Non-Fickian modeling of pollutant mixing during initial period in open channels

Inhwan Park, Il Won Seo

Thursday 2 july 2015

17:33 - 17:36h at Asia (level 0)

Themes: (T) Hydro-environment, (ST) Impacts of pollutants on the water environment, Poster pitches

Parallel session: Poster pitches: 13G. Environment - Impact

Field studies on the 2D mixing indicates that the pollutant cloud has skewed distribution in the initial mixing period and gradually transformed to the Gaussian distribution. Thus, in the initial period, Fickian dispersion model following the Taylor’s assumption cannot be applied, and the initial period is maintained in quite long time. To analyze the soluble pollutant mixing in the initial period, previous researchers developed analytic solutions using statistical distributions, which cannot be employed in natural streams. In this research, a 2D dispersion model, named as PDM-2D (Particle Dispersion Model-2D) was developed based on the physical interpretation of the shear flow dispersion in the open channel. Since this model was not using the Fickian dispersion assumption, it is applicable both in the initial and in the Taylor period without inputting the dispersion coefficients. In PDM-2D, pollutant mixing was divided into the shear advection stage and the vertical diffusion stage. In the shear advection stage, pollutant particles were translated in longitudinal and transverse directions following velocity formulas, and then the transported particles were evenly distributed in vertical layer during each time step in the vertical diffusion stage. The end result is the scattering of pollutant particles in the longitudinal and transverse directions. PDM-2D was applied to the straight channel and the simulation results were compared with the Fickian dispersion model. Concentration curves from this study show skewed distribution in the initial period (Pe > 2.5) and turned into the symmetric distribution after the balance between shear advection and vertical diffusion was achieved (Pe < 1). Skewness was rapidly decreased in the intermediate period. From the simulation concentration distributions, dispersion coefficients were calculated using the routing method. In the advection-dominant mixing region, non-dimensional longitudinal dispersion coefficients were larger than 5.93, which was suggested by Elder (1959), and values were converged to Elder’s value in the diffusion-dominant mixing region. PDM-2D was also applied to the meandering channel, and simulation results were compared with the experimental data. The breakthrough curves were well fitted with the experimental results on the apex region.