Only a handful of field studies have examined turbulent flow structure at discordant confluences; the dynamics of flow at such confluences have mainly been examined in the laboratory. This paper reports results of a field‐based investigation of turbulent flow structure at a discordant river confluence. These results support the hypothesis that flow at a discordant alluvial confluence with a velocity ratio greater than 2 exhibits jet‐like characteristics. Scaling analysis shows that the dynamics of the jet core are quite similar to those of free jets but that the complex structure of flow at the confluence imposes strong effects that can locally suppress or enhance the spreading rate of the jet. This jet‐like behavior of the flow has important implications for morphodynamic processes at these types of confluences. The highly energetic core of the jet at this discordant confluence is displaced away from the riverbed, thereby inhibiting scour; however, helical motion develops adjacent to the jet, particularly at high flows, which may promote scour. Numerical experiments demonstrate that the presence or absence of a depositional wedge at the mouth of the tributary can strongly influence detachment of the jet from the bed and the angle of the jet within the confluence.

The effects of temperature induced stratification on flow hydrodynamics, thermal mixing, and the capacity of the flow to entrain sediment at a medium‐size stream confluence with a highly discordant bed are investigated. To isolate the effects due to differences in the temperature/density of the incoming streams, two simulations were conducted with identical flow conditions (mean velocity ratio = 0.41 and temperature difference between the two streams ΔT = 4.7°C). In the first case the Richardson number was Ri = 0 (no coupling between the temperature and the momentum equations via the Boussinesq approximation), while in the second simulation Ri = 0.67. Even in the Ri = 0 case the structure of the mixing interface (MI) was different from the one expected for concordant bed confluences with a similar confluence angle and velocity ratio. The MI contained only corotating eddies shed in the shear layer forming on the fast‐speed side of the confluence apex. In the Ri = 0.67 case no wake region was present but a large recirculation eddy formed not far from the confluence apex. In both cases, the flow near the upstream part of the MI was found to be highly 3‐D and to allow the passage of particles from one side of the confluence to the other. While in the Ri = 0 case mixing was driven by the MI eddies, in the Ri = 0.67 case mixing was controlled by large near‐bed intrusions of heavier fluid from the tributary containing colder water and also by the fluid advected in and out of the recirculation eddy.

Abstract