<p> The interaction of biological and physical processes has resulted in distinct morphological and biological zonation across the Subarctic macrotidal tidal flats at the head of Frobisher Bay. The tidal flats have been divided into six morphological zones which are closely related to the three biological communities found there. </p> <p> Faunal species of the Upper Flat inhabit the beach and fines flat morphological zones where ice action during breakup and freezeup has the greatest influence. Species inhabiting these zones are hardy, and freshwater tolerant. Many are highly motile and recolonize the area after ice breakup. </p> <p> The Middle Flat extends from 5.0 m ALLT to 2.2 m ALLT. It is inhabited by motile polychaetes at its upper end (bouldery flat >4.5 m ALLT). with more sedentary species appearing towards its lower end (very bouldery flat). Below 2.2 m ALLT, on the Lower graded flat, sedentary infauna such as Cyrtodaria kurriana, Mya truncata and sabellid polychaetes, dominate the substrate. </p> <p> The three major processes acting on the tidal flats are, in order of importance, tidal, ice and wave action. Exposure indices, generated from tidal data, reveal 2 critical tidal heights at around 4 m and 7.5 m ALLT, in Frobisher Bay. The boundary between motile and less motile fauna, and bouldery and very bouldery morphological zones, occurs around 4.0 m ALLT. Of the flora and fauna only Fucus evanescens is found beyond the 7.5 m ALLT limit. </p> / Thesis / Master of Science (MSc)

Thesis advisor: Noah P. Snyder / Salt marshes are hotspots for biodiversity, important carbon sinks, pollutant filtration systems, and buffers which strengthen coastal resiliency. Detailed geomorphic and sedimentary observations are vital to understanding the stability and sustainability of salt marshes in response to sediment placement restoration projects. Marsh stability is dependent on the suspended sediment and nutrients that are delivered to the marsh platform by meandering tidal channels. This study observed suspended-sediment fluxes at the mouth of Southeast Creek on Gull Island at Seven Mile Island Innovation Lab in coastal, southern New Jersey. The mesotidal, almost entirely low marsh, southern Gull Island received a recent dredge placement in fall 2020 of approximately 30,600 cubic meters of sediment from the adjacent New Jersey Intracoastal Waterway. Wetland instrumentation platforms to monitor post-dredge conditions were deployed summer 2021 and were removed in summer 2022. This instrumentation measured current velocity and depth time series at all platforms and suspended-sediment concentration using acoustic backscatter techniques at the mouth of Southeast Creek. This study is motivated by understanding the ongoing transport and deposition of dredged material placed on the marsh island. I found that velocities were ebb-dominant during all conditions. Concentrations on the flood phase of the tide were approximately half the magnitude of those seen on the ebb phases of the tide for normal circumstances while elevated concentrations were observed on ebbing and flooding currents for storms. A net export of 6.27 x 105 kg suspended sediment was observed through the tidal channel for all timescales throughout the deployment period. / Thesis (MS) — Boston College, 2023. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

