Free and forced morphodynamics of river bifurcations

Abstract

Water and sediment distribution by river bifurcations is often highly unbalanced. This may result from a variety of factors, such as migration of bars, channel curvature and backwater effects, which promote an uneven partition of flow and sediment fluxes in the downstream branches, which we call ‘forcings’. Bifurcations also display an intrinsic instability mechanism that leads to unbalanced configurations, as occurs in the idealized case of a geometrically symmetric bifurcation, which we call ‘free’, provided the width‐to‐depth ratio of the incoming flow is large enough. Most frequently, these free and forced mechanisms coexist; however, their controlling roles in bifurcation dynamics have not been investigated so far. In this paper we address this question by proposing a unified free‐forced modelling framework for bifurcation morphodynamics. Upstream channel curvature and different slopes of downstream branches (slope advantage) are specifically investigated as forcing effects typically occurring in bifurcations of alluvial channels. The modelling strategy is based on the widely used two‐cell model of Bolla Pittaluga et al. (Water Resources Research, 2003, 39(3), 1–13), here extended to account for the spatially non‐uniform fluxes entering the bifurcation node. Results reveal that the relative role of free and forced mechanisms depends on the width‐to‐depth ratio falling above or below the resonant threshold that controls the stability of free bifurcations: when the main channel is relatively wide and shallow (super‐resonant regime) the bifurcation invariably evolves towards unbalanced configurations, whatever the combination of curvature and slope advantage values, which instead control the bifurcation response under sub‐resonant conditions. Detection of the resonant aspect ratio as a key threshold also releases the modelling approach from the need for parameter calibration that characterized previous approaches, and allows for interpreting under a unified framework the opposite behaviours shown by gravel‐bed and sand‐bed bifurcations for increasing Shields parameter values. © 2018 John Wiley & Sons, Ltd.

Flow and Suspended Sediment Division at Two Highly Asymmetric Bifurcations in a River Delta: Implications for Channel Stability

“…The asymmetry of bifurcations can be predicted from their width‐to‐depth ratio (Redolfi et al., ). In general, bifurcations are predicted to be the more asymmetric for larger width‐to‐depth ratios.…”

“…Two factors have been proposed for the steering: first, the transverse transport of sediment by the secondary flow toward the inner bend (Kleinhans et al., ; van Denderen et al., ) and, second, coarsening of bed material toward the outer bend (Frings & Kleinhans, ; Sloff & Mosselman, ), Interestingly, practical engineering methods disagree with mathematical models about the division of sediment in bends. While it is a common engineering practice to minimize the amount of diverted sediment by diverting water from outer bends (Raudkivi, ), mathematical models predict that most sediment is diverted into the outer bend branch (Kleinhans et al., ; Redolfi et al., ). In addition, the division of sediment depends on the river stage (Edmonds, ) and can vary during the passage of a flood wave (Frings & Kleinhans, ).…”

“…Tides may stabilize river bifurcations, as deltas under mixed influence of river and tidal flow typically have more channels than fluvially dominated deltas. Tides influence sediment transport, channel geometry, and the division of river discharge (Buschman et al., ; Redolfi et al., ; Sassi et al., , ). The mechanism by which tides stabilize deltaic river bifurcations is not fully understood.…”

“…The transverse slope models (Bolla Pittaluga et al., , ) require a parameter that specifies the length of the upstream reach that influences the bifurcation. Redolfi et al., (, ) developed a method based on the linearized shallow water equations to determine this parameter. This allows one to predict the equilibrium configuration of bifurcations without calibration.…”

“…This allows one to predict the equilibrium configuration of bifurcations without calibration. The models by Bolla Pittaluga et al., (, ) and Redolfi et al., (, ) interestingly predict either a symmetric or an asymmetric stable equilibrium but cannot predict unstable configurations where one of the branches silts up and is abandoned entirely. It has been suggested that overland flow during extreme discharge events is important for the sedimentation of derelict channels (Toonen et al., ).…”