COMBINED CONVECTION-DIFFUSION MODELLING OF SEDIMENT ENTRAINMENT

Peter Nielsen

Abstract


A new, quantitative framework is presented for the modelling of suspended sediment concentration distributions. It accounts for large scale mixing (convection) as well as small scale mixing. The small scale mixing is modelled in terms of gradient diffusion as usual. It is shown how the new, combined convection-diffusion model can explain the different c(z)-distributions of different sediment sizes in the same flow. Also, the dependence of the c(z)-distribution shape on wave period and sediment settling velocity is explained for oscillatory flow over ripples. It is shown that individual Fourier components of cn{z,i) behave differently in diffusion dominated and in convection dominated flows. This makes it possible to determine the dominant entrainment mechanism (convection or diffusion) on the basis of concentration time series taken simultaneously at different levels.

Keywords


sediment entrainment; convection modeling; diffusion modeling

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