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The primary motivation of this study is to analyze the 1D-2DH hydrodynamic model of the San Francisco Bay and Sacramento-San Joaquin Delta (SFBD) outlined in Nederhoff et al. (2021). I compared model water level data to 70 tidal records from the National Oceanic and Atmospheric Association (NOAA), the United States Geological Survey (USGS), the California Data Exchange Center (CDEC), and from local municipalities throughout the Bay Area to investigate how the model captures water levels and tidal constituent amplitudes. While the Nederhoff et al (2017) model analyzed an extended time period from 1950-2019, I analyzed M2 amplitude and tidal water levels for the water year of 2017 (WY2017) with a larger dataset that extended into the Sacramento-San Joaquin Delta. Because WY2017 was a high river flow year for the Sacramento Delta, the model was able to be evaluated throughout a large range of flow regimes. I used harmonic analysis through the MATLAB package UTide (Codiga et al. 2011) to assess the model’s ability to replicate M2 amplitudes. I assessed the error for these M2 values as well as for tidal water levels. The average RMSE for M2 amplitude is 0.111 m across the entire model domain during WY2017, performing fairly consistent throughout the model. The one exception being the shallow and complex Grizzly Bay, which performed significantly worse, with RMSE values around 0.5 m. The model better replicated water levels in the 2DH grid representation of the San Francisco Bay ( Attempts to improve the model were mostly unsuccessful. I tried to increase the grid resolution at the Carquinez Strait to improve tidal propagation upstream, but altering the grid caused the coupling between the 2DH grid and 1D network to detach. This prevented the propagation of water flow in either direction at the coupling near Collinsville. The software required to fix this coupling was non-standard and unavailable for my usage, so I was unable to resolve the issue. I also attempted to create a new wind forcing file using in-situ data rather than the ERA5 reanalysis. This new wind forcing made negligible difference in water level and M2 model skill. An experiment in removing river flow showed that riverine impacts on elevating extreme water levels only have effects (>0.05 m) east of the Carquinez Strait. Extreme water levels west of this point in the San Pablo, Central, and South Bays are dominated by tides, storm surge, and to a lesser extent local wind. A decrease in tidal amplitude by river flow potentially decreases flood risk in some parts of the Bay during times of high outflow from the Sacramento-San Joaquin Delta. I also investigated maximum equilibrium effects of constant wind in the two prevailing wind directions (southerly and westerly) of the San Francisco Bay. The wind setup effect become more prominent (>0.05 m) at and above a steady 10 m/s in both directions. This study also showed that wind likely exerts a small influence on tidal properties, especially for winds greater than 10 m/s.

Consistent shoreline development and urbanization have historically resulted in the loss of wetlands. However, some construction activities have inadvertently resulted in the emergence of new tidal wetlands, with prominent examples of such anthropogenic wetlands found within the Hudson River Estuary. Here, we utilize two of these human-induced tidal wetlands to explore the sedimentary and hydrologic conditions driving wetland development from a restoration perspective. Tivoli North Bay is an emergent freshwater tidal marsh, while Tivoli South Bay is an intertidal mudflat with vegetation restricted to the seasonal growth of aquatic vegetation during summer months. Using a combination of sediment traps, cores, and tidal flux measurements, we present highly resolved sediment budgets from two protected bays and parameterize trapping processes responsible for their divergent wetland evolution. Utilizing a 16-year tidal flux dataset, we observe net sediment trapping in Tivoli North for most years, with consistent trapping throughout the year. Conversely, sediment flux measurements at Tivoli South reveal net sediment loss over the study period, with trapping constrained to the summer months before being surpassed by large sediment exports in the fall and early spring. The timing of the transition from sediment import to export marks the end of the invasive water chestnut growing season and the onset of the associated exodus of both sediment and organic material from Tivoli South. When sediment cores collected for this study are compared to sediment cores collected in 1996, 137Cs profiles confirm little to no sediment accumulation in Tivoli South over the previous two decades. These results support the hypothesis that water chestnut is serving to inhibit sediment trapping and facilitate sediment erosion, preventing marsh development in Tivoli South. The longevity of this dataset highlights the capacity of aquatic vegetation to regulate sediment exchange and geomorphology in enclosed bays when provided an opportunity to colonize. Results of this project provide evidence to inform the management of restoration projects in river systems with freshwater tidal wetlands, especially those affected by invasive species of aquatic vegetation. In bays where tidal sediment supply is not limited, water chestnut removal may present a viable strategy to facilitate marsh restoration.

