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Channel bifurcations are a fundamental element of a broad variety of flowing freshwater environments worldwide, such as braiding and anabranching rivers, river deltas and alluvial fans. River bifurcations often develop asymmetrical configurations with uneven discharge partition and a bed elevation gap between the downstream anabranches. This has been reproduced by one-dimensional (1-D) analytical theories which, however, rely on the empirical calibration of one or more parameters and cannot provide a clear and detailed physical explanation of the observed dynamics. We propose a novel two-dimensional (2-D) solution for the flow and bed topography in channel bifurcations based on an innovative application to a multi-thread channel configuration of the 2-D steady linear solution developed decades ago to study river bars and meandering in single thread river settings. The resonant value of the upstream channel aspect ratio, corresponding to the theoretical resonance condition of regular river meanders (Blondeaux & Seminara, J. Fluid Mech., vol. 157, 1985, pp. 449-470) is the key parameter discriminating between symmetrical and asymmetrical bifurcations, in quantitative agreement with experimental observations and numerical simulations, and qualitatively matching field observations. Only when the aspect ratio of the upstream channel of the bifurcation exceeds resonance, is the bifurcation node able to trigger the upstream development of a steady alternate bar pattern, thus creating an unbalanced configuration. Ultimately, the work provides an analytical explanation of the intrinsic legacy between bifurcation asymmetry and the phenomenon of 2-D upstream morphodynamic influence discovered by Zolezzi & Seminara. [ABSTRACT FROM AUTHOR]

The evolution of natural river channels is strongly affected by the interplay between the altimetric pattern and the planimetric configuration acting contemporary in branches of braided rivers. Very few quantitative observations are presently available to characterize such interactions. In the present contribution we discuss the results of experimental runs performed with both uniform and graded sediments. The experiments were aimed at describing quantitatively the evolution of a single laterally unconstrained channel until the occurrence of the first bifurcation. An objective criterion for the occurrence of the bifurcation has been established using the data provided by the Fourier analysis of the evolving bank profiles; the procedure enabled us to characterize the morphodynamic sequence leading to flow and channel bifurcation. The sought outcome of the investigation is to derive a suitable description of the bifurcation process to be implemented in predictive models for braiding evolution, for which physically based nodal point conditions would be highly desirable. [ABSTRACT FROM AUTHOR]

Abstract: Large gravel-bed braided rivers show a highly dynamic evolution, where even moderate flow pulses can produce significant morphological adjustment. An estimation of network variations is needed by river authorities as well as by ecologists for better managing river ecosystems. However, few field investigations have been performed to measure the effect of unsteady discharges in braided rivers. In this paper we report on the morphological changes induced by six different floods in a 1-km-long reach of the Tagliamento River, Italy. Topographic surveys were performed after each significant flood, and an automatic digital camera continuously monitored the planimetric configuration. Repeated measurements of cross section profiles allowed the quantification of the transported sediments volume and, in particular, provided a basis to estimate the percentage of the floodplain subject to morphological evolution. Data analysis showed a good relationship between peak flow level, active width, and mean bed level changes. Similarly, lateral channel shifting is likely to increase for larger floods. Monitoring of a range of discharges up to bankfull conditions revealed a fundamental difference between flow pulses (events on the average occurring one or more times per year) and flood pulses (with a return period larger than two years). The morphological evolution in the former case is mainly limited to a few active branches and is locally driven by bank erosion at the bend apex. On the other hand, larger floods induce a complete reworking of the network configuration, where bifurcations and confluences play a crucial role. Single morphological units (like branches, nodes, bars) are no longer recognizable after this type of event. Braided rivers are therefore characterized by the occurrence of two different spatial and temporal scales that force the ecosystem dynamics. [Copyright &y& Elsevier]

Abstract: Prediction of bed load flux remains a significant problem in understanding braided river morphodynamics for geomorphic and engineering applications. Two sets of data from laboratory experiments on braided networks performed at the University of Trento (Italy) and at the University of Alberta (Canada) provide the basis for development of a dimensionless bed load function and for testing a simple predictive model. Measured total sediment transport rates (time-averaged) at equilibrium channel configuration collapse to a single dimensionless relationship based on dimensionless stream power. Bed load fluxes predicted by the Parker and Bagnold functions and cross-section average hydraulic parameters under-predict the bed load flux, particularly at low shear stress. This is consistent with previous observation and theory demonstrating the significant influence of transverse variability of the hydraulic parameters in controlling and predicting bed load flux. A simple method for adjusting for this lateral variation is proposed for computing the sediment transport rate using topographic cross-sections of braided rivers. Results show good agreement with the measured values, suggesting that simple assumptions combined with the mean morphology of the channel may be sufficient to estimate mean bed load flux. Model predictions also highlight the significance of active bed width, rather than bed shear stress, as a fundamental ingredient in the prediction of the bed load rate and therefore, as an important morphological property to be modelled. The model also predicts similar behaviour in the at-a-station variability of bed load and in the importance of variation of active width, relative to bed shear stress, in the transport process. [Copyright &y& Elsevier]