Effects of Hydrodynamic Conditions on DO Transfer at a Rough Sediment Surface

A numerical model was developed to calculate the rate of dissolved-oxygen (DO) diffusion across a sediment surface taking into account the surface roughness and biochemical reactions of the sediment. Estimates of DO transfer rate from the model were compared with results from laboratory experiments conducted in a rectangular flume using roughness elements. In experiments, there was maximum value for the nondimensionalized DO transfer rate (Stanton number, St) in the transitional region of surface roughness, in which the mass flux was two to five times larger than that of the smooth surface. The reproducibility of the experimental results by numerical analysis was significantly improved by including terms for flushing frequency of water in cavities between the roughness elements and for nonsteady variations in the diffusion rate due to step changes in DO concentration in the flushed region. A simple method to estimate enhancement effect for St caused by nonsteady variations was also presented.