This study addresses the mixing of two free-surface flows with variable density due to temperature, turbidity, or salinity differences. It focuses on river confluences and the mixing of effluents downstream of discharge channels from Nuclear Power Plant (NPP) cooling systems. The processes of material transfer in rivers influence the mixing downstream of discharge outlets and confluences, as well as the liquid-solid fractionation and the exchange of pollutants between the water column and sediments.

There are many descriptions of flow behavior at confluences, based on both laboratory experiments and field measurements, characterizing the impact of confluence parameters on hydrodynamics. However, studies dealing with the processes and efficiency of water mixing are more limited. Currently, predicting downstream mixing at confluences under varying hydrological conditions remains difficult. This issue requires understanding the effects of lateral shear and density effects (stratification) on mixing. In an operational context, understanding these mechanisms is crucial for quantifying impacts on ecology and water usage.

This study is being carried out in collaboration with IRSN, INRAE, and LMFA.

Scientific Objectives are:

1. To understand how hydrodynamics and mixing are affected by density effects, for different water heights and velocities.
2. To identify a reliable numerical parameter adapted to confluences that can establish a macroscopic scale indicating the relative influence of the two processes on mixing.
3. To predict the value of the lateral mixing coefficient using a Fisher-type formula or by calculating it from the 3D velocity field, integrating density effects.

The project will include experiments in a hydraulic channel, controlling the density difference by adding salt and monitoring conductivity using four probes. A cooling process, which has no significant impact on density, will create a temperature gradient by adding ice to one of the two flows. This will allow visualization of the mixing process with a thermal camera positioned above the channel. Field measurements at confluences and at discharge channels of hot water from NPPs will be conducted using CTD probes and an aDcp mounted on a boat that performs crossings of both upstream tributaries and the river downstream of the confluence, with DGPS positioning.