A hydrodynamic characterization of saltmarsh ecosystems
Young, David Louis
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In the studies presented in this thesis, we examine hydrodynamics of saltmarsh ecosystems at two spatial scales. First is an assessment of tidally-driven surface flow through the saltmarsh. We find substantial water level differences between the estuary channel and the neighboring saltmarsh and correspondingly large pressure gradients, which strongly affect the flow in the marsh. At one marsh location, where the flow is unrestricted by bathymetry, the pressure gradient governs the flow through the marsh vegetation. At this location, the flow is effectively modeled as a balance between the pressure gradient force and the drag force due to marsh vegetation and bottom stress using the Darcy-Weisbach/Lindner's equations. Second is an examination of the concentration field and mixing of meandering turbulent plumes, commonplace in natural settings and engineering applications. Much is known about the concentration field and turbulent mixing in straight plumes, yet little of the meandering plume dynamics is understood - partially due to the difficulty in separating the plume meander from the turbulent fluctuations. To address this, we acquired simultaneous laser induced fluorescence (LIF) and particle image/tracking velocimetry (PIV/PTV) data of a phase-locked meandering plume as well as LIF for a straight plume for comparison. Analysis of the LIF data reveals that, compared to the straight plume, the centerline concentration of the meandering plume decreases more rapidly with distance downstream, and the plume width increases more rapidly with distance downstream, resulting in more rapid dilution of tracer concentration. The PIV velocity analysis indicates the large-scale alternating vortices induced by the diverting plate are the dominant feature of the meandering plume, forcing the plume meander and governing the spatial variation of the mean concentration and turbulence characteristics. Comparison of the turbulent flux measurements with the mean concentration field indicates that the eddy-diffusion hypothesis effectively models the turbulent flux in the plume. As turbulence plays a key role in both flow through vegetation and the dynamics of turbulent plumes, the ability to describe and quantify turbulence is a key aspect of any hydrodynamic analysis in saltmarsh ecosystems. Unfortunately for field studies in estuarine and coastal settings, calculating turbulence characteristics, such as the Reynolds stress, TKE, and turbulence intensity, is complicated by the orbital velocity of surface waves remaining after subtraction of the mean velocity. In the final portion of this thesis, a new single-instrument method for removing wave bias from Reynolds stress estimates is proposed. This method is compared to two frequency-domain-based single-instrument techniques and a two-instrument method that uses a linear-filtration procedure of data collected from an adjacent instrument to remove the wave signal. The method proposed in this study offers superior performance over the other single-instrument techniques in shallow water, comparing favorably with the two-instrument method and thereby offering a more financially and logistically efficient means of removing wave bias.