FullyCoupledSolverforHyproehciExchange

Numerical models have been extensively used to understand and predict flow and reactive transport processes in the hyporheic zone. However, most models focus on fully saturated riverbeds without accounting for surface water stage fluctuations related to precipitation and flooding. To capture the complete picture of hyporheic processes in riverbeds and riverbanks, we developed a fully-coupled multiphase reactive transport solver using the Open Source Field Operation And Manipulation (OpenFOAM) platform. This solver captures surface water stage fluctuations and partially-saturated flow in fluvial sediment using VoF two-phase flow and extended-Darcy's Law two-phase flow models for surface and subsurface domains, respectively. The transport models designed for partially saturated conditions in both domains are implemented. A geochemical reaction module, PhreeqcRM, is integrated into the solver to facilitate complex geochemical reaction networks. A two-way conservative flux boundary condition is implemented at the surface-subsurface interface to realistically map fluxes. The solver's capability is illustrated through a variety of hyporheic-related problems across spatial scales. These include laboratory experiments and reactive transport in two and three dimensions, from the bedform scale to multiscale riverbeds and riverbanks with fluctuating surface water flow. This novel solver allows for quantifying dynamics in the hyporheic zone with fewer simplifications. Based on the code structure and parallel design of OpenFOAM, the solver can simulate large, three-dimensional (3D) multiscale cases. The code, examples, and pre- and post-processing scripts are all open source, providing community access to use and modify them as desired.