Voulgaris George

Last Name

Voulgaris

First Name

George

Full item page

Search Results

Now showing 1 - 12 of 12

Tidal asymmetry and residual circulation over linear sandbanks and their implication on sediment transport : a process-oriented numerical study

(American Geophysical Union, 2007-12-22) Sanay, Rosario ; Voulgaris, George ; Warner, John C.

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 magnitude of the generated residual flow is always larger than that of the locally generated overtides. Also, net sediment transport is always dominated by the nonlinear interaction of the residual flow and the semidiurnal tidal currents, although cross-bank sediment transport can occur even in the absence of a cross-shore residual flow. On the other hand, net sediment divergence/convergence increases as the bottom drag decreases and as bank orientation increases. The sediment erosion/deposition is not symmetric about the crest of the bank, suggesting that originally symmetric banks would have the tendency to become asymmetric.
Tidal circulation and flushing characteristics of the Nauset Marsh System : report to the Town of Orleans

(Woods Hole Oceanographic Institution, 1997-07) Aubrey, David G. ; Voulgaris, George ; Spencer, Wayne D. ; O'Malley, Stephen P.

Various interested bodies (i.e., National Park Servce, Cape Cod Commssion, and the Town of Orleans) charged with management of the Nauset Marsh system on Cape Cod, MA, commissioned a study of the estuarine circulation within the Nauset system. Recent signficant morphological changes in the system have changed mixing processes and residence times for the embayment. This study specifically addressed the differing water circulation and residence times arising from a migrating single inlet (dominant condition) and dual inlet (1992-1996) situations. These residence times are to be used by the Cape Cod Commission to identify nitrogen-sensitive sub-embayments based on various assumptions of build-out and nutrient loading. The Nauset Marsh system has experienced considerable development in recent years; proper management of this resource area requires knowledge of the consequences of such development. Application of field observations of bathymetry, sea surface elevation, temperature, salinity and currents, leads to better understanding the physics of the system. These data, analyzed in various forms, served as input data for a numerical, two-dimensional circulation model of the embayment. The circulation model provided flow and discharge data with which the residence times were calculated. Bathymetric measurements defined the volumes of the various sub-embayments to be used in the calculation of residence times. Residence times were calculated for six sub-embayments of the system, defined on the basis of their common hydrodynamic and morphologic characteristics. Two scenarios were evaluated: one for the present single-inlet system, which is near typical for most system states, and one for a dual inlet system such as existed for a period of time from 1992 through 1996. Residence times were evaluated for twelve cases, to demonstrate the range of residence times that can be defined based on varing assumptions. For instance, residence times can be defined on the basis of mean low water volumes or mean water levels, the latter being the more conservative (yielding a longer residence time). In addition, residence times depend on whether spring tides, neap tides, or average tidal conditions are used. We provide data on all three conditions: the neap tidal case is the most conservative in the sense of providing a longer residence time. This case can serve as the basis for flushing if conservatism is desired. Finally, residence time can be defined based on the amount of time it takes for water to renew itself with water from adjacent sub-embayments, or more conservatively assuming renewal from the offshore waters (which are presumed to be cleaner). Based on these various inputs, assumptions and calculations, residence times for Salt and Mill ponds under conditions of a single inlet are the longest of the various sub-embayments. Town Cove is still relatively quickly renewed, though not as fast as the main channels serving the system. Flow pattern under dual-inlet condition does seem to be partioned well, with the northern inlet serving the northern part of the system and the southern inlet serving the southern part of the system, with litte hydrodynamc communication between the two divisions. This new hydrodynamc behavior results in shorter residence times under dual inlets than under a single inlet. Calculations indicate that the slowest flushing occurs in Mill and Salt ponds. The main body of the embayment, consisting of narrow channels between well-flushed salt marsh and tidal flats, flushes rapidly. Two-dimensional calculations show that Town Cove also flushes relatively rapidly, on average. However, its greater depth and occasional temperature stratification create conditions which might accumulate nutrients in bottom sediments, which, when released, can cause decrease in water quality (such as plankton blooms). A more sophisticated low-trophic level ecosystem model combined with vertical hydrodynamic structure could clarfy the dynamics of this process. This study provides a defensible basis for evaluating nutrient loading and potential eutrophication arising from development in the watershed around Nauset embayment. However, since morphological changes occur on a rapid basis in this area, the issue of residence time should be re-examined periodically. For instance, rapid onshore migration of the southern barrier beach is threatening closure of the south chanel, a condition which could adversely affect water quality in Nauset Harbor in the near futue. A process should be established to examine the sensitivity of residence times for rapidly changing morphology.
Storm-induced inner-continental shelf circulation and sediment transport : Long Bay, South Carolina

(Elsevier B.V., 2012-05-10) Warner, John C. ; Armstrong, Brandy ; Sylvester, Charlene S. ; Voulgaris, George ; Nelson, Timothy R. ; Schwab, William C. ; Denny, Jane F.

