Impact of bed roughness and sediment permeability on mass exchange across sediment-water interface

The mass exchange process across sediment-water interface (SWI) plays an important role in contaminant migration and transformation in natural waters. The mass exchange across the SWI for rough bed depends on both bed roughness and sediment permeability. By means of laboratory annular flume experiment, quantitative measurements of interfacial mass exchange flux at the SWI and its variations have been performed under the rough bed condition. A parameterization method has been adopted to analyze the dependence of effective diffusion coefficient on its main controlling parameters. The experimental results indicate that within the variation range of the present flow and sediment conditions, the effective diffusion coefficient shows an obvious piecewise variation feature from hydraulically smooth, transitional up to fully rough regimes due to the joint effect of mean flow velocity, bed roughness and sediment permeability. The dependence of the effective diffusion coefficient on its main controlling parameters can be described consistently by adopting the permeability Reynolds number. Based on an analysis of dual parameters (i.e. the roughness and permeability Reynolds numbers), the corresponding threshold values for different flow regimes have been determined to reasonably characterize the comprehensive impact of the bed roughness and sediment permeability on the interfacial mass exchange characteristics.