Near‐bed turbulence and hydrodynamic control of diffusional mass transfer at the sea floor

Abstract

A simple closure scheme for turbulent transport yields mean‐velocity and dissolved‐substance concentration profiles just above and solute fluxes across a sediment‐water interface underlying uniform, steady flow. In the case of near‐bed turbulence characterization, this approach retains both turbulent and viscous terms in expressions for important structural aspects of the near‐bed regions of turbulent boundary layers. Moreover, a modified turbulent kinetic energy balance is used to define eddy viscosity in the viscous‐dominated region of flow very near the bed. In the case of characterization of mass distribution and diffusional transfer rate, the approach partitions important factors that control mass transfer rate: nearbed turbulent transport, interfacial‐flux boundary condition, and reaction kinetics. Diffusive sublayers overlying smooth beds exposed to typical deep‐sea conditions are predicted to be ∼ 1 mm thick, a result in agreement with the sublayer thickness observed in situ and the inferred thickness used in many hydrodynamic models. One application of the new closure scheme indicates that typical biogenic roughness in fine‐sediment marine environments enhances solute exchange rates threefold over those expected for a smooth bed. This increase is due to enhanced turbulent transport in the vicinity of the rough bed and occurs in spite of viscous ponding of “dead water” among roughness elements.