Effects of emergent vegetation on lateral diffusion in wetlands

Constructed wetlands are widely used for a variety of environmental applications, such as wastewater treatment and recharge, and their efficacy is largely determined by the hydrodynamic characteristics of the flow system. An experimental study was carried out to quantify the lateral dispersion of passive substances in shallow zones of a constructed wetland wherein water flows though the interstices of the distributed vegetation. The experimental set up was designed to mimic the Tres Rios constructed wetland located in Phoenix, Arizona. The major emphasis was on the lateral diffusivity Kt of a shallow zone with randomly distributed vegetation. The results are presented in the context of a simple theoretical model where Kt is expressed in terms of the diameter of the plant stalk Dv, the characteristic distance between the plants dv, the flow velocity U and the drag coefficient CD as (Kt/UDv)(dv/Dv)=βCD, where β is a dimensionless constant. Fitting of data to the above model indicate that CD, in general, is a function of the Reynolds number (Re). The data are also compared with a model proposed by Nepf et al. (Water Res 35 (1999) 479).