[1] We investigate the equilibrium configurations and the stability of river bifurcations in gravel braided networks. Within the context of a one-dimensional approach, the nodal point conditions play a crucial rule, as pointed out by Wang et al. [1995] who propose an empirical relationship relating water and sediment flow rates into the downstream branches. In the present paper, an alternative formulation of nodal point conditions is proposed based on a quasi two-dimensional approach. The results show that, if the Shields parameter of the upstream channel is large enough, the system only admits of one solution with both branches open, which is invariably stable. As the Shields parameter of the upstream channel decreases, two further stable solutions appear characterized by a different partition of water discharge into the downstream branches: in this case, the previous solution becomes unstable. Theoretical findings are confirmed by the numerical solution of the nonlinear onedimensional equations.

Bifurcating rivers shape natural landscapes by distributing water and sediments on fluvial plains and in deltas. Symmetrical bifurcations were often found to be unstable so that one branch downstream of the bifurcation enlarged while the other dwindled. A unified theory able to predict bifurcation stability in both gravel bed and sand bed rivers is still lacking. Here we develop a new theory for the stability of bifurcations for the entire range of gravel bed to sand bed rivers. The theory indicates opposite behavior of gravel bed and sand bed rivers: we predict that symmetrical bifurcations are inherently stable for intermediate Shields stresses but are inherently unstable for the low and high Shields stresses found in the majority of rivers on Earth. In the latter conditions asymmetrical bifurcations are stable. These predictions are corroborated by observations and have ramifications for many environmental problems in fluviodeltaic settings.

[1] This paper tackles the problem of morphodynamic equilibrium of tidal channels and tidal inlets. We report a laboratory investigation of the process whereby an equilibrium morphology is established in a tidal system consisting of an erodible channel connected through an inlet to a tidal sea. Observations suggest that a morphodynamic equilibrium is eventually established both in the inlet region and in the channel. The latter exhibits a weakly concave bed profile seaward, a weakly convex profile landward, and the formation of a ''beach'' close to the landward end of the channel. A second set of observations concerns the formation and development of both small-and large-scale bed forms. In particular, small-scale forms are found to develop in the channel and in the basin, while larger-scale forms, i.e., tidal bars, develop in the channel. A last observation concerns the formation of an outer delta in the ''sea'' basin. Results concerning the long-term equilibrium of the bed profile in the channel compare fairly satisfactorily with recent theoretical results. The nature and characteristics of the observed small-scale forms appear to be consistent with theoretical predictions and field observations concerning ''fluvial'' ripples and tidal dunes; bars show features in general accordance with recent results of a stability theory developed for tidal bars. The hydrodynamics of the inlet region exhibits a strongly asymmetric character, as observed in the field and predicted in early theoretical works, while the overall characteristics of the outer delta conform to available empirical relationships.Citation: Tambroni, N., M. Bolla Pittaluga, and G. Seminara (2005), Laboratory observations of the morphodynamic evolution of tidal channels and tidal inlets,

We investigate the morphodynamic equilibrium of tidally dominated alluvial estuaries, extending previous works concerning the purely tidal case and the combined tidal‐fluvial case with a small tidal forcing. We relax the latter assumption and seek the equilibrium bed profile of the estuary, for a given planform configuration with various degrees of funneling, solving numerically the 1‐D governing equation. The results show that with steady fluvial and tidal forcings, an equilibrium bed profile of estuaries exists. In the case of constant width estuaries, a concave down equilibrium profile develops through most of the estuary. Increasing the amplitude of the tidal oscillation, progressively higher bed slopes are experienced at the mouth while the river‐dominated portion of the estuary experiences an increasing bed degradation. The fluvial‐marine transition is identified by a “tidal length” that increases monotonically as the river discharge and the corresponding sediment supply are increased while the river attains a new morphological equilibrium configuration. Tidal length also increases if, for a fixed river discharge and tidal amplitude, the sediment flux is progressively reduced with respect to the transport capacity. In the case of funnel‐shaped estuaries the tidal length strongly decreases, aggradation is triggered by channel widening, and tidal effects are such to enhance the slope at the inlet and the net degradation of the river bed. Finally, results suggest that alluvial estuaries in morphological equilibrium cannot experience any amplification of the tidal wave propagating landward. Hence, hypersynchronous alluvial estuaries cannot be in equilibrium.

[1] We develop a nonlinear asymptotic theory of flow and bed topography in meandering channels able to describe finite amplitude perturbations of bottom topography and account for arbitrary, yet slow, variations of channel curvature. This approach then allows us to formulate a nonlinear bend instability theory, which predicts several characteristic features of the actual meandering process and extends results obtained by classical linear bend theories. In particular, in agreement with previous weakly nonlinear findings and consistently with field observations, the bend growth rate is found to peak at some value of the meander wave number, reminiscent of the resonant value of linear stability theory. Moreover, a feature typical of nonlinear waves arises: the selected wave number depends on the amplitude of the initial perturbation (for given values of the relevant dimensionless parameters), and in particular, larger wavelengths are associated with larger amplitudes. Meanders are found to migrate preferentially downstream, though upstream migration is found to be possible for relatively large values of the aspect ratio of the channel, a finding in agreement with the picture provided by linear theory. Meanders are found to slow down as their amplitude increases, again a feature typical of nonlinear waves, driven in the present case by flow rather than geometric nonlinearities. The model is substantiated by comparing predictions with field observations obtained for a test case. The potential use of the present approach to investigate a number of as yet unexplored aspects of meander evolution (e.g., chute cutoff) is finally discussed.