Abstracts Category : Other

Flow structure of an asymmetric large river confluence

by Kevin Kimball Johnson

The focus of this study is on flow structure and confluence morphodynamics at a large river confluence with an asymmetrical planform. Five sets of field measurements of three-dimensional velocity components and bed topography at the confluence of the Wabash River and Embarras River along the Illinois-Indiana border reveal a complex hydrodynamic environment that responds dynamically to changes in momentum flux ratio of the incoming flows and to the effects of secondary circulation cells. The scour hole location and geometry within this confluence hydrodynamic zone are subject to change depending on the relative momentum of the two rivers with the zone of scour shifting outward and enlarging as the momentum flux of the lateral tributary (Embarras River) increases relative to the momentum flux of the main stem (Wabash River). When the momentum of the Embarras River is relatively small, this river has little influence on flow structure in the confluence and flow in the Wabash River resembles patterns observed in a gently curving meandering river. When the momentum of the Embarras River is relatively large, the high-angle convergence of the two rivers leads to the development of a distinct shear layer where the orientation of cross-stream velocities changes abruptly, characterized by descending flow. Data on secondary flow components defined by the Rozovskii method reveal counter-rotating helical cells with strong downwelling along the shear layer. Patterns of backscatter intensity, used here as a surrogate for sediment concentration, along with data on surficial water temperatures indicate that the mixing interface between the two rivers corresponds to the position of the shear layer. The research shows how three-dimensional velocity data along with the ancillary information obtainable from acoustic Doppler current profilers can play a vital role in developing a better understanding of complex hydrodynamic environments at large river confluences.Advisors/Committee Members: Rhoads, Bruce L (advisor), Best, James L (committee member), Cienciala, Piotr (committee member).