Diffusional Mass Transfer at Sediment-Water Interface

Abstract

Utilizing a miniature, Clark-type, dissolved oxygen (DO) microprobe and a laser-Doppler velocimeter (LDV), laboratory experiments were conducted to elucidate the effect of the turbulent flow field on the diffusive sublayer thickness, mass transfer coefficient, and DO flux over a smooth bed. Both an artificial and a natural sediment were tested under flow conditions ranging in Reynolds number from 0 to 7,000, for a total of 17 experiments. The vertical resolution achieved with the microprobe enabled measurement of DO concentrations within the diffusive boundary layer and provided a direct measurement of the concentration sublayer thickness. Velocity profile measurements obtained with the LDV were used to estimate the depth-averaged velocity and the shear stress velocity. Analysis of the data included formulation of dimensionless groups and the obtaining of empirical relationships that can facilitate the prediction of the diffusive sublayer thickness, mass transfer coefficient, and mass flux at the sediment-water interface. Although the experimental work focuses on DO transport, the approach undertaken represents a generalized theory of waterside-controlled mass transfer at the sediment-water interface in the presence of a moving fluid.