Additional longitudinal displacement for contaminant dispersion in wetland flow

• An initial point release at the wetland surface is considered.
• The additional longitudinal displacement of the contaminant cloud is determined.
• The displacement is analytically incorporated for concentration distribution.
• The displacement can greatly alter the concentration distribution.

SummaryWhen there is a sudden and uniform release of contaminant over the depth of wetland, the centroid of resulted solute cloud will travel downstream at the cross-sectional mean velocity of the flow. However, if the initial release is not uniform, there will be an additional longitudinal displacement of the centroid, which is important for predicting the concentration distribution but cannot be revealed by the classical one-dimensional Taylor dispersion model. For the most typical case of an initial point source release at the free-water-surface of the wetland, an idealized case modeling accidental leakage of toxic chemicals in waters, in the present paper we analytically deduce the longitudinal displacement by the method of concentration moment. The result is then incorporated in the analytical solutions of concentration distribution, which are further verified by our numerical simulations. The effects of the longitudinal displacement on the concentration distribution are analyzed in detail. It is shown that without considering the displacement, for vertical planes close to the edges of the contaminant cloud, the analytical solution can over- or under-estimate the vertical distribution of concentration for over 20% of the maximum concentration in the plane even at a large dimensionless time of t∗=5t∗=5. The longitudinal displacement is shown to decrease with the increase of the important damping factor α, which characterizes the effects of vegetation in wetlands. A simple application is given at the end of this paper to illustrate the evolution of the additional longitudinal displacement.