Hydrodynamic Effects on Sediment Oxygen Demand

Abstract

Sediment oxygen demand (SOD) is a rate at which oxygen is consumed by sediments from the overlying water column. It is well known that SOD is the largest dissolved oxygen (DO) sink within natural waters. DO dynamics are fundamental to water quality analysis and relate the relative health of aquatic environments. Advances in water quality modeling require not only accurate SOD rates, but also an understanding of the dynamic relationship that links the changes in these rates to the hydrodynamic framework within the water quality models. Several experiments have been conducted over smooth artificial sediments for the range of Reynolds' numbers from 0 to 3,000 (typical of lakes, reservoirs, and slow moving streams). Water was recirculated within a laboratory channel that was sealed at the beginning of each experiment. An ultraviolet (UV) light was employed in the re-circulation path to limit water-borne microbial growth that might induce extraneous oxygen demands. SOD flux was determined directly from time-averaged DO readings taken by two Clark-type DO probes placed at either end of the channel. Depth-averaged velocity profiles and turbulence parameters were obtained by use the of a 3D acoustic-Doppler velocimeter (ADV). Micro-oxygen profiles of the near bed region were obtained by the use of a miniature Clark-type DO microprobe mounted to a computer-controlled micro-manipulator. Preliminary results suggest that SOD flux increases as a power function with increasing Reynolds' number. Furthermore, the diffusive boundary layer (DBL) thickness (localized region that is presumed to impede mass transfer to the sediments) decreases with increasing Reynolds' number. These observations support a water-side control theory for mass transfer at the sediment-water interface.