Deltas are fascinating landforms subject to fluvial and marine forcing. Bifurcations are common features in deltas, governing the distribution of water and sediment fluxes among the distributary channel network. Recently, it has been observed that tide‐influenced deltas tend to display less numerous but more stable branches in comparison to their riverine counterparts. River bifurcations subject to unidirectional flow have been widely studied in the last decades. In contrast, the acting physical mechanisms and factors controlling the stability of bifurcations in tide‐influenced deltas 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 bifurcations in distributary deltaic systems could be affected by the tides, we investigate, through an analytical model, the equilibrium configurations and stability of tidal bifurcations under the hypothesis of small monochromatic tidal oscillations. In particular, we build on previous works on river bifurcations, incorporating the solution for the equilibrium of a single‐river‐dominated estuary. We find that higher tidal amplitudes and a closer proximity of the junction node to the sea tend to hamper the development of unbalanced solutions, reducing the asymmetries in water and sediment fluxes between branches. This stabilizing effect exerted by the tidal action is associated with the erosive character of the tidal currents that promotes channel deepening and increases the capacity of the system to keep morphodynamically active both bifurcates in comparison with the purely fluvial case. Preliminary field observations of natural deltas corroborate our findings.

Effect of Small Tidal Fluctuations on the Stability and Equilibrium Configurations of Bifurcations

Ragno, Niccolò;Tambroni, Nicoletta;Bolla Pittaluga, Michele
2020

Abstract

Deltas are fascinating landforms subject to fluvial and marine forcing. Bifurcations are common features in deltas, governing the distribution of water and sediment fluxes among the distributary channel network. Recently, it has been observed that tide‐influenced deltas tend to display less numerous but more stable branches in comparison to their riverine counterparts. River bifurcations subject to unidirectional flow have been widely studied in the last decades. In contrast, the acting physical mechanisms and factors controlling the stability of bifurcations in tide‐influenced deltas 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 bifurcations in distributary deltaic systems could be affected by the tides, we investigate, through an analytical model, the equilibrium configurations and stability of tidal bifurcations under the hypothesis of small monochromatic tidal oscillations. In particular, we build on previous works on river bifurcations, incorporating the solution for the equilibrium of a single‐river‐dominated estuary. We find that higher tidal amplitudes and a closer proximity of the junction node to the sea tend to hamper the development of unbalanced solutions, reducing the asymmetries in water and sediment fluxes between branches. This stabilizing effect exerted by the tidal action is associated with the erosive character of the tidal currents that promotes channel deepening and increases the capacity of the system to keep morphodynamically active both bifurcates in comparison with the purely fluvial case. Preliminary field observations of natural deltas corroborate our findings.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11567/1019694
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