Long Bay is a sediment-starved, arcuate embayment located along the US East Coast connecting both South and North Carolina. In this region the rates and pathways of sediment transport are important because they determine the availability of sediments for beach nourishment, seafloor habitat, and navigation. The impact of storms on sediment transport magnitude and direction were investigated during the period October 2003–April 2004 using bottom mounted flow meters, acoustic backscatter sensors and rotary sonars deployed at eight sites offshore of Myrtle Beach, SC, to measure currents, water levels, surface waves, salinity, temperature, suspended sediment concentrations, and bedform morphology. Measurements identify that sediment mobility is caused by waves and wind driven currents from three predominant types of storm patterns that pass through this region: (1) cold fronts, (2) warm fronts and (3) low-pressure storms. The passage of a cold front is accompanied by a rapid change in wind direction from primarily northeastward to southwestward. The passage of a warm front is accompanied by an opposite change in wind direction from mainly southwestward to northeastward. Low-pressure systems passing offshore are accompanied by a change in wind direction from southwestward to southeastward as the offshore storm moves from south to north. During the passage of cold fronts more sediment is transported when winds are northeastward and directed onshore than when the winds are directed offshore, creating a net sediment flux to the north–east. Likewise, even though the warm front has an opposite wind pattern, net sediment flux is typically to the north–east due to the larger fetch when the winds are northeastward and directed onshore. During the passage of low-pressure systems strong winds, waves, and currents to the south are sustained creating a net sediment flux southwestward. During the 3-month deployment a total of 8 cold fronts, 10 warm fronts, and 10 low-pressure systems drove a net sediment flux southwestward. Analysis of a 12-year data record from a local buoy shows an average of 41 cold fronts, 32 warm fronts, and 26 low-pressure systems per year. The culmination of these events would yield a cumulative net inner-continental shelf transport to the south–west, a trend that is further verified by sediment textural analysis and bedform morphology on the inner-continental shelf.
Predicting wave-induced ripple equilibrium geometry

(John Wiley & Sons, 2013-06-28) Nelson, Timothy R. ; Voulgaris, George ; Traykovski, Peter A.

A comprehensive database of existing (since 1954) field and laboratory measurements of ripple geometry is compiled and combined with newly collected field data to examine the performance of ripple equilibrium predictors. Reanalysis of this enlarged ripple geometry data set reveals that ripples formed from monochromatic waves scale differently than ripples formed from random waves for many existing ripple predictors. Our analysis indicates that ripple wavelengths from the two data sets collapse into a single scaling when the semiorbital excursion and sediment grain diameter are used as normalizing factors. Ripple steepness remains relatively constant for both regular and irregular wave conditions, and it only slightly increases for shorter ripple wavelengths. These findings allowed for the development of a new equilibrium ripple predictor suitable for application in a wide range of wave and sediment conditions.
Inner-shelf circulation and sediment dynamics on a series of shoreface-connected ridges offshore of Fire Island, NY

(Springer, 2014-10-24) Warner, John C. ; List, Jeffrey H. ; Schwab, William C. ; Voulgaris, George ; Armstrong, Brandy ; Marshall, Nicole

