Stratification effects on hydrodynamics and mixing at a river confluence with discordant bed

Abstract

Stratification effects induced by density differences between the incoming flows are investigated at a medium-size stream confluence with a highly discordant bed. The relatively large temperature difference ΔT = 4.7 °C between the main, high-discharge tributary and the low-discharge tributary together with the relatively small velocity of the flow in the main channel translate in a very high Richardson number, Ri = 1.89, which suggests that stratification effects have an important effect on flow hydrodynamics and thermal mixing. Simulation results confirm this and show that for the case when the velocity of the low-discharge, lower-temperature tributary is much smaller than that of the high-discharge tributary, the denser fluid from the low-discharge tributary plunges rapidly toward the bottom and the confluence apex. It then moves as a near-bed current of denser fluid across the main-tributary side of the main channel until the opposing bank where it reaches the free surface. It then continues to move parallel to the bank line. Meanwhile, the less-dense fluid from the main tributary moves over the near-bed current of denser fluid into the central part of the main channel. This induces a two-layer structure of the flow inside the upstream part of the confluence, which explains the very different mean flow patterns near the bed and near the free surface. Flow hydrodynamics and mixing in the Ri = 1.89 simulation are found to be very different when compared to those observed in the no-density-effects (Ri = 0) case where there is no coupling between the temperature and the momentum equations via the Boussinesq approximation. In this case, the fluid from the low-discharge tributary remains on the corresponding side of the main channel. Away from the confluence apex, the volume of mixed fluid is several times larger in the Ri = 1.89 simulation, which demonstrates that under certain conditions two streams of unequal densities can mix much faster compared to the case when the two streams have the same density. The large-scale flow patterns, eddy dynamics and volume of mixed fluid in the Ri = 1.89 simulation were also found to present major differences with a case where the main, high-discharge tributary contained the lower-temperature (denser) fluid.