Controls on Flow Separation at the River Bifurcation Head

Abstract

Flow separation zones are common hydrodynamic features in river bifurcations, strongly influencing flow partitioning and channel morphology. While the inner‐bank separation zones have been widely recognized, we identify a previously underreported separation zone at the bifurcation head. The formation mechanism and controlling factors of this feature remain poorly understood. In this study, using a boundary‐layer theoretical model and a high‐resolution hydrodynamic model of an idealized Y‐shaped bifurcation, we demonstrate that the bifurcation angle and the inflow attack angle associated with the asymmetric flow partitioning are two key parameters for controlling its occurrence. An adverse pressure gradient at the bifurcation head is a necessary condition for the development of this separation zone. Among these governing parameters, the inflow attack angle at the bifurcation head plays a dominant role in its appearance. The separation zone forms only when the inflow attack angle exceeds a critical value of 0.87 radians (), indicating a threshold behavior in its onset. The bifurcation angle plays a secondary role, as smaller bifurcation angles allow this threshold to be reached under a lower asymmetry of flow partitioning. Moreover, the spatial extent of this flow separation zone becomes larger with increasing asymmetry of flow partitioning, suggesting a complementary role to the inner‐bank separation zone and providing a negative feedback on both the increase in flow partitioning asymmetry and the stability of bifurcation morphology.