A stability diagram for fine-grained, cohesive fluvial-channel bifurcations

Although the evolution of fine-grained fluvial distributive networks depends upon the stability of channel bifurcations, neither the stability field nor the stabilizing processes of bifurcations are currently well-known. Here we define the theoretical stability field for fine-grained bifurcations using Delft3D, a morphodynamic numerical model, and test the model predictions using field data collected on eight natural bifurcations in the Mossy River Delta, Saskatchewan, Canada. The numerical experiments start with a generic, preformed, bifurcation that is symmetrical about the channel centerline, has fixed walls, and constant bathymetry in each bifurcate. Upstream boundary conditions include a constant incoming water and sediment discharge with equilibrium sediment concentrations and both steady and equal water surface elevations downstream. Computations proceed until the discharge ratio between bifurcate channels ( Qr) does not change for one morphological time unit (defined as cross-sectional area divided by sediment transport rate per unit width), at which time the system is defined as stable. Similar to the stability diagram for coarse-grained distributive networks (Bolla Pittaluga et al., 2003; Miori et al., 2006), stable, equilibrium, fine-grained bifurcation configurations are asymmetrical in their Qr. However, unlike coarse-grained systems, Qr becomes more asymmetrical with increasing Shields theta|n When the eight natural bifurcations are plotted on the stability diagram, three plot in ¡§unstable¡¨ space. Even though these ¡§unstable¡¨ bifurcations have been active for 40 years, analyses of their evolution from serial maps of the Mossy Delta shows that there is significant bar growth and change in channel area compared to the five ¡§stable¡¨ bifurcations. The implications of these findings for our understanding of channel bifurcation dynamics, evolution and avulsion will be discussed.