Monitoring thermal regimes along rivers has become a key issue due to its impact on aquatic populations and pathogen development. Monitoring fine sediments is equally important when considering their deposition, transfer through river infrastructures, or their impact on fish populations. River confluences are locations of significant exchanges of matter and heat, with flow structures being highly complex and involving various flow patterns.The goal of this PhD thesis is to study the influence of these flow structures on the mixing of heat and suspended sediments through experimental methods. It will improve our understanding of the mechanisms involved in sediment transport and heat dynamics in rivers, which is crucial for assessing, maintaining, or restoring the ecological health of these environments.

This research will primarily rely on measurements taken in the large channel of the Hydraulic and Hydromorphology Laboratory (HHLab) at INRAE Lyon-Grenoble Auvergne-Rhône-Alpes. This 18-meter long, 2-meter wide channel is designed to model flows formed at confluence points with zero angle. Starting with reference flows (i.e., without suspended sediments or temperature differences between the tributaries) characterized in the PhD thesis of B. Cerino (2021-2024), we will introduce either a difference in suspended sediment concentration or a temperature difference between the two tributaries. Flow velocity fields and flow structures will be characterized using Acoustic Doppler Velocimetry (ADV) probes. Fine sediments will be modeled using silt, with their concentrations measured using optical and acoustic methods. Surface temperatures will be measured using an infrared camera, and water column temperatures will be measured at specific points using thermocouples. These measurements will allow us to compare the mixing of suspended sediments and temperature in the presence or absence of different flow structures, and to quantify the effects of sediment concentration differences, temperature gradients, and overall mixing on the flow.

This PhD will, to our knowledge, be the first to link flow structures with the mixing of matter and heat across a wide range of hydraulic conditions. The use of temperature and sediment concentration to quantitatively measure mixing at confluences will be an innovative approach, requiring experimental development.

Financement

Ministry of Ecological Transition, Biodiversity, Forestry, Marine, and Fisheries (Doctoral Training for IPEF)