Stage-related changes in flow structure at three small confluences: implications for process-form interactions

Abstract:

The combining of flows from separate tributaries at confluences results in complex hydrodynamic conditions that play an important role in confluence morphodynamics. The hydrodynamic conditions are characterized by highly three-dimensional patterns of fluid motion controlled mainly by the confluence planform and momentum ratio of the incoming flows. Despite abundant previous work, understanding of 3D flow structure at confluences and the influence of this structure on channel form remain poorly constrained, preventing the development of a universal conceptual model. The purpose of this study is to analyze how 3D flow structure scales with flow stage. The analysis was performed at three confluences with different planform configurations, to capture the effect of different planform and bed morphology. Previous work at these confluences has examined flow structure at low stages that were not capable of mobilizing bed material, and thus did not directly influence morphodynamics. Measurements of three-dimensional velocities were obtained along several cross sections at each confluence using a Sontek M9 acoustic Doppler current profiler and were analyzed using the Velocity Mapping Toolbox (VMT) (Parsons et al., 2013). The structure of depth-averaged and cross-sectional downstream velocities reveals marked flow convergence and the presence of a stagnation zone near the upstream junction corner that extends downstream as a region of velocity deficit separating the two confluent flows. Localized streamwise helicity is evident for some flows, but not for others. Whereas helicity can be related to flow curvature in some cases, its cause in other cases is not readily apparent. The 3D velocity measurements indicate that flow patterns for these transport-effective flows do not necessarily conform to patterns documented during low flows. Thus, inferences about mutual adjustment between process and form at confluences based on low-stage measurements may in some instances be misleading.