Morphodynamic equilibrium of tidal bifurcations

Deltas are fascinating landforms subject to riverine (input of water and sediments) and marine processes (waves, tides) where bifurcations are the building block controlling the distribution of water, nutrient and sediment fluxes among the distributary channels of the network. In this work we focus on the role of tides as a key factor in controlling bifurcation behaviour. Recently it has been suggested and observed that tidal deltas (i.e. delta influenced or totally dominated by the tides) have the tendency to less numerous but more stable branches in comparison to fluvial-dominated deltas [Hoitink et al., 2017]. River bifurcations subject to unidirectional flow have been widely studied in the last decades. However, in the case of tidal bifurcations, the acting physical mechanisms and controlling factors are still not well understood, and a theoretical framework is still lacking. In order to fill this gap and understand how the stability and evolution of a delta could be affected by the tides, we investigate through an analytical model, the equilibrium configurations and stability conditions of a tidal bifurcation under the hypothesis of small tidal oscillations. In particular, we extend to the tidal case the previous works of Bolla Pittaluga et al. [2015] and Seminara et al. [2012] relative to the equilibrium and stability of a single bifurcation, and to the equilibrium of a single river dominated estuary, respectively. Results show that higher tidal amplitude and a closer position of the junction node to the sea, tends to hamper the development of unstable solutions, reducing the asymmetries in water and sediment fluxes between branches obtained when the upstream width-to-depth ratio falls above a critical value. Field observations of natural deltas seems to corroborate our findings.