 2013 John Wiley & Sons, Ltd.     

MEANDERING OF RIVERS

Abstract

Among various factors that have influence on the meandering of an alluvial channel, the most significant are valley slope, discharge, bed material, and time. The necessary condition for the origin and development of meandering of an alluvial channel is the erosion of bed material and deposition of the eroded material downstream. The criterion for the development of the meandering is that the discharge must be equal to or greater than the critical discharge (i.e., discharge corresponding to critical shear velocity). The initial channel section has an effect on the development of meandering. The meandering in the V-shaped channels starts from the center (deepest point) of the channel and works inside the banks (inside meandering) before it windens the banks, While the meandering in the rectangular channels starts with the widening of the banks (outside meandering). Maender width increases with the increase in the increase discharge and slope, and decreases with the increase in size of bed material. The meander development continues with time the meander reaches the final stage and equilibrium condition.     

Character of channel planform change and meander development: Luangwa River,Zambia

Air photo interpretation and field survey were used to examine rates and patterns of planform change over the last 40 years on an 80 km reach of the Luangwa River, Zambia. The river, a tributary of the Zambezi, is a 100–200 m wide, medium sinuosity sand‐bed river (sinuosity index 1·84). High rates of channel migration (<33 m a⁻¹) and cutoffs on meandering sections are frequent. Some meandering reaches, however, have remained relatively stable. A form of anastomosing with anabranches up to 14 km in length is also a characteristic. Patterns of meander development vary between bends but all can be described in relation to traditional geomorphic models; change occurs by translation, rotation, double‐heading, concave bank bench formation and cutoff causing river realignment. At the local scale spatial variability in bank resistance, induced by floodplain sedimentology, controls rate of bank erosion, and valley‐side channel ‘deflection’ is also apparent. Copyright © 2000 John Wiley & Sons, Ltd.     

Influence of junction angle on three‐dimensional flow structure and bed morphology at confluent meander bends during different hydrological conditions

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.     

Advances in computational morphodynamics using the International River Interface Cooperative (iRIC) software

Results from computational morphodynamics modeling of coupled flow–bed–sediment systems are described for 10 applications as a review of recent advances in the field. Each of these applications is drawn from solvers included in the public-domain International River Interface Cooperative (iRIC) software package. For mesoscale river features such as bars, predictions of alternate and higher mode river bars are shown for flows with equilibrium sediment supply and for a single case of oversupplied sediment. For microscale bed features such as bedforms, computational results are shown for the development and evolution of two-dimensional bedforms using a simple closure-based two-dimensional model, for two- and three-dimensional ripples and dunes using a three-dimensional large-eddy simulation flow model coupled to a physics-based particle transport model, and for the development of bed streaks using a three-dimensional unsteady Reynolds-averaged Navier–Stokes solver with a simple sediment-transport treatment. Finally, macroscale or channel evolution treatments are used to examine the temporal development of meandering channels, a failure model for cantilevered banks, the effect of bank vegetation on channel width, the development of channel networks in tidal systems, and the evolution of bedrock channels. In all examples, computational morphodynamics results from iRIC solvers compare well to observations of natural bed morphology. For each of the three scales investigated here, brief suggestions for future work and potential research directions are offered. © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd     

A simple model of river meandering and its comparison to natural channels

We develop a new method for analysis of meandering channels based on planform sinuosity. This analysis objectively identifies three channel‐reach lengths based on sinuosity measured at those lengths: the length of typical, simple bends; the length of long, often compound bends; and the length of several bends in sequence that often evolve from compound bends to form multibend loops. These lengths, when normalized by channel width, tend to fall into distinct and clustered ranges for different natural channels. Mean sinuosity at these lengths also falls into distinct ranges. That range is largest for the third and greatest length, indicating that, for some streams, multibend loops are important for planform sinuosity, whereas for other streams, multibend loops are less important. The role of multibend loops is seldom addressed in the literature, and they are not well predicted by previous modelling efforts. Also neglected by previous modelling efforts is bank–flow interaction and its role in meander evolution. We introduce a simple river meandering model based on topographic steering that has more in common with cellular approaches to channel braiding and landscape evolution modelling than to rigorous, physics‐based analyses of river meandering. The model is sufficient to produce reasonable meandering channel evolution and predicts compound bend and multibend loop formation similar to that observed in nature, in both mechanism and importance for planform sinuosity. In the model, the tendency to form compound bends is sensitive to the relative magnitudes of two lengths governing meander evolution: (i) the distance between the bend cross‐over and the zone of maximum bank shear stress, and (ii) the bank shear stress dissipation length related to bank roughness. In our simple model, the two lengths are independent. This sensitivity implies that the tendency for natural channels to form compound bends may be greater when the banks are smoother. Copyright © 2002 John Wiley & Sons, Ltd.     

