Stability and Asymmetry of Tide-Influenced River Bifurcations

Abstract

Bifurcations are important geomorphological features in tide-influenced deltas. At bifurcations, river flow and tides distribute sediment over the channel network and determine the morphodynamic evolution of the entire delta. Using a one-dimensional numerical model, we study the effects of tides on the morphological evolution of bifurcations from river-dominated to tide-dominated systems. In accordance with previous studies, bifurcations with small tidal influence, in which the flood flow hardly drives morphological change, have a larger range of Shields stress and width-to-depth ratio conditions for which symmetric bifurcations are stable to depth perturbations, compared to their river-dominated counterparts. We extended the existing studies to tide-dominated conditions. When bifurcations become increasingly tide-dominated, the range of conditions under which balance discharge partition (symmetric morphology) can exist, shrinks. Under these conditions, the bed can also change during the flood phase and growth of the bed asymmetry is larger than the decay during ebb. However, the bed asymmetry in equilibrium becomes less pronounced with increasing tidal dominance. We conclude that tides reduce the tendency of closure and abandonment of one of the downstream channels compared to river-dominated bifurcations, either by inhibiting the instability or by reducing asymmetry.