Wargula Anna E.

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Wargula
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Anna E.
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  • Article
    Wave-driven along-channel subtidal flows in a well-mixed ocean inlet
    (John Wiley & Sons, 2014-05-20) Wargula, Anna E. ; Raubenheimer, Britt ; Elgar, Steve
    Observations of waves, flows, and water levels collected for a month in and near a long, narrow, shallow (∼ 3000 m long, 1000 m wide, and 5 m deep), well-mixed ocean inlet are used to evaluate the subtidal (periods > 30 h) along-inlet momentum balance. Maximum tidal flows in the inlet were about 1.5 m/s and offshore significant wave heights ranged from about 0.5 to 2.5 m. The dominant terms in the local (across the km-wide ebb shoal) along-inlet momentum balance are the along-inlet pressure gradient, the bottom stress, and the wave radiation-stress gradient. Estimated nonlinear advective acceleration terms roughly balance in the channel. Onshore radiation-stress gradients owing to breaking waves enhance the flood flows into the inlet, especially during storms.
  • Article
    Curvature‐ and wind‐driven cross‐channel flows at an unstratified tidal bend
    (John Wiley & Sons, 2018-04-19) Wargula, Anna E. ; Raubenheimer, Britt ; Elgar, Steve
    Observations of currents, water levels, winds, and bathymetry collected for a month at an unstratified, narrow (150 m), shallow (8 m), 90° tidal inlet bend are used to evaluate an analytical model for curvature‐driven flow and the effects of local wind on the cross‐channel circulation. Along‐channel flows ranged from −1.0 to 1.4 m/s (positive is inland), and the magnitudes of cross‐channel flows were roughly 0.1–0.2 m/s near the outer bank of the bend. Cross‐channel observations suggest the lateral sea‐surface gradients and along‐channel flows are tidally asymmetric and spatially variable. The depth‐averaged along‐channel dynamics are consistent with a balance between the surface tilt and centrifugal acceleration. The vertical structure and magnitude of cross‐channel flows during weak winds are consistent with a one‐dimensional depth‐varying balance between centrifugal acceleration, bottom stress, and diffusion. Low‐passed (to remove tides) surface and bottom cross‐channel flows are correlated (r2 = 0.5–0.7) with cross‐channel wind velocity, suggesting that winds can enhance or degrade the local‐curvature‐induced, two‐layer flow and can drive three‐layer flow. The observed flow response to the wind is larger than that expected from a one‐dimensional balance, suggesting that two‐dimensional and three‐dimensional processes may be important.
  • Article
    Tidal flow asymmetry owing to inertia and waves on an unstratified, shallow ebb shoal
    (John Wiley & Sons, 2018-09-22) Wargula, Anna E. ; Raubenheimer, Britt ; Elgar, Steve ; Chen, Jia-Lin ; Shi, Fengyan ; Traykovski, Peter A.
    Observations of water levels, waves, currents, and bathymetry collected for a month at an unstratified tidal inlet with a shallow (1 to 2 m deep) ebb shoal are used to evaluate the asymmetry in flows and dynamics owing to inertia and waves. Along‐channel currents ranged from −1.5 to 0.6 m/s (positive inland) inside the main (3 to 5 m deep) channel crossing the ebb shoal. Net discharge is negligible, and ebb dominance of the channel flows is owing to inflow and outflow asymmetries near the inlet mouth. Offshore wave heights ranged from 0.5 to 2.5 m. During moderate to large wave events (offshore significant wave heights >1.2 m), wave forcing enhanced onshore mass flux near the shoal edge and inside the inlet, leading to reduced ebb flow dominance. Momentum balances estimated with the water depths, currents, and waves simulated with a quasi 3‐D numerical model reproduce the momentum balances estimated from the observations reasonably well. Both observations and simulations suggest that ebb‐dominant bottom stresses are balanced by the ebb‐dominant pressure gradient and the tidally asymmetric inertia, which is a sink (source) of momentum on flood (ebb). Simulations with and without waves suggest that waves drive local and nonlocal changes in the water levels and flows. Specifically, breaking waves at the offshore edge of the ebb shoal induce setup and partially block the ebb jet (local effects), which leads to a more onshore‐directed mass flux, changes to the advection across the ebb shoal, and increased water levels inside the inlet mouth (nonlocal effects).
  • Thesis
    Wave-, wind-, and tide-driven circulation at a well-mixed ocean inlet
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2017-06) Wargula, Anna E.
    The effects of waves, wind, and bathymetry on tidal and subtidal hydrodynamics at unstratified, shallow New River Inlet, NC, are evaluated using field observations and numerical simulations. Tidal flows are ebb-dominated (-1.5 to 0.6 m/s, positive is inland) inside the main (2 to 5 m deep) channel on the (1 to 2 m deep) ebb shoal, owing to inflow and outflow asymmetry at the inlet mouth. Ebb-dominance of the flows is reduced during large waves (> 1 m) owing to breaking-induced onshore momentum flux. Shoaling and breaking of large waves cause depression (setdown, offshore of the ebb shoal) and super-elevation (setup, on the shoal and in the inlet) of the mean water levels, resulting in changes to the cross-shoal pressure gradient, which can weaken onshore flows. At a 90-degree bend 800-m inland of the inlet mouth, centrifugal acceleration owing to curvature drives two-layered cross-channel flows (0.1 to 0.2 m/s) with surface flows going away from and bottom flows going toward the bend. The depthaveraged dynamics are tidally asymmetric. Subtidal cross-channel flows are correlated (r2 > 0.5) with cross-channel wind speed, suggesting that winds are enhancing and degrading the localcurvature- induced two-layer flow, and driving three-layer flow.
  • Article
    Upward swimming of competent oyster larvae Crassostrea virginica persists in highly turbulent flow as detected by PIV flow subtraction
    (Inter-Research, 2013-08-15) Wheeler, Jeanette D. ; Helfrich, Karl R. ; Anderson, Erik J. ; McGann, B. ; Staats, P. ; Wargula, Anna E. ; Wilt, K. ; Mullineaux, Lauren S.
    Investigating settlement responses in the transitory period between planktonic and benthic stages of invertebrates helps shape our understanding of larval dispersal and supply, as well as early adult survival. Turbulence is a physical cue that has been shown to induce sinking and potentially settlement responses in mollusc larvae. In this study, we determined the effect of turbulence on vertical swimming velocity and diving responses in competent eastern oyster larvae Crassostrea virginica. We quantified the behavioural responses of larvae in a moving flow field by measuring and analyzing larval velocities in a relative framework (where local flow is subtracted away, isolating the behavioural component) in contrast to the more common absolute framework (in which behaviour and advection by the flow are conflated). We achieved this separation by simultaneously and separately tracking individuals and measuring the flow field around them using particle image velocimetry in a grid-stirred turbulence tank. Contrary to our expectations, larvae swam upward even in highly turbulent flow, and the dive response became less frequent. These observations suggest that oyster larvae are stronger swimmers than previously expected and provide evidence that turbulence alone may not always be a sufficient cue for settlement out of the water column. Furthermore, at a population level, absolute velocity distributions differed significantly from isolated larval swimming velocities, a result that held over increasing turbulence levels. The absolute velocity distributions indicated a strong downward swimming or sinking response at high turbulence levels, but this observation was in fact due to downwelling mean flows in the tank within the imaging area. Our results suggest that reliable characterization of larval behaviour in turbulent conditions requires the subtraction of local flow at an individual level, imposing the technical constraint of simultaneous flow and behavioural observations.