Solute transport in laminar channel flows with floating vegetation islands: Dispersion and vegetation absorption

In view of the concerns regarding contaminant transport and removal after water pollution incidents, this study focuses on the temporal and spatial evolutions of concentration distribution and removal performance in channel flows with floating vegetation islands (FVIs). For a typical case of instantaneous and uniform contaminant release, the analytical solution of the spatial concentration distribution is deduced based on the multi-scale perturbation method. With the combined effects of advection and vegetation absorption, a tremendous concentration gradient forms on cross-sections and cannot be smeared out until the contaminant is completely removed. Both the vertical and longitudinal concentration distributions are reshaped in different degrees due to the different intensities of vegetation absorption. The stronger the vegetation absorption is, the greater the vertical distribution of the mass proportion deviates from that for pure dispersion. By establishing the relationships of the removal rate and removal efficiency with the residence time, distance and absorption parameter based on the control volume analysis, the temporal and spatial evolutions of the removal performances with different absorption intensities are assessed quantitatively. The significantly reduced critical lengths and durations of influenced regions confirm the ecological effects of FVIs from the perspective of hydraulic research.