The upper reach of the Amazon River has a very dynamic morphology, with the highest rates of migration observed in the entire Amazon River. It has an anabranching channel pattern which alternates between a condition of single channel and anabranching structures; in particular, the anabranching structure near Iquitos City shows an interesting channel behavior. Its channels migrate at different rates, where there are processes of narrowing and widening, and also collision and development of new channels. The temporal evolution of the Iquitos anabranching structure is described during the period from 1985 to 2014. The study is carried out by using satellite images to track the migration patterns, which are contrasted to the underlying geological units in the valley. Bathymetry of the structure and several velocity transects were obtained during a field campaign prior to the 2012 historic flood event. This information allowed for numerical modeling in order to compute the hydrodynamic flow field that complements the temporal analysis, aiming to understand the planform migration patterns after the 2012 flood event. It is observed that the geological units play an important role in modulating the migration rates and planform development of the channels. The channels in the structure are in contention to be the main channel, which become the secondary channel after migration. This causes the channels to experience a rise in bed elevation and narrowing of the channel itself; if this trend continues for several more years, these channels will detach from the Iquitos anabranching structure, thus forming paleo‐channels. This geomorphic process is important for horizontal and vertical soil heterogeneity along the floodplain. In general, the analysis shows a complex interaction between the underlying geological units, flow structure, morphology of the bed and planform migration. Copyright © 2016 John Wiley & Sons, Ltd.

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 À 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.

[1] Fluvial channels present bed forms such as dunes and ripples that alter instantaneous hydrodynamics parameters such as flow velocities, water surface profiles, bed shear stresses, and Reynolds stresses and create turbulent coherent structures that are significantly different from those presented in flat bed conditions. It is known that LES-based models are more suitable than RANS models to reproduce the complex hydrodynamics around bed forms. Herein, a LES model is applied to describe the mean and turbulent flow structure under superimposed bed forms. Three cases were simulated: RUN I (train of ripples), RUN II (superimposed bed forms), and RUN III (amalgamated bed forms). The LES modeling was performed using a free surface condition to allow the model to develop undulations and boils on the water surface caused by effect of the bed forms. Some important conclusions from this study are: the division of high and low shear stresses on the stoss side of the dune, the progression of the flow field topology from RUN I and RUN III, and the type of turbulent coherent structures found in each stage. The region of high shear stresses was related to turbulence production, in which the streamwise velocity fluctuations (where strips structures are related to streaks) were associated to the modification of the bed morphology. The turbulence Horseshoes Vortices (THV) were more frequent in RUN I than in the other two cases (where streamwise rolls were more frequent). Finally, the frequency of the bursting events increased from RUN I to RUN II and decreased from RUN II to RUN III. Implications of detailed hydrodynamics into bed forms processes are also presented and discussed.Citation: Frias, C. E., and J. D. Abad (2013), Mean and turbulent flow structure during the amalgamation process in fluvial bed forms, Water Resour. Res., 49,[6548][6549][6550][6551][6552][6553][6554][6555][6556][6557][6558][6559][6560]

We present a study to relate the sinuosity of the main channel and its effect on the dynamics of the secondary channels of anabranching structures. For this purpose, two locations of the Peruvian Amazon River were selected: (1) a site with a medium to high-sinuosity main channel (MS site: Muyuy, Peru) and (2) a site with a low-sinuosity main channel (LS site: at the triple boundary between Brazil, Colombia, and Peru). The main channels for both the MS and LS anabranching structures have freedom to migrate in the lateral direction, while at least one of their secondary channels is adjacent to the geological valley. For MS and LS sites, temporal analysis of planform evolution was carried out using 30 years of satellite imagery from which metrics such as width, sinuosity, and annual maximum migration rates of main and secondary channels were calculated. Additionally, detailed hydrodynamic and bed morphology field measurements were carried out, and a two-dimensional shallow water numerical model was developed. For a medium to high-sinuosity main channel anabranching structure, the secondary channels present a dominant mechanism for reworking the floodplain, while for the low-sinuosity anabranching structure, the main channel planform dynamics is dominant. Flow velocities along the main and secondary channels for low, transition, and high-flow discharges describe that for MS (LS) site, the velocities are much higher along the secondary (main) channels.

The Ucayali River is one of the most dynamic large rivers in the world, with high rates of channel migration regularly producing cutoffs. In the lower portion of the Ucayali River, before its confluence to the Marañon River where the Amazon River is born, the increase in water and sediment discharge triggers bends with secondary channels (transitional stage from purely meandering to anabranching), which influence the planform migration rates and patterns of the sinuous channels. Based on remote sensing analysis, a comparison of planform dynamics of bends with and without secondary channels is presented. For the case of a bend with secondary channels (Jenaro Herrera, JH), detailed field measurements for bed morphology, hydrodynamics, bed and suspended load are performed for low-, transitional- and high-flow conditions (August, February and May, respectively). Additionally, a two-dimensional depth average hydraulic model is utilized to correlate observed migrating patterns with the hydrodynamics. Results indicate that the secondary channels have disrupted typical planform migration rates of the main meandering channel. However, at high amplitudes, these secondary channels reduce their capacity to capture flow and start a narrowing process, which in turn increases migration rates of the main channels (meandering reactivation process), suggesting that an imminent cutoff along the JH bend is underway by pure lateral migration or by the collapse into the existing paleochannels.

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