Effects of emergent and submerged natural vegetation on longitudinal mixing in open channel flow

An awareness of mixing processes is imperative in understanding the transport of pollutants in open channel flows, important for environmental impact studies. To date, controlled laboratory studies of the effects of vegetation on mixing processes have used simulated plants. This may neglect some of the important variables introduced by the presence of natural vegetation. In this study natural vegetation was planted within a laboratory channel, and a series of experiments quantifying velocity, turbulence, and longitudinal mixing were conducted over a time period sufficient to allow growth of the vegetation to impact on the mixing processes. In emergent conditions the results generally confirmed previous artificial vegetation and modeling studies, showing that vegetation reduces the magnitude of longitudinal shear dispersion. Additionally, measureable change in longitudinal mixing was observed primarily as a function of flow depth but also of plant age. Normalization using previously suggested parameter combinations failed to yield predictive trends. Submerged tests uniquely covered natural vegetation with a significant wake zone, and from this it was observed that longitudinal mixing is primarily a function of the degree of submergence. Overall, this paper presents a new data set quantifying the effects of natural vegetation on longitudinal mixing processes and illustrating deficiencies in previous understanding and predictive expressions based on idealized artificial vegetation. © 2010 by the American Geophysical Union.