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Bimodality of Hydraulic Residence Time in Conceptualized Wetland as a Consequence of Vegetation Roughness

Author(s): Tommaso Musner; Andrea Bottacin-Busolin; Andrea Marion

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Abstract: Constructed wetlands for waste water treatment are often designed on the basis of an average water residence time, but this can lead to significant inaccuracies in the prediction of the efficiency of contaminant removal. In order to perform a more accurate description of the residence times, a conceptual wetland characterized by a main flow channel with lateral vegetated transects is modeled here using a stationary two-dimensional depth-averaged hydrodynamic model coupled with a time dependent two-dimensional advection-dispersion equation. The overall effect of vegetation is described as a flow resistance represented in the hydrodynamic model as a prescribed roughness coefficient. Simulations are performed for a given flow discharge considering increasing values of the ratio between the roughness coefficient in the vegetated zones and in the main channel. Residence time distributions of a non-reactive tracer are derived from numerical simulations of the solute breakthrough curves associated to a continuous concentration input. Results show that increasing vegetation densities produce an increasingly pronounced bimodality of the RTDs. At longer times, the RTDs plotted in a log scale show a linear decrease with different slope as a function of the roughness ratio. The overall residence time distribution can be decomposed into the sum of two main components: a first component associated to the relatively fast transport in the main channel, and a second component associated to the slow transport within the vegetated transects.


Year: 2012

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