Browsing by Author "Williams, Megan E."

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    Biological-physical interactions are fundamental to understanding and managing coastal dynamics
    (2023) Solan, Martin; Spencer, Tom; Paterson, David M.; Unsworth, Christopher A.; Christie, Elizabeth K.; Blight, Andrew J.; Brown, Jenny; Brooks, Helen; Lichtman, I. Dougal; Wei, Xiaoyan; Li, Xiaorong; Thorne, Pete; Leyland, Julian; Godbold, Jasmin A.; Thompson, Charlie; Williams, Megan E.; Plater, Andrew; Moller, Iris; Amoudry, Laurent O.
    There is an urgent need to address coastal dynamics as a fundamental interaction between physical and biological processes, particularly when trying to predict future biological-physical linkages under anticipated changes in environmental forcing. More integrated modelling, support for observational networks and the use of management interventions as controlled experimental exercises should now be vigorously pursued.
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    Flow characterization and turbulence in the eastern section of the Strait of Magellan, Southern Chile
    (2024) Suarez, Leandro; Guerra, Maricarmen; Williams, Megan E.; Escauriaza, Cristian; Lozovatsky, Iossif; Coppersmith, Ronald; Fernando, Harindra Joseph S.
    The Strait of Magellan connects the Pacific and Atlantic oceans in South America's southern region, and it has been recognized for centuries as an important transoceanic navigation route as well as a unique marine environment with a rich ecological diversity. Evaluations of the impact of human activities in the channel and multiple potential future developments require abetter understanding of the physical environment to design sustainable strategies aimed at preserving these characteristics. In this investigation, we study the flow near the Atlantic inlet of the Strait where the dynamics is characterized by the interactions of the tide propagation within two narrows, which are the predominant features of the channel morphology. Tides amplified by the Patagonian shelf generate strong currents through these narrows and control the exchange between the Atlantic and central regions of the Strait. We employ bottom-mounted and vessel-mounted Acoustic Doppler Current Profilers (ADCPs) with tide gauges to analyze the mean flow, tidal propagation, and turbulence, complementing the data with previous available measurements. The analysis reveals residual flows directed toward ebb flow at the channel center and flood near the edges, showing a significant spring-neap variation. Turbulence statistics in the second narrows exhibit a significant variability between ebb and flood, with a balance between production and dissipation observed only during ebb phases.
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    Observations of river-wave interactions at a small-scale river mouth
    (2024) Roco, Alonso; Flores, Raul P.; Williams, Megan E.; Saldias, Gonzalo S.
    Small coastal rivers often discharge directly into the surfzone, where the fate of freshwater and river-borne materials (e.g., sediment, nutrients and contaminants) is primarily determined by interactions between buoyant plume processes and the nearshore wave forcing. Understanding river-wave interactions in the surfzone is essential to assess coastal water quality and impacts on ecosystem health along adjacent shorelines. These interactions are modulated by the variability in river discharge, offshore wave climate, tidal forcing, and surfzone dynamics, and therefore occur over a wide range of timescales. However, the different timescales at which river-wave interactions occur have not been fully investigated. Here we use in-situ and remote (drone-based) observations collected at the mouth of the Maipo River, a small-scale river system in central Chile, to investigate interactions between the river outflow and the incident wave forcing that may influence the distribution of freshwater along the coast. We focus on interactions occurring at infragravity (wave groups), tidal, and synoptic (offshore wave climate) timescales. The observations included inlet and surfzone deployments, and documented low river discharge conditions (QR similar to 20-27 m3s-1) with variable wave forcing (Hs similar to 1-3 m). We observed that the salinity signal at the river mouth is strongly modulated by the tide, with a freshwater plume forming solely during the ebb. Large oscillations associated to infragravity (IG) motions are continuously observed on top of the tidal variability. These oscillations are evidenced in surfzone salinity, inlet water levels, and inlet velocity during the early ebb when the freshwater plume starts to develop. As the plume evolves into a strong outflow jet during the late ebb, they are not observed in the inlet and become restricted to the surfzone. The combined analysis of drone imagery and in-situ observations suggests that the observed variability in surfzone salinity at IG frequencies is associated with the arrival of wave groups and the propagation of wave fronts in the plume area. In particular, during the late ebb, the large oscillations in surfzone salinity (amplitude of 15-20 psu) are explained by a contraction and expansion of the outflow plume in response to the variability in the onshore wave forcing at the timescale of wave groups. On synoptic timescales (hours to days), the surfzone salinity responded to the intensity of the offshore wave forcing (similar to 20 m depth), with higher waves promoting lower salinity as freshwater is mostly retained near the coast. A wave-river momentum comparison confirms that the wave forcing had the potential to trap freshwater in the surfzone. This study shows that nearshore waves may influence the dynamics of small river plumes and the fate of freshwater (and associated terrestrial materials) on a wide range of timescales, from infragravity (1-5 min) to hours and days.