Effect of tides on stability of bifurcations in river deltas

Abstract

In river channel networks, the sediment transport division determines the morphodynamic development of bifurcations. In sand-bed systems, avulsion often occurs, meaning asymmetric division of sediment transport until one branch closes off. In contrast, it has been claimed that bifurcations in tidal deltas tend to be stable such that both downstream channels keep conveying a major portion of the water. In this study, we investigate the effect of tides on stability of river bifurcations. A schematized bifurcation was built in a 1D morphodynamic model, with one upstream channel that splits into two downstream channels. The system is driven by river discharge upstream and symmetrical tidal water level variation from downstream channels. The model was validated against Delft3D in 2DH mode and yielded similar morphological evolution in much shorter time. Subsequently, the 1D model was used to study the morphological stability of bifurcations for a wide range of Shields values, width-to-depth ratios and relative role of tidal forcing over river forcing. We found that tides increase the range Shields stress and width-to-depth ratios for which for a stable bifurcation can be found for increasing tidal dominance. This is because the tide-induced changes of water depth and flow velocity cause a time dependent stability condition. The duration and growth rate of the bed level asymmetry during each condition determine the stability of the bifurcation. Due to the tides the bifurcation is going through periods of stable and unstable conditions, but tidally averaged the bifurcation is stable for a wider range of Shields numbers and width-to-depth ratios. This explains why avulsions are rarely observed in tide-influenced deltas.