Graduate Thesis Or Dissertation
 

Engineering Services of Submerged Aquatic Vegetation (SAV): Investigation of the Land-Building and Energy-Attenuating Properties of SAV Habitats Across Spatial Scales

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/r494vs997

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  • Submerged aquatic vegetation (SAV) species, such as seagrasses, are highly valued in estuaries because of the many economic, ecological, and cultural services that they provide, including shelter for fisheries, minimizing water turbidity, and improving am-bient water quality. SAV can also alter its physical environment by attenuating wave and current velocities, changing the magnitude and/or direction of sediment trans-port, and reducing fluid stress on erodible shorelines. The energy attenuating and sediment accreting properties of SAV are commonly referred to as their engineering services, as they can act as a first line of defense against coastal flooding and erosion hazards. This thesis is comprised of three manuscripts that investigate engineering services of SAV in combined wave and current environments, particularly for the case of spatially discontinuous habitats. Throughout the work, the abbreviation “SAV” is used to concisely refer to two similar species of seagrasses commonly found in the United States, Zostera marina and Zostera japonica. Collectively, the work addresses critical knowledge gaps related to the design of habitat restoration at various stages. In the first manuscript, we modify a regional scale depth-averaged numerical model of waves and circulation (ADCIRC+SWAN) to quantify the control of discontinuous SAV habitats on tidal and circulation processes in the Coos Bay Estuary (Oregon, United States). To answer our research question, we develop a dynamic friction routine to improve the ability of ADCIRC+SWAN to capture time-varying flow attenuation by flexible SAV species, which deform under wave and current forces. Using our dynamic friction routine, we show that neglecting SAV deflection leads to exces-sive attenuation of flow velocities within canopies and enhanced flow velocities in the unvegetated areas adjacent to the canopies, increasing the tendency for sediment transport. The novel dynamic friction routine can be easily adapted into other re-gional scale numerical models to improve prediction of the larger-scale hydrodynamic impacts of discontinuous SAV habitats in other settings. In the second manuscript, we present the results of a full-scale laboratory experiment investigating the sediment accreting capability of finite SAV patches in oscillatory flow. The results of this experiment show that even when flexible SAV mimics are deflected by waves to less than one-third their upright length, depositional mounds shoreward of the patch form with lengths that scale with patch diameter. Further-more, results indicate that provided a minimal stem density is used, larger diameter, relatively sparse SAV patches lead to larger depositional mounds than smaller, very dense patches. We also provide evidence that the depositional mounds are generated by the suspension and transport of sediment within the canopy by between-stem turbulence and canopy streaming; thus, predicting sediment transport around and within finite SAV patches requires inclusion of these two mechanisms. Finally, the third manuscript uses a depth-resolving regional scale numerical model of SAV, hydrodynamics, and sediment transport interactions (COAWST) to quantify the influence of SAV habitat design parameters on key restoration outcomes. In doing so, we integrate the foci and expand the parameter space explored in the two previous manuscripts. Results indicate that habitat depth and cross-shore length have a leading order influence on the modification of wave and current velocities over the habitat. Although findings suggest that habitat shading depends more on water depth (a function of cross-shore habitat extent) than suspended sediment turbidity, incorporating the SAV-induced transport mechanisms identified in the second manuscript into the COAWST modeling framework could improve prediction of within-patch turbidity, as well as prediction of longer term morphological impacts of proposed restorations. Collectively, the body of work reiterates the necessity of SAV habitat design to match the scale of target outcomes, and it provides numerical tools and frameworks to support scale-appropriate SAV restoration siting and design. In conjunction with recent advances in understanding SAV biogeochemical responses to changing climates, the outcomes of this dissertation can be used to protect the long-term survival of SAV habitats and the myriad of services they provide to all inhabitants of coastal estuaries.
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