Lorenzo Durante

River bifurcations are a ubiquitous characteristic of both gravel-bed and sand-bed fluvial systems, such as braided networks, anabranches, and deltas. The morphology and development of bifurcations shape fluvial plains and deltas, which influence flood-prone areas, as well as land loss and gain. Bifurcations worldwide are frequently unstable to any perturbation of their current state, leading to highly asymmetric discharge partitions between the branches or ultimately to the complete closure of one of them. However, in tide-influenced deltas, it has been observed that bifurcations tend to exhibit more stable branches, keeping all channels active. Therefore, despite the morphodynamic equilibrium of bifurcations being strongly influenced by the characteristics of the upstream channel, only recently has effort been made to study the action exerted by external forcings on the downstream channels. Present linear stability analyses predict that the stability of these systems is dependent on the Shields stress and the half-width to depth ratio of the upstream channel. Moreover, when downstream forcings are accounted for, the length of the downstream channels and the tidal amplitude affect the overall stability.

The objective is to conduct a comprehensive investigation of the stability of individual bifurcations through linear stability analyses, which will consider new configurations and subsequently apply them to deltas, viewed as a sequence of bifurcations. These analyses will also be validated through numerical simulations, with the aim of surpassing the constraints imposed by current theoretical models.

The aforementioned objectives will be primarily addressed through linear stability analyses, which will be based on the established two-cell model proposed by Bolla Pittaluga et al. (2003). Additionally, numerical simulations will be conducted using the extensively utilized code Delft3D, in order to further substantiate the theoretical results.

A novel linear stability analysis has been conducted to investigate the impact of branch length on the stability of riverine and estuarine bifurcations. The results suggest that the stability of bifurcations increases as the length of the branches decreases. Furthermore, possible asymmetries in the system were examined, revealing the existence of counterintuitive stable configurations where the unfavored channel dominates. The numerical simulations performed are in fairly good agreement with these findings.