Field and laboratory observations of small-scale dispersion in wetlands

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2004.Includes bibliographical references (p. 156-160).Estimating longitudinal dispersion in wetlands is a necessary first step in predicting the behavior of dissolved species and suspended particles. However, many processes are involved, and they can interact in nonlinear ways. Relevant processes include turbulent diffusion, which describes net solute flux created by turbulent eddies. Other dispersive processes result from the retardation of a portion of the solute relative to the rest of a cloud. This retardation can be provided by trapping in the vortex structure behind stems (hold-up dispersion), velocity deficits well downstream of stems (stem-wake dispersion), or transverse gradients in longitudinal velocity (shear dispersion). To better understand the relative magnitude of these various dispersive processes, measurements were taken of velocity, vertical diffusion, and longitudinal dispersion in both the laboratory and the field. Laboratory flume experiments were conducted using an emergent canopy of rigid cylinders with different cylinder densities over depth. Field experiments were conducted in a natural salt marsh. Drag due to local stem density was found to control horizontal velocity in both the lab and field studies over most of the depth. The resulting non-uniform velocity profile generated shear dispersion, which controlled dispersion at longer distances (> 250 cm) downstream of a slug release. For distances < 250 cm downstream, wake shear dispersion was found to be most important.by Anne F. Lightbody.S.M

The physical role of transverse deep zones in improving constructed treatment wetland performance

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 254-274).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.(cont.) 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.by Anne F. Lightbody.Ph.D