Three-Dimensional Finite Element Modeling of Thermal Stratification in the Riba-Roja Reservoir Confluence: A Fluid–Thermal Multiphysics Approach

This study presents a numerical investigation of the hydrodynamic and thermal interactions at the confluence of the Ebro and Segre Rivers in the Riba-roja Reservoir, using the Finite Element Method (FEM) within the Kratos Multiphysics framework. The coupled Navier–Stokes and energy equations were solved, employing the variational multi-scale (VMS) method for stabilization. Field data from thermal imaging and in situ measurements in March and October 2011 were used for model validation. The results revealed complex mixing and stratification dynamics influenced by regulated and unregulated flows, seasonal temperature variations, and reservoir geometry. Despite some discrepancies in temperature predictions due to the decoupled energy equation, the model effectively captured the system’s thermal behavior. This work represents a first step toward incorporating a fully coupled energy equation and exploring the effects of thermal mixing and stratification on suspended sediment transport. This study enhances understanding of fluid–thermal interactions in reservoir systems, with implications for water quality management and ecological conservation.