Calculation of primary and secondary flow and boundary shear stresses in a meandering channel

Turbulent flow in a meandering channel is computed with two Computational Fluid Dynamics (CFD) codes solving the Navier–Stokes equations by employing different turbulence closure approaches. The first CFD code solves the steady Reynolds-Averaged Navier–Stokes equations (RANS) using an isotropic turbulence closure. The second code is based on the concept of Large Eddy Simulation (LES). LES resolves the large-scale turbulence structures in the flow and is known to outperform RANS models in flows in which large-scale structures dominate the statistics. The results obtained from the two codes are compared with experimental data from a physical model study. Both, LES and RANS simulation, predict the primary helical flow pattern in the meander as well as the occurrence of an outer-bank secondary cell. Computed primary as well as secondary flow velocities are in reasonably good agreement with experimental data. Evidence is given that the outer-bank secondary cell in a meander bend is the residual of the main secondary cell of the previous bend. However, the RANS code, regardless of the turbulence model employed, overpredicts the size and strength of the outer-bank secondary cell. Furthermore, only LES is able to uphold the outer-bank second secondary cell beyond the bend apex until the exit of the bend as turbulence anisotropy contributes to its persistence. The presence of multiple secondary cells has important consequences for the distribution of shear stresses along the wetted perimeter of the channel, and thereby the sediment transport in meandering channels. Consequently, even though LES is expected to compute the bed-shear stresses along the wetted perimeter of the channel with a higher degree of accuracy than the RANS model, comparisons between LES and RANS computed wall shear stresses agree well. These findings are useful for practitioners who need to rely on RANS model predictions of the flow in meandering channels at field scale.     

Temporal and spatial adjustments of channel migration and planform geometry: responses to ENSO driven climate anomalies on the tropical freely‐meandering Aguapeí River,São Paulo,Brazil

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 r_c/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.     

Timescale dependence in river channel migration measurements

Accurately measuring river meander migration over time is critical for sediment budgets and understanding how rivers respond to changes in hydrology or sediment supply. However, estimates of meander migration rates or streambank contributions to sediment budgets using repeat aerial imagery, maps, or topographic data will be underestimated without proper accounting for channel reversal. Furthermore, comparing channel planform adjustment measured over dissimilar timescales are biased because short- and long-term measurements are disproportionately affected by temporary rate variability, long-term hiatuses, and channel reversals. We evaluate the role of timescale dependence for the Root River, a single threaded meandering sand- and gravel-bedded river in southeastern Minnesota, USA, with 76 years of aerial photographs spanning an era of landscape changes that have drastically altered flows. Empirical data and results from a statistical river migration model both confirm a temporal measurement-scale dependence, illustrated by systematic underestimations (2–15% at 50 years) and convergence of migration rates measured over sufficiently long timescales (> 40 years). Frequency of channel reversals exerts primary control on measurement bias for longer time intervals by erasing the record of observable migration. We conclude that using long-term measurements of channel migration for sediment remobilization projections, streambank contributions to sediment budgets, sediment flux estimates, and perceptions of fluvial change will necessarily underestimate such calculations. © 2019 John Wiley & Sons, Ltd.     

From straight to deeply incised meandering channels: Slope impact on sinuosity of confined streams

Meandering channels and valleys are dominant landscape features on Earth. Their morphology and remnants potentially indicate past base-level fluctuations and changing regional slopes. The prevailing presence of meandering segments in low-slope areas somewhat confuses the physically based relationships between slope and channel meandering. This relationship underlies a fundamental debate: do incised sinuous channels actively develop during steepening of a regional slope, or do they inherit the planform of a preexisting sinuous channel through vertical incision? This question was previously explored through reconstructed evolution of meandering rivers, numerical simulations, and controlled, scaled-down laboratory experiments. Here, we study a rare, field-scale set of a dozen adjacent perennial channels, evolving in recent decades in a homogeneous erodible substrate in response to the Dead Sea level fall (> 30 m over 40 years). These channels are fed by perennial springs and have no drainage basin or previous fluvial history; they initiated straight and transformed into incising meandering channels following the emergence of the preexisting lake bathymetry, which resulted in increased channel lengths and regional slopes at different rates for each channel. This field setting allows testing the impact of changing regional slope on the sinuosity of a stream in the following cases: (a) relatively long and low-gradient shelf-like margins, (b) a sharp increase in the basinward gradient at the shelf-slope transition, and (c) gradually steepening slopes. Under a stable and low valley slope, the channels mainly incise vertically, inheriting a preexisting sinuous pattern. When the regional slope steepens, the channels start to meander, accompanying the vertical incision. The highest sinuosity evolved in the steepest channel, which also developed the deepest and widest valley. These results emphasize the amplifying impact of steepening regional slope on sinuosity. This holds when the flow is confined and chute cutoffs are scarce.     

