Theoretical analysis of pollutant mixing zone considering lateral distribution of flow velocity and diffusion coefficient.

Theoretical formulae have shown significant advantages in describing the characteristic geometric scales of the pollutant mixing zone (PMZ) formed by offshore pollutant discharged by a single general form. They, however, fail to predict the influence of the lateral inhomogeneity of the river flow because constant flow velocity and the lateral diffusion coefficient are assumed during the derivation. The realistic flow velocity in a river is fitted by an exponential law in this study and the lateral diffusion coefficient is proposed to have the same form. Similar idea has been used in previous studies on the vertical dispersion of scalar in the lower atmosphere. Pollutant discharged from a steady onshore point source into a wide straight open channel is examined to characterize the concentration taking into consideration of these lateral variations. Theoretical formulae describing the maximum length, maximum width and its corresponding longitudinal position, as well as the area of the PMZ are derived. A non-dimensional standard curve equation for the isoconcentration boundary of PMZ is also obtained. The results show that the shape of the dimensionless standard curve of PMZ depends only on the exponential constants in the exponential laws. The exponential profiles that fit the near-shore velocity give good prediction, while the ones that match the entire lateral range up to the center of the river underpredict the PMZ significantly. These findings are of great importance for practitioners to characterize the geometry of the PMZ in rivers and for water quality modeling.

Calculation method for steady-state pollutant concentration in mixing zones considering variable lateral diffusion coefficient.

Prediction of the pollutant mixing zone (PMZ) near the discharge outfall in Huangshaxi shows large error when using the methods based on the constant lateral diffusion assumption. The discrepancy is due to the lack of consideration of the diffusion coefficient variation. The variable lateral diffusion coefficient is proposed to be a function of the longitudinal distance from the outfall. Analytical solution of the two-dimensional advection-diffusion equation of a pollutant is derived and discussed. Formulas to characterize the geometry of the PMZ are derived based on this solution, and a standard curve describing the boundary of the PMZ is obtained by proper choices of the normalization scales. The change of PMZ topology due to the variable diffusion coefficient is then discussed using these formulas. The criterion of assuming the lateral diffusion coefficient to be constant without large error in PMZ geometry is found. It is also demonstrated how to use these analytical formulas in the inverse problems including estimating the lateral diffusion coefficient in rivers by convenient measurements, and determining the maximum allowable discharge load based on the limitations of the geometrical scales of the PMZ. Finally, applications of the obtained formulas to onsite PMZ measurements in Huangshaxi present excellent agreement.

A new method of microskin autografting with a Vaseline-based moisture dressing on granulation tissue.

In the conventional method of microskin autografting, aggressive early excision is adopted, followed by coverage with a microskin-allograft complex to close extensive burn wounds. However, early excision is always associated with a defect of viable tissue, resulting in massive blood loss and causing high risk to aged patients or those with other systemic diseases. We developed a new method in which an eschar thinning operation was first adopted, followed by raising granulation tissue and microskin autografting, which was covered by a Vaseline-based moisture dressing. A total of 52 patients were included in this study and randomly assigned to the control group (n=26) and the experimental group (n=26) for the conventional method and the new method, respectively. The re-epithelisation rate on the 21st day after autografting indicated that there was no significant difference between both groups. There was also no significant difference between the two groups when the re-epithelialisation rate was compared with the type of organisms cultured. However, the Vancouver Burn Skin Score (VBSS) results demonstrated a significant improvement of cosmetic appearance in the experimental group (score=2.1) as compared to the control group (score=3.9). The new method also showed other advantages, including less blood loss, shorter surgical duration and lower cost of surgery. From this prospective study, it can be concluded that the new method can be an alternative to the conventional microskin autografting procedure.