Numerical Modeling of Bifurcation Evolution in a Sand-bed Braided River

River bifurcations are key units in a braided river. Although simple bifurcations are well understood and can be analyzed by 1D models (e.g. Bolla Pittaluga et al., 2003 and Kleinhans et al., 2008), predicting the stability and dynamics of multiple interacting bifurcations in a braided river with migrating bars requires understanding of the interaction between braid bars, channel network and bifurcations, in particular the upstream curvature and downstream backwater effects. Our objective is to understand the evolution of bifurcations at migrating bars in a braided river and the effects on bar evolution. We used the 3D numerical morphodynamic model Delft3D to produce a dynamically braiding sand bed river. This model solves the 3D-flow and computes sediment transport and bed level change accounting for effects of transverse bed slope. It includes a simple bank erosion model to reactivate emerged areas. The morphology of mid-channel bars produced by the model was analyzed and the partitioning of water and sediment over the bifurcating channels are compared with a 1D model concept. Next, the evolution of bars is linked to that of the bifurcations, in order to infer relations between bar morphology and bifurcation evolution. We find that upstream bar dynamics have a major effect on the stability of bifurcations. Migration and elongation of bars can close the upstream entrance of a bifurcation channel, independent of the stability of the bifurcation. Moreover, bifurcation angle and upstream curvature can be affected by upstream bar migration and elongation, which steers flow and sediment partitioning at the bifurcation. At the same time, the partitioning of water and sediment over a bifurcation affects bar shape. Sediment eroded at one of the bar sides just downstream of the bifurcation deposits downstream of the braid bar in the form of tail bars. Hence bar shape as observable on imagery contains useful information about the evolution of the upstream bifurcation and may be used to predict the development of downstream bifurcations and bars.