Physical Sciences and Mathematics Commons^™

Controls On Floc Size In A Continental Shelf Bottom Boundary Layer, Paul Hill, George Voulgaris, John Trowbridge

George Voulgaris

Simultaneous in situ observations of floc size, waves, and currents in a continental shelf bottom boundary layer do not support generally accepted functional relationships between turbulence and floc size in the sea. In September and October 1996 and January 1997, two tripods were deployed in 70 m of water on the continental shelf south of Woods Hole, Massachusetts. On one a camera photographed particles in suspension 1.2 m above the bottom that had equivalent circular diameters larger than 250 um, and on the other, three horizontally displaced acoustic current meters measured flow velocity 0.35 m above the bottom. The tripods …

Cross-Shore Variation Of Wind-Driven Flows On The Inner Shelf In Long Bay, South Carolina, United States, Benjamin Gutierrez, George Voulgaris, Paul Work

George Voulgaris

The cross-shore structure of subtidal flows on the inner shelf (7 to 12 m water depth) of Long Bay, South Carolina, a concave-shaped bay, is examined through the analysis of nearly 80 days of near-bed (1.7–2.2 m above bottom) current observations acquired during the spring and fall of 2001. In the spring and under northeastward winds (upwelling favorable) a two-layered flow was observed at depths greater than 10 m, while closer to the shore the currents were aligned with the wind. The two-layered flow is attributed to the presence of stratification, which has been observed under similar conditions in the …

Evaluation Of The Acoustic Doppler Velocimeter (Adv) For Turbulence Measurements, George Voulgaris, John Trowbridge

George Voulgaris

Accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in this paper. Simultaneous measurements of open-channel flow were undertaken in a 17-m flume using an ADV and a laser Doppler velocimeter. Flow velocity records obtained by both instruments are used for estimating the true (‘‘ground truth’’) flow characteristics and the noise variances encountered during the experimental runs. The measured values are compared with estimates of the true flow characteristics and values of variance (^u92&, ^w92&) and covariance (^u9w9&) predicted by semiempirical models for open-channel flow. The analysis showed that the ADV sensor can measure mean velocity and Reynolds stress within …

Tidal Asymmetry And Residual Circulation Over Linear Sandbanks And Their Implication On Sediment Transport: A Process-Oriented Numerical Study, Rosario Sanay, George Voulgaris, John Warner

George Voulgaris

A series of process-oriented numerical simulations is carried out in order to evaluate the relative role of locally generated residual flow and overtides on net sediment transport over linear sandbanks. The idealized bathymetry and forcing are similar to those present in the Norfolk Sandbanks, North Sea. The importance of bottom drag parameterization and bank orientation with respect to the ambient flow is examined in terms of residual flow and overtide generation, and subsequent sediment transport implications are discussed. The results show that although the magnitudes of residual flow and overtides are sensitive to bottom roughness parameterization and bank orientation, the …

Shelf Edge Tide Correlated Eddies Along The Southeastern United States, Dana Savidge, Jonathan Norman, Colton Smith, Julie Amft, Trent Moore, Catherine Edwards, George Voulgaris

George Voulgaris

High frequency radar observations in the Southeastern United States have revealed sequences of small short‐lived cyclonic eddies along the shoreward edge of the Gulf Stream, that spin up as the local tide turns alongshelf antiparallel to the Stream. Eddies propagate equatorward along the shelf edge, sometimes progressing shoreward before dissipating one to three hours later. They are distinctly different from Gulf Stream meander eddies, which propagate poleward. In this article, radar and mooring data are used to establish three important aspects of these neweddies: they represent an instability process operating at a previously unidentified frequency, scale, and cross‐Stream position; they …

Wind Speed Dependence Of Single-Site Wave-Height Retrievals From High-Frequency Radars, Brian Haus, Lynn Shay, Paul Work, George Voulgaris, Rafael Ramos, Jorge Martinez-Pedraja

George Voulgaris

Wave-height observations derived from single-site high-frequency (HF) radar backscattered Doppler spectra are generally recognized to be less accurate than overlapping radar techniques but can provide significantly larger sampling regions. The larger available wave-sampling region may have important implications for observing system design. Comparison of HF radar–derived wave heights with acoustic Doppler profiler and buoy data revealed that the scale separation between the Bragg scattering waves and the peak energy-containing waves may contribute to errors in the single-site estimates in light-to-moderate winds. A wave-height correction factor was developed that explicitly considers this scale separation and eliminates the trend of increasing errors …

Subtidal Inner Shelf Currents Off Cartagena De Indias, Caribbean Coast Of Colombia, Mauro Maza, George Voulgaris, Bulusu Subrahmanyam

George Voulgaris

Seasonal trends in inner shelf subtidal circulation off the coast of Cartagena de Indias, Colombia, are examined through the analysis of current profiles, hydrographic, meteorological and satellite data collected from 1999 to 2002. During the dry season (December–April) the water column is well-mixed and along-shelf currents flow southwestward following the steady trade winds. In the rainy season (May –November) the water column experiences continuous events of stratification and the along-shelf currents flow northeastward, opposing the weak southwestward winds. In the dry season the along shelfcirculation is mostly driven by wind forcing, while in the rainy season, the circulation is set …

