The Physical Role of Transverse Deep Zones in Improving Constructed Treatment Wetland Performance

Velocity heterogeneity is often present in wetland systems and results in some influent water remaining in the wetland for less than the expected residence time. This phenomenon, known as short-circuiting, alters the distribution of the chemical and biological transformations that occur within the wetland and decreases performance in constructed treatment wetlands. In this thesis, field observations, experiments in a laboratory physical model, and mathematical modeling are used to explore the ability of transverse deep zones to mitigate the negative effect of short-circuiting on constructed wetland performance. Field observations were used to quantify short-circuiting in a 360-acre constructed treatment wetland in Augusta, Georgia. In each of the three marsh sections examined, between three and six narrow flowpaths were found that together carried 20–70% of the flow at a velocity at least ten times faster than the rest of the marsh. One known method for offsetting the deleterious effect of short-circuiting flowpaths is to include several transverse deep zones within each wetland cell. To study the physical mechanisms behind this proposed strategy, laser-induced fluorescence (LIF) was used within a laboratory scale model of a short-circuiting wetland with a transverse deep zone. Water exiting a fast flowpath formed a jet that initially entrained co-flowing fluid and spread laterally but then, due to the drag present within the system, reached a final width that depended on the width of the upstream flowpath. Finally, the understanding of flow patterns gained by the field and laboratory experiments were combined into an analytical streamtube model. Modeled results revealed that a transverse deep zone can offset the adverse impact of short-circuiting flowpaths through two separate mechanisms. When lateral mixing is present within the deep zone, it dilutes the water that has traveled through the fast flowpath. In addition, deep zones likely reduce the probability that fast flowpaths will align throughout the entire wetland, which increases the probability that all water will receive some treatment even when no lateral mixing is present within the deep zones. These results indicate that deep zones may improve performance when properly sized and located within a constructed treatment wetland. Thesis Supervisor: Heidi M. Nepf Title: Professor of Civil and Environmental Engineering