<p>Globular clusters have linear sizes (tidal radii) which are theoretically de- termined by their mass and by the gravitational potential of their host galaxy. However observationally, cluster sizes are simply a determination of where the cluster’s surface brightness profile becomes zero. This distance is also known as the limiting radius. While it is commonly assumed that the tidal radius and the limiting radius of a globular cluster are the same thing, it has yet to be validated. The purpose of this thesis is to explore the assumption that cluster tidal radii and limiting radii are equal by comparing the tidal radii of an observed and simulated globular cluster population.</p> <p>An established link between cluster tidal radii and limiting radii will yield new methods of utilizing globular clusters as tools for studying galaxies. If cluster sizes are truly imposed by the tidal field of the host galaxy, then tidal radii measurements can be used to trace the mass distribution within a galaxy, including the dark matter halo. Additionally, as we will demonstrate in this thesis, cluster sizes can also be used a tracer for the orbital anisotropy profile of a galaxy.</p> <p>To explore the assumption that tidal radii and limiting radii are equal, we utilize the globular cluster population of the Virgo giant M87. Unusually deep, high signal-to-noise images of M87 are used to determine the radius for approximately 2000 globular clusters. To compare with these observations, we simulate a globular cluster population that has the same characteristics to the observed M87 cluster population. These characteristics include cluster radial distribution, mass distribution, central concentration distribution and line of sight velocity dispersion. Placing these simulated clusters in the well-studied tidal field of M87, the orbit of each cluster is solved and the theoretical tidal radius of each cluster is determined. We compare the predicted relationship between cluster size and projected galactocentric distance found in our sim- ulation to observations in order to test whether a cluster’s tidal radius and limiting radius are equal. We find that for an isotropic distribution of cluster velocities, theoretical tidal radii are approximately equal to observed limiting radii. The simulation predicts the observed increase in cluster size with galac- tocentric distance, which is expected if tidal radii are dependent on the tidal field. Additionally, simulated cluster sizes are of the same order of magnitude as observed cluster sizes. However the simulation does underestimate cluster sizes in the inner regions of M87. To minimize the discrepancy between theory and observations, we further explore the effects of orbital anisotropy on cluster sizes, and suggest a possible orbital anisotropy profile for M87 which yields the best fit between theory and observations. Finally, we suggest multiple future studies which will aid in our understanding of tidal theory and in establishing a stronger link between tidal radii and limiting radii.</p> / Master of Science (MSc)

The predictive capabilities of blade-resolved unsteady Reynolds averaged Navier-Stokes (URANS) and detached eddy simulation (DES), the most commonly used hybrid RANS/large eddy simulation (LES) model, are assessed for hydrokinetic turbine performance and mean and turbulent flows in the intermediate-wake region, and results for a range of tip-speed ratio encompassing design and off-design conditions are analyzed to understand the wake recovery mechanism. The performance predictions compared within 5% of the experimental data. Both URANS and DES models performed reasonably well for the near wake predictions, where the errors were < 15%. DES outperformed URANS for both mean wake deficit and turbulence predictions in the intermediate-wake region and both quantities compared within 10% of the experiments. The improved prediction by DES is because of 1) its ability to predict the tip vortex breakdown, which plays a critical role in the wake recovery, especially for higher tip speed ratios; 2) the presence of the free-surface which created an upper bypass region of accelerated flow. The study reveals that the tip vortex breakdown mechanism depends on tip speed ratio. For lower values of tip speed ratio, instabilities generated in the root vortex core are identified to be the cause of breakdown. For higher values, the breakdown occurred because of the instabilities generated during the vortex filament entanglement. The presence of the free-surface led to an early vortex breakdown and the interaction between the wake and free-surface is initiated by the interaction of stanchion with the free-surface. Future work should focus on investigation of other hybrid RANS/LES models to address the limitations of the DES models, and extension of the study to include wave effects.

The isotopic composition of the organic components of an animal's body, with respect to carbon and nitrogen, reflect the weighted average of the isotopic compositions of the animal's food sources, with a certain degree of enrichment in the heavier isotopes. Thus, by comparing the isotopic compositions of the animal and all the potential food sources, it is possible to ascertain the relative proportions of each availible food source in its diet, if the various food sources are sufficiently isotopically distinct. This approach is particularly usefull in estuarine communities where food-webs tend to be complex and where there are several sources of primary productivity. In this study it was used on two types of claw in an arctic and a temperate estuarine tidal flat in order to assess the importance of terrestrially fixed organic matter to each community. The results indicated that while marine and terrestrial organics were important food sources in the arctic tidal flat, the clams in the temperate site depended mostly on marine organics. / Thesis / Bachelor of Science (BSc)