Radon As An Indicator Of Limited Cross-Shelf Mixing Of Submarine Groundwater Discharge Along An Open Ocean Beach In The South Atlantic Bight During Observed Hypoxia, Clayton Mccoy, Richard Viso, Richard Peterson, Susan Libes, Brent Lewis, John Ledoux, George Voulgaris, Erik Smith, Denise Sanger

George Voulgaris

Amongst other ocean state parameters, the development of a wave measurement program was supported as part of the Southeast U.S. Atlantic Coastal Ocean Observing System (SEACOOS). The program focused on supporting nearshore wave measurements using both cabled and autonomous systems but also examined the feasibility of using HF Radar systems for remote estimation of wave parameters. The nearshore stations have provided a significant database on directional wave climate for a number of nearshore locations in the region that provide valuable information to coastal engineers and managers for sustainable development along the coast of the southeastern United States. The ability of …

Wind-Speed Inversion From Hf Radar First-Order Backscatter Signal, W Shen, Klaus-Werner Gurgel, George Voulgaris, Thomas Schlick, D Stammer

George Voulgaris

Land-based high-frequency (HF) radars have the unique capability of continuously monitoring ocean surface environments at ranges up to 200 km off the coast. They provide reliable data on ocean surface currents and under slightly stricter conditions can also give information on ocean waves. Although extraction of wind direction is possible, estimation of wind speed poses a challenge. Existing methods estimate wind speed indirectly from the radar derived ocean wave spectrum, which is estimated from the secondorder sidebands of the radar Doppler spectrum. The latter is extracted at shorter ranges compared with the first-order signal, thus limiting the method to short …

A Case History Of The Science And Management Collaboration In Understanding Hypoxia Events In Long Bay, South Carolina, Usa., Denise Sanger, Debra Hernandez, Susan Libes, George Voulgaris, Braxton Davis, Erik Smith, Rebecca Shufford, Dwayne Porter, Eric Koepfler, Joseph Bennet

George Voulgaris

Communication of knowledge between the scientific and management communities is a difficult process complicated by the distinctive nature of professional career goals of scientists and decision-makers. This article provides a case history highlighting a collaboration between the science and management communities that resulted from a response to a 2004 hypoxia, or low dissolved oxygen, event in Long Bay, off Myrtle Beach, South Carolina. A working group of scientists and decision-makers was established at the time of the event and has continued to interact to develop a firm understanding of the drivers responsible for hypoxia formation in Long Bay. Several factors …

Exploring The Persistence Of Sorted Bedforms On The Inner-Shelf Of Wrightsville Beach, North Carolina, Benjamin Gutierez, George Voulgaris, Robert Thieler

George Voulgaris

Geological studies offshore of Wrightsville Beach, North Carolina reveal subtle large-scale regions of coarse sand with gravel and shell hash (widths between 100 and 200 m and negative relief of ∼1 m) that trend obliquely to the coast. It was previously suggested that these regions serve as conduits for sand exchange between the shoreface and inner shelf during storm-associated downwelling. Consequently they were classified as rippled scour depressions. More recently, the role of alongshore flows and self-organization as a result of inhibited settling of fine sand has been discussed. In this study, 45 days of near-bed current measurements were analyzed …

Effect Of Channel Bifurcation On Residual Estuarine Circulation: Winyah Bay, South Carolina, Yong Kim, George Voulgaris

George Voulgaris

The residual circulation pattern of Winyah Bay, the fourth largest estuary on the eastern coast of the US, is examined using stationary and shipborne current measurements during periods of low freshwater discharge. The estuary has a complex morphology with a single channel and narrow banks at the river entrance and the bay mouth, and a bifurcated channel system (main and western channels, respectively) in the middle part that appears to affect the residual circulation. Overall, the upper (single channel morphology) and middle (dual-channel morphology) parts of the estuary exhibit a baroclinic residual circulation. The presence of bifurcated channels in the …

Internal Waves Revealed By Synthetic Aperture Radar (Sar) Imagery In The Vicinity Of The Eastern Cretan Arc Straits (Eastern Mediterranean), Adonis Velegrakis, E. Oikonomou, A. Theocharis, M Collins, H Kontoyannis, V Papadopoulos, George Voulgaris, T. Wells, E. Balopoulos

George Voulgaris

Internal waves have been detected on ERS-1 SAR images obtained during late summer over the eastern Cretan Straits, an area characterised by complex regional physiography, bottom topography, flow regime and stratification patterns of the upper part of the water column. Analysis of the imaged characteristics of the internal waves has revealed a strong diversity in form, propagation direction and type of sea surface modulation, which indicates various mechanisms of generation. Analysis of the currents recorded over the area shows that, although semi-diurnal tidal currents are present, these are of low magnitude in comparison with the overall flow and, therefore, tidal …

The In Situ Passive Acoustic Measurement Of Shingle Movement Under Waves And Currents: Instrument (Tosca) Development And Preliminary Results, George Voulgaris, Michael Wilkin, Michael Collins

George Voulgaris

An instrumented platform (TOSCA) is described for monitoring coarse-grained sediment transport in high (wave and tidal) energy environments. The system consists of a number of sensors which measure: instantaneous current velocities at three levels; sea surface elevation; and self-generated noise (SGN) caused by intercollision of particles in transit. SGN is related to transport rates, on the basis of laboratory calibration; this, in turn, allows a correlation with simultaneous near-bed shear stress to be established. During the field measurements, PC-based real-time data transmission is realised through the use of a (buoyed) radio telemetry system.

TOSCA has been deployed in eastern Christchurch …