River bifurcations: key processes, timescales and evolutionary trajectories

River bifurcations are constituent components of multi-thread fluvial systems, playing a crucial role in their morphodynamic evolution. As highlighted by several field studies and laboratory investigations, natural bifurcations tend to display unbalanced configurations, in which one branch carries most of the incoming water and sediment supply. This tendency often results in the partial or complete abandonment of one of the bifurcates (known as avulsion), leading to the diversion of the majority or all the incoming water and sediment discharges towards the other bifurcate. Most studies in the literature either focused on the long-term equilibrium configurations of bifurcations or tailored their analysis on specific case studies, rarely addressing avulsion conditions. Moreover, only a few works aimed at quantifying the characteristic timescales with which a bifurcation evolves towards its long-term configuration. Among the different fluvial contexts in which the dynamics of bifurcations is relevant, river deltas are of particular interest from both a geomorphological and coastal management viewpoints. Interestingly, available relations for sediment division at bifurcations mostly concern bedload transport, while within deltas most of the sediments are transported in suspension. The first part of this PhD thesis focuses on the key physical processes underlying the evolution of river bifurcations, along with their characteristic timescales and the different resulting evolutionary trajectories. These analyses are carried out mostly by means of a novel one-dimensional numerical model of a bifurcation, which is formulated by combining the solution of the free-surface profiles and Exner equations along the branches with existing nodal point relations for sediment partitioning. The numerical model is firstly employed to explore the conditions in which one branch has a vanishing transport capacity, to overcome the limitations of existing equilibrium models. Model results show that when the difference in the flow discharges among the bifurcates is so high that one branch no longer transport sediments (partial avulsion conditions), the other branch undergoes significant degradation due to an autogenic process, which leads to the abandonment of the non-transporting branch when the bifurcates are sufficiently long. To complement the numerical findings, a new analytical model is formulated to reproduce the essential characteristics of the partial avulsion equilibrium and to identify the key parameters that control the transition between different long-term configurations. The same numerical model is then used to quantify the evolutionary timescale of bifurcations, looking at the velocity with which small perturbations are amplified as a result of the free instability of the system. Firstly, an operative definition of the intrinsic timescale of a bifurcation is provided; then, its dependence on the governing parameters, namely the flow conditions in the upstream channel and the length of the bifurcates, is investigated. By combining the numerical results with those obtained through a linear analytical model, the bifurcation timescale is then compared with those of external forcings, namely migrating bars and time-varying flow discharge. The second part of the thesis concerns bifurcations belonging to river-dominated deltas. Specifically, an in-depth analysis of the flow and sediment division at several bifurcations belonging to the Wax Lake Delta (US) is performed, by means of a Lagrangian particle tracking model and a 2D numerical hydrodynamic model. First, sediment particles are routed on previously calibrated hydrodynamic simulations performed for the whole delta. The resulting sediment partitioning at each bifurcation satisfactorily compared against available field data on suspended sediment concentration. To extend our findings to similar deltaic contexts, the same numerical tools are then used on a synthetic geometry, which allows for a broad analysis of the role of differences in channel width, branching angle and inlet bed elevation between the bifurcates on water and sediment partitioning. The insights gained from these analyses provide a foundation for further investigations on natural bifurcations and may offer valuable information for the design of sustainable river restoration projects in different fluvial contexts. Moreover, our investigations on deltaic bifurcations help to understand the preferential pathways for sediment transport in river deltas, with noteworthy implications for delta management.