3D NUMERICAL INVESTIGATION OF TIDAL FORCING ON THE STABILITY OF BIFURCATIONS

River bifurcations are ubiquitous features of both gravel-bed and sand-bed fluvial systems, including braiding networks, anabranches and deltas. As such, their morphology and development shape fluvial plains and deltas dictating flood risk areas as well as land loss and land gain. The morphodynamic equilibrium of bifurcations is strongly affected by the characteristics of the upstream channel, such as width-to-depth ratio (Bolla Pittaluga et al., 2003), curvature (Kleinhans et al., 2008), bar presence (Bertoldi et al., 2009; Redolfi et al., 2019), and related sediment transport mechanisms (Bolla Pittaluga et al., 2015; Redolfi et al., 2016), which regulate water and sediment partitioning into the two downstream branches. Interestingly, in nature the downstream boundary conditions may also influence the bifurcation dynamics due to their oscillatory behaviour with significant backwater effects. It has been observed that tide‐influenced deltas tend to exhibit more stable branches keeping all channels active. Factors such as the length of the downstream channels or tidal ranges strongly affect the evolution of the bifurcations. On the basis of an analytical 1D model developed under the hypothesis of small monochromatic tidal oscillations, Ragno et al. (2020) proved that even small-amplitude tides have a stabilizing effect on the bifurcation due to the erosive character of weak tidal ebb flows. These flows keep both branches morphodynamically active, preventing the development of unbalanced solutions. The present work aims to corroborate Ragno et al. (2020) findings by means of fully 3D numerical simulations performed through Delft3D and to extend the analysis to the case of strongly tide influenced bifurcations. Moreover, the full description of the flow field along the vertical direction allows for a more thorough evaluation of sediment and flow partitions at the bifurcation and to quantify 3D effects at the node on the evolution and the equilibrium configuration of the system.