Observations of Mixing Processes Downstream from the Confluence of the Mississippi and St. Croix Rivers

Abstract

The initial lateral mixing downstream from a confluence of two rivers with different water proper ties and velocities has a near-field mixing zone that is typically 5-10 river widths long and visually similar to the free shear-flow zone in an infinitely deep fluid. Vertical and horizontal profiles, and time-series measurements of temperature and specific conductance were collected in the near-field mixing zone at three distances downstream from the confluence of the Mississippi and St. Croix Riv ers on April 11, 1992. The vertical interface between the two fluids was characterized by step-like gradients in color, temperature, specific conductance, and velocity. Mixing in the near-field zone con sisted of stirring and diffusion processes. The difference in velocity between the two fluids (0.18 m/ s) created lateral shear instabilities that were visible at the surface and appeared to have 3-dimensional structure. These instabilities stirred the water, increasing the surface area of the interface, and created horizontal intrusions of water with temperature and specific-conductance properties that con trasted with the surrounding water. Twenty meters downstream from the confluence (about 0.05 river widths), the downstream size of these intrusions was 10-80 m and the vertical size was 0.2-0.5 m. There were sharp gradients of temperature and specific conductance across the horizontal interface between the intrusions and surrounding fluid (differences of 3.4°C and 500 uS/cm), and measure ments made inside some intrusions suggest that mixing at the edges of these intrusions was double diffusive. Double diffusion may have coupled with the shear (estimated gradient Richardson number was 0.2) to produce triple diffusion. Turbulent instabilities continued to stir the water causing the intrusions to decrease in size downstream such that the final measurements of temperature and spe cific conductance about 8 river widths downstream from the confluence were relatively smooth, monotonic functions of the cross-channel distance.

Mixing Dynamics at the Large Confluence Between the Yangtze River and Poyang Lake

“…Water conductivity was a suitable tracer to illustrate the mixing processes at the confluence of the Yangtze River and the Poyang Lake, which naturally have a large difference in water conductivity. Moreover, water conductivity is a conservative parameter (Gaspar, 1987) and can be continuously recorded in situ using multiparameter water chemistry meter, as suggested by Moody (1995) and Pouchoulin et al. (2020).…”

Predicting Transverse Mixing Efficiency Downstream of a River Confluence

“…Second, electrical conductivity is a conservative parameter (Gaspar, 1987), thus allowing identifying the mixing coefficient directly from its value within the range of the two upstream rivers. Last, electrical conductivity can be continuously recorded in situ using conventional conductivity‐temperature‐depth (CTD) loggers, as already done by Moody (1995), for instance. However, electrical conductivity is expressed as a specific conductance for a given reference temperature (here 25°C), hence requiring concurrent temperature measurements for correction.…”

“…Defining analytical or numerical solutions of the 2-D advection-diffusion equation to predict the lateral distribution of concentration downstream of a confluence is problematic due to the complexity of near-field mixing processes (Moody, 1995). Transverse mixing due to molecular diffusion is usually negligible compared to shear dispersion and advective effects over a range of spatial scales.…”

“…However, their survey did not focus on the near-field section downstream of the confluence, and the resolution of their current-meter measurements and tracer sampling does not provide a detailed insight on mixing processes throughout the explored cross sections. Moody (1995) did provide observations of mixing processes in the near field of the Mississippi and St. Croix confluence but with similar measurement limitations and no application of a mixing model. On the other hand, many authors have recently investigated the mixing processes downstream of confluences of small (Rhoads & Sukhodolov, 2001), medium (Herrero et al, 2018), to very large (Gualtieri et al, 2019;Lane et al, 2008;Laraque et al, 2009) rivers using modern measurement techniques, especially high-resolution multibeam echosounders and hydroacoustic velocity profilers.…”

“…However, their survey did not focus on the near‐field section downstream of the confluence, and the resolution of their current‐meter measurements and tracer sampling does not provide a detailed insight on mixing processes throughout the explored cross sections. Moody (1995) did provide observations of mixing processes in the near field of the Mississippi and St. Croix confluence but with similar measurement limitations and no application of a mixing model.…”