Locations along the inner-continental shelf offshore of Fire Island, NY, are characterized by a series of shoreface-connected ridges (SFCRs). These sand ridges have approximate dimensions of 10 km in length, 3 km spacing, and up to ∼8 m ridge to trough relief and are oriented obliquely at approximately 30° clockwise from the coastline. Stability analysis from previous studies explains how sand ridges such as these could be formed and maintained by storm-driven flows directed alongshore with a key maintenance mechanism of offshore deflected flows over ridge crests and onshore in the troughs. We examine these processes both with a limited set of idealized numerical simulations and analysis of observational data. Model results confirm that alongshore flows over the SFCRs exhibit offshore veering of currents over the ridge crests and onshore-directed flows in the troughs, and demonstrate the opposite circulation pattern for a reverse wind. To further investigate these maintenance processes, oceanographic instruments were deployed at seven sites on the SFCRs offshore of Fire Island to measure water levels, ocean currents, waves, suspended sediment concentrations, and bottom stresses from January to April 2012. Data analysis reveals that during storms with winds from the northeast, the processes of offshore deflection of currents over ridge crests and onshore in the troughs were observed, and during storm events with winds from the southwest, a reverse flow pattern over the ridges occurred. Computations of suspended sediment fluxes identify periods that are consistent with SFCR maintenance mechanisms. Alongshore winds from the northeast drove fluxes offshore on the ridge crest and onshore in the trough that would tend to promote ridge maintenance. However, alongshore winds from the southwest drove opposite circulations. The wind fields are related to different storm types that occur in the region (low-pressure systems, cold fronts, and warm fronts). From the limited data set, we identify that low-pressure systems drive sediment fluxes that tend to promote stability and maintain the SFCRs while cold front type storms appear to drive circulations that are in the opposite sense and may not be a supporting mechanism for ridge maintenance.
Tidal observations at Ria Formosa, Algarve, Portugal

(Woods Hole Oceanographic Institution, 2000-05) Salles, Paulo ; O'Malley, Stephen P. ; Voulgaris, George ; Aubrey, David G.

The apparent persistence and stability of multiple tidal inlets in coastal lagoons are important for a variety of reasons, such as water quality, navigability and beach/barrier stability. To identify and study the processes controlling the persistence of multiple tidal inlets, the hydrodynamics of the system have to be better understood. This project is part of a larger study (INDIA) examining general tidal inlet processes. The present components consist of a numerical simulation study of processes controlling multiple inlet stability, combined with exhaustive field measurements. This report addresses only the second component. The analysis uses as study site and main source of data the Ria Formosa lagoon in Portugal, which has multiple and historically persistent inlets. For the numerical simulation model, field measurements are needed to provide (i) updated bathymetry of the inlets, in situ measurements of (ii) water level fluctuations within the estuary and (iii) flow velocities through the inlets. This report gives first a brief description of the instrumentation used in the field (section 2), then describes the methods used to deploy the instruments, perform the surveys and gather the data (section 3), explains the procedures for data reduction and show some results (section 4 and Annex).
Turbulence in the shallow nearshore environment during SANDYDUCK '97

(Woods Hole Oceanographic Institution, 2001-02) Fredericks, Janet J. ; Trowbridge, John H. ; Voulgaris, George

An array of five acoustic Doppler velocimeters (ADV), which produce high quality measurements of the three-dimensional velocity vector in a sample volume with a scale of one centimeter, was deployed from late August through late November of 1997 at a water depth of approximately 4.5 m off Duck, North Carolina. The sensors were deployed near the sea floor but above the centimeters-thick wave boundary layer, and the sampling scheme was designed to resolve turbulence statistics averaged over tens of minutes, much longer than typical wave periods but shorter than time scales associated with variablity of energetic wind-driven and wave-driven alongshore flows.
Alongshore momentum balance analysis on a cuspate foreland

(John Wiley & Sons, 2013-10-15) Kumar, Nirnimesh ; Voulgaris, George ; List, Jeffrey H. ; Warner, John C.

Nearshore measurements of waves and currents off Cape Hatteras, North Carolina, U.S.A, are used to investigate depth-averaged subtidal circulation and alongshore momentum balances in the surf and inner shelf region around a cuspate foreland. Data were collected on both sides of the cape representing shorefaces with contrasting shoreline orientation (north-south vs. northwest-southeast) subjected to the same wind forcing. In the nearshore, the subtidal flow is aligned with the local coastline orientation while at the cape point the flow is along the existing submerged shoal, suggesting that cape associated shoals may act as an extension of the coastline. Alongshore momentum balance analysis incorporating wave-current interaction by including vortex and Stokes-Coriolis forces reveals that in deep waters surface and bottom stress are almost in balance. In shallower waters, the balance is complex as nonlinear advection and vortex force become important. Furthermore, linearized momentum balance analysis suggests that the vortex force can be of the same order as wind and wave forcing. Farther southwest of Cape Hatteras point, wind and wave forcing alone fail to fully explain subtidal flow variability and it is shown that alongshore pressure gradient as a response to the wind forcing can close the momentum balance. Adjacent tide gauge data suggest that the magnitude of pressure gradient depends on the relative orientation of local coastline to the wind vector, and in a depth-averaged sense the pressure gradient generation due to change in coastline orientation even at km length scale is analogous to the effect of alongshore variable winds on a straight coastline.
Implementation of the vortex force formalism in the coupled ocean-atmosphere-wave-sediment transport (COAWST) modeling system for inner shelf and surf zone applications

