Vertical heterogeneity and flexible root dynamics in pollutant transport: A hybrid lattice Boltzmann method - random displacement model approach for optimizing artificial floating bed design.

Artificial floating beds are widely recognized as an effective ecological approach for river water quality management. However, prior research has predominantly focused on pollutant retention efficiency across vegetation types, leaving the pollutant diffusion dynamics influenced by flexible-rooted vegetation underexplored. This study bridges this gap by investigating solute transport mechanisms in artificial floating bed channels with flexible vegetation roots through integrated indoor flume experiments and numerical simulations. A novel hybrid model, combining the lattice Boltzmann method for hydrodynamic simulation and the random displacement model for solute transport, was developed to quantify the vertical heterogeneity of pollutant diffusion coefficients. Experiments involved three bionic vegetation types with varying root morphologies, and solute transport was monitored using planar laser-induced fluorescence. Key findings revealed that flow velocity within the vegetation root zone was significantly reduced, particularly for vegetation with higher drag coefficients (e.g., Plant 2). The characteristic root diameter d50 was identified as the optimal parameter for simulating diffusion coefficients, achieving high accuracy. Vertical root distribution variance was incorporated into the diffusion model, enhancing simulation precision. Results demonstrated distinct pollutant dispersion patterns depending on source depth and vegetation type, with non-vegetated zones adhering to Gaussian concentration distributions. This study provides critical theoretical insights into pollutant transport mechanisms in flexible-rooted artificial floating beds, offering a foundation for optimizing artificial floating beds design and placement to improve water quality management strategies. Future work should validate these findings through outdoor experiments and integrate pollutant retention modules for practical applications.