Author(s): Makoto Higashino; Heinz G. Stefan

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Keywords: Boundary layer; diffusion; ; k; –ε turbulence model; mass transfer; sediment/water interaction; sedimentary oxygen demand; turbulence

Abstract: It is well-known that sediment/water interaction affects water quality in natural water bodies in a variety of ways. Diffusional mass transfer, across a sediment/water interface can contribute significantly to the mass balances of dissolved substances such as oxygen, phosphorus, nitrogen and sulfate, especially in lakes, reservoirs, detention basins, navigation canals and estuaries. At low flow velocities above a sediment bed, diffusional mass transfer across a sediment/water interface becomes limited by lack of turbulence in the boundary layer. In this paper, three boundary layer turbulence models are used to quantify the limiting effect of turbulence near a sediment bed on diffusional mass transfer across a sediment/water interface: One is Dade's formula, the second is a model by Myong and Kasagi (MK model), and the third is a model by Nagano and Tagawa (NT model). The latter two are low-Reynolds number k–ε turbulence models that have been successfully used for the prediction of turbulent heat transfer near a wall. Dade's formula gives larger eddy diffusivity/viscosity values near the sediment/water interface than the other two turbulence models, but the velocity profile obtained by Dade's formula is only slightly different from the universal velocity profile given by the linear law and the log–law. Simulated solute concentration profiles and vertical diffusive fluxes (Sherwood numbers Sh) at the sediment/water interface were found to be only weakly dependent on the choice of turbulence model when material sinks or sources in the sediments were not explicitly quantified. Sherwood numbers simulated by the NT model were less than 40% smaller than those obtained by Dade's formula and theMKmodel, when the Schmidt number Sc > 10, which is the case for most materials in aquatic solution. The mass transport modelwas also applied to the estimation of sedimentary oxygen demand (SOD) by including a microbial oxygen sink in the sediments. Simulated oxygen uptake rate, i.e. SOD (gm-2 d-1), was virtually the same for all three turbulence models, except at low shear velocity (U* < 0.5 cm), and at high microbial activity inside the sediment (Xmax > 100 mg/l). Even then the lowestSODvalues obtained with theNTturbulence model were within 20% of the higher values estimated with the other two models. One can conclude that any of the three models used to describe turbulence near the sediment bed, is appropriate forSODestimation, because microbial activity inside the sediment is as limiting as near-bed turbulence

DOI: https://doi.org/10.3826/jhr.2008.2769

Year: 2008