On Shallow Mixing Interfaces and Their Relevance for Understanding Mixing at River Confluences

Abstract

In many environmental fluid mechanics applications, the spatial development of mixing layers is significantly affected by the friction at the channel bottom. This motivates the present study of mixing interfaces developing under shallow flow conditions between parallel and nonparallel streams with a velocity ratio larger/smaller than one (mixing layer mode) and with a velocity ratio close to one (wake mode). Such mixing interfaces are very important to understand flow and mixing at river confluences where shallow conditions are generally observed. Given that at most river confluences the incoming streams have different temperatures and suspended sediment loads, the paper also discusses stratification induced by density differences between the incoming streams. For sufficiently small densimetric Froude numbers, a spatially developing lock-exchange-like flow develops away from the confluence apex. In such cases, mixing at large distance from the confluence apex is controlled by the lock-exchange-like flow rather than the vertically oriented, mixing interface vortices.