Effect of bed absorption on contaminant transport in wetland channel with rectangular cross-section

Abstract

Predicting the evolution of concentration distribution for contaminant transport in wetland flows has a wide range of ecological applications. To account for the ecological effect such as substrate absorption, the influence of bed absorption on the dispersion process of contaminant transport in a wetland channel with rectangular cross-section is analytically investigated in this study by the technique of long-time asymptotic expansion. The combined effects of bed absorption, free-water-surface-effect and the bank-sidewall-effect are demonstrated, by parameter analysis in terms of the bed absorption coefficient and the aspect ratio of the channel section. The studied cases illustrate that bed absorption decreases the total mass of the contaminant cloud retained in the flow and causes the contaminant cloud to concentrate more in the upper layer. The mean advection velocity of the contaminant cloud increases as the intensity of bed absorption amplifies. Environmental dispersivity is augmented with bed absorption in a deep wetland channel, which is inverse in a shallow channel. The effect of bed absorption weakens the positive skewness of the cross-sectional mean concentration distribution when the wetland channel approaches square, while strengthens the negative skewness of the cross-sectional mean concentration distribution when the channel is relatively shallow. Moreover, the cross-sectional mean concentration decays faster under the effect of bed absorption. For applications, the critical length and duration of the influenced region are presented with bed absorption.