Density driven confluence mixing pattern identified using high-resolution aerial images at Chicago Sanitary and Ship Canal

Abstract:

Through analysis of high-resolution image at Chicago Sanitary and Ship Canal(CSSC) -- Calumet Sag(Cal--Sag) river confluence, new types of river confluence mixing patterns, that involves flow density effect, are found. With data from USGS measurement and numerical model, these patterns have been categorized as four different types based on Richardson number Ri. Ri=0 denotes the classic vertical unstable mixing interface (MI): the plane turbulent mixing layer. When Ri becomes larger, MI turns to horizontal (type ii mixing), partially unstable horizontal (type iii mixing) and full plunging (type iv mixing) types. The trigger of this transition is weakly varied flow density between the two branches and the incoming flow momentum from opposite side. Formation of density currents is found when incoming flow density difference is around 0.75 [g/L], which links directly to the surface mixing patterns. A more generalized confluence mixing theory by considering weak buoyancy effect is proposed, which provides the potential of identifying river mixing condition directly through high resolution satellite/aerial image without taking any field measurement.