Morphodynamics of multi-thread rivers Theory and Observations of Channel Loops

Using satellite and aerial imagery, the planform of a large number of bifurcations is gathered and interpreted in terms of quantifiable parameters. The majority of the observed bifurcations displays a conspicuous asymmetry in terms of bifurcation angle, upstream channel curvature, length of the downstream bifurcates. The first part of this work focuses on the effect of the differences in channel width between the upstream channel and the bifurcates added together and between the bifurcates themselves. Therefore, two parameters are derived, namely the bifurcation enlarge- ment that accounts for the primer factor and the channel width asymmetry that accounts for the latter factor. Within the data considered, the two derived factors are poorly correlated and each factor is also poorly correlated to other planform and morphodynamic parameters. Especially the lack of correlation between the observed bifurcation enlargement and channel width asymmetry indicates that the bifurcates do not obey a regime equilibrium configuration by which the channel carrying more water and sediment is also larger. The second part then deals with the mechanisms underlying the equilibrium configurations of bifurcations, especially how the channel width differences affect these configurations, using a well-known simplifying physical relationship that couples the distributions of water and sediment discharges with the planimetry and the bathymetry of the bifurcation node. The two planform param- eters appear to compete against each other in terms of equilibrium stability. The bifurcation enlargement associated with a shallower and steeper downstream flow destabilizes the bifurcation as the flow condition in both anabranches is closer to the critical sediment motion condition. On the other hand, the channel width asymmetry is associated with an increase of the transport capacity in the narrower dominated channel. Finally, the stability threshold of the observed bifurcation is computed for each observed bifurcation, showing that the bifurcation planform notably affects the stability of the configuration in natural settings.