MASS-TRANSFER MECHANISM IN A POROUS RIVERBED

The mass transfer mechanism in a porous riverbed was investigated in two experimental open channels which have porous beds composed of 1.9 and 4.08 cm diameter ceramic balls, respectively. Vertical diffusion coefficients in the porous bed were determined by analyzing the diffusional pattern of tracer (NaCl) from the overlying water into the deeper region of the porous bed. Velocity profiles both over and under the water-bed interface and turbulence at the interface was measured. The diffusion coefficients in the porous bed were expressed by the product of velocity component and mixing length along every depth of the porous bed. Near the interface, the diffusion coefficient was expressed by the product of the turbulent intensity and the void scale of the porous layer. In the deeper region of the porous bed, the diffusion coefficient was expressed by the product of the time-averaged velocity and the void scale. © 1990.