Hydrologic control of waveforms on small meandering rivers

Small river channels within the Humber Basin of Southern Ontario exhibit irregular meandering patterns; however, contemporary river hydrology appears capable of controlling the scale of regular waveform development. Recent changes in river planform are assessed through a bi‐temporal comparison of channel planform using orthorectified aerial photography and statistical analysis of curvature series based on autocorrelation and spectral analysis. Calculated meander wavelengths are acceptable given traditional relationships between wavelength and mean annual flood discharge. It is also evident that changes in stream power are highly correlated with changes in the dominant wavelengths between channel reaches in this study. Gradual development of small scale waveforms following a rare hydrological and geomorphological event in 1954 further confirms that these forms can be attributed to the typical discharge regime. This paper argues that the scaling of wavelengths with discharge can be considered a strong factor controlling planform evolution on some small meandering river systems, despite manifest irregular planforms. Copyright © 2007 John Wiley & Sons, Ltd.     

EXPERIMENTAL STUDY ON BED SHEAR STRESS AND SEDIMENT TRANSPORT IN A MEANDERING LABORATORY CHANNEL

1 INTRODUCTION River erosion is a complex phenomenon. The rate of bank retreat is determined by flow, bed topography, sediment transport, bank properties, and water quality. Prediction of future river planform changes and the knowledge of river erosion and river meandering are required for land use planning in alluvial river valleys and determining locations for bridges and hydraulic structures. The control of riverbank erosion requires prediction of flow and bed features in a meanderin…     

The effects of ice cover on flow characteristics in a subarctic meandering river

The effects of ice cover on flow characteristics in meandering rivers are still not completely understood. Here, we quantify the effects of ice cover on flow velocity, the vertical and spatial flow distribution, and helical flow structure. Comparison with open‐channel low flow conditions is performed. An acoustic doppler current profiler (ADCP) is used to measure flow from up to three meander bends, depending on the year, in a small sandy meandering subarctic river (Pulmanki River) during two consecutive ice‐covered winters (2014 and 2015). Under ice, flow velocities and discharges were predominantly slower than during the preceding autumn open‐channel conditions. Velocity distribution was almost opposite to theoretical expectations. Under ice, velocities reduced when entering deeper water downstream of the apex in each meander bend. When entering the next bend, velocities increased again together with the shallower depths. The surface velocities were predominantly greater than bottom/riverbed velocities during open‐channel flow. The situation was the opposite in ice‐covered conditions, and the maximum velocities occurred in the middle layers of the water columns. High‐velocity core (HVC) locations varied under ice between consecutive cross‐sections. Whereas in ice‐free conditions the HVC was located next to the inner bank at the upstream cross‐sections, the HVC moved towards the outer bank around the apex and again followed the thalweg in the downstream cross‐sections. Two stacked counter‐rotating helical flow cells occurred under ice around the apex of symmetric and asymmetric bends: next to the outer bank, top‐ and bottom‐layer flows were towards the opposite direction to the middle layer flow. In the following winter, no clear counter‐rotating helical flow cells occurred due to the shallower depths and frictional disturbance by the ice cover. Most probably the flow depth was a limiting factor for the ice‐covered helical flow circulation, similarly, the shallow depths hinder secondary flow in open‐channel conditions. Copyright © 2016 John Wiley & Sons, Ltd.     

Hydraulic and geomorphic processes in an overbank flood along a meandering,gravel‐bed river: implications for chute formation

Hydraulic interactions between rivers and floodplains produce off‐channel chutes, the presence of which influences the routing of water and sediment and thus the planform evolution of meandering rivers. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel‐bedded, meandering river as a field laboratory for studying these mechanisms at a realistic scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off‐channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer‐grained floodplains. The controls on chute formation at these locations include the flood magnitude, river curvature, floodplain gradient, erodibility of the floodplain sediment, and the flow resistance provided by riparian vegetation. Copyright © 2015 John Wiley & Sons, Ltd.