NEAR-BED TURBULENCE AND HYDRODYNAMIC CONTROL OF DIFFUSIONAL MASS-TRANSFER AT THE SEA-FLOOR

A simple closure scheme for turbulent transport yields mean-velocity a nd dissolved-substance concentration profiles just above and solute fl uxes across a sediment-water interface underlying uniform, steady flow . In the case of near-bed turbulence characterization, this approach r etains both turbulent and viscous terms in expressions for important s tructural aspects of the near-bed regions of turbulent boundary layers . Moreover, a modified turbulent kinetic energy balance is used to def ine eddy viscosity in the viscous-dominated region of flow very near t he bed. In the case of characterization of mass distribution and diffu sional transfer rate, the approach partitions important factors that c ontrol mass transfer rate: near-bed turbulent transport, interfacial-f lux boundary condition, and reaction kinetics. Diffusive sublayers ove rlying smooth beds exposed to typical deep-sea conditions are predicte d to be approximately 1 mm thick, a result in agreement with the subla yer thickness observed in situ and the inferred thickness used in many hydrodynamic models. One application of the new closure scheme indica tes that typical biogenic roughness in fine-sediment marine environmen ts enhances solute exchange rates threefold over those expected for a smooth bed. This increase is due to enhanced turbulent transport in th e vicinity of the rough bed and occurs in spite of viscous ponding of ''dead water'' among roughness elements.