hyporheicFoam: An Open Source Solver for Fully Coupled Simulation of Flow and Reactive Transport in Rivers and Riverbed

Rivers and their surrounding beds are important conduits and media for aquatic reactions, e.g., respiration and denitrification. Although the fundamental reaction process is known, the quantification and mechanistic prediction through high-fidelity modeling are still difficult. Previous common modeling practice is to simulate the river flow first, and then the simulated pressure field is imposed as a boundary condition to drive the porous media flow in the riverbed. The coupling is only one-way, i.e., only the surface flow affects the subsurface. More advanced models should be able to simulate the concurrent motion of water and water-borne quantities (e.g., solutes and heat) in both domains, and more importantly the two-way mass exchange across their common boundaries. We present the development and application of an open-source solver, hyporheicFoam, based on the OpenFOAM platform. This solver can simulate the full surface-subsurface coupling under dynamic conditions. hyporheicFoam is designed and implemented with several capabilities. It solves the Reynolds-Averaged Navier-Stokes equations and the modified Richard equations for turbulent open-channel flows and fluid motion in the surrounding geological deposits, respectively. The biogeochemical reaction network can be specified by the user with a simple text file. The coupling between two domains is implemented by mapping conservative flux boundary conditions at the interface through an iterative algorithm. The new model provides opportunities to study coupling effects which were impossible to model previously. This new model is validated with experimental data. With this new model, we can not only demonstrate the necessity of performing fully coupled simulation but also quantifying the errors induced with the previous segregated approach.