( 2012-01) Kumar, Nirnimesh ; Voulgaris, George ; Warner, John C. ; Olabarrieta, Maitane

The coupled ocean-atmosphere-wave-sediment transport modeling system (COAWST) enables simulations that integrate oceanic, atmospheric, wave and morphological processes in the coastal ocean. Within the modeling system, the three-dimensional ocean circulation module (ROMS) is coupled with the wave generation and propagation model (SWAN) to allow full integration of the effect of waves on circulation and vice versa. The existing wave-current coupling component utilizes a depth dependent radiation stress approach. In here we present a new approach that uses the vortex force formalism. The formulation adopted and the various parameterizations used in the model as well as their numerical implementation are presented in detail. The performance of the new system is examined through the presentation of four test cases. These include obliquely incident waves on a synthetic planar beach and a natural barred beach (DUCK’ 94); normal incident waves on a nearshore barred morphology with rip channels; and wave-induced mean flows outside the surf zone at the Martha’s Vineyard Coastal Observatory (MVCO). Model results from the planar beach case show good agreement with depth-averaged analytical solutions and with theoretical flow structures. Simulation results for the DUCK’ 94 experiment agree closely with measured profiles of cross-shore and longshore velocity data from Garcez-Faria et al. (1998, 2000). Diagnostic simulations showed that the nonlinear processes of wave roller generation and wave-induced mixing are important for the accurate simulation of surf zone flows. It is further recommended that a more realistic approach for determining the contribution of wave rollers and breaking induced turbulent mixing can be formulated using non-dimensional parameters which are functions of local wave parameters and the beach slope. Dominant terms in the cross-shore momentum balance are found to be the quasi-static pressure gradient and breaking acceleration. In the alongshore direction, bottom stress, breaking acceleration, horizontal advection and horizontal vortex forces dominate the momentum balance. The simulation results for the bar / rip channel morphology case clearly show the ability of the modeling system to reproduce horizontal and vertical circulation patterns similar to those found in laboratory studies and to numerical simulations using the radiation stress representation. The vortex force term is found to be more important at locations where strong flow vorticity interacts with the wave-induced Stokes flow field. Outside the surf zone, the three-dimensional model simulations of wave-induced flows for non- breaking waves closely agree with flow observations from MVCO, with the vertical structure of the simulated flow varying as a function of the vertical viscosity as demonstrated by Lentz et al. (2008).
Holocene sediment distribution on the inner continental shelf of northeastern South Carolina : implications for the regional sediment budget and long-term shoreline response

(Elsevier B.V., 2013-02-26) Denny, Jane F. ; Schwab, William C. ; Baldwin, Wayne E. ; Barnhardt, Walter A. ; Gayes, Paul T. ; Morton, Robert A. ; Warner, John C. ; Driscoll, Neal W. ; Voulgaris, George

High-resolution geophysical and sediment sampling surveys were conducted offshore of the Grand Strand, South Carolina to define the shallow geologic framework of the inner shelf. Results are used to identify and map Holocene sediment deposits, infer sediment transport pathways, and discuss implications for the regional coastal sediment budget. The thickest deposits of Holocene sediment observed on the inner shelf form shoal complexes composed of moderately sorted fine sand, which are primarily located offshore of modern tidal inlets. These shoal deposits contain ∼67 M m3 of sediment, approximately 96% of Holocene sediment stored on the inner shelf. Due to the lack of any significant modern fluvial input of sand to the region, the Holocene deposits are likely derived from reworking of relict Pleistocene and older inner-shelf deposits during the Holocene marine transgression. The Holocene sediments are concentrated in the southern part of the study area, due to a combination of ancestral drainage patterns, a regional shift in sediment supply from the northeast to the southwest in the late Pleistocene, and proximity to modern inlet systems. Where sediment is limited, only small, low relief ridges have formed and Pleistocene and older deposits are exposed on the seafloor. The low-relief ridges are likely the result of a thin, mobile veneer of sediment being transported across an irregular, erosional surface formed during the last transgression. Sediment textural trends and seafloor morphology indicate a long-term net transport of sediment to the southwest. This is supported by oceanographic studies that suggest the long-term sediment transport direction is controlled by the frequency and intensity of storms that pass through the region, where low pressure systems yield net along-shore flow to the southwest and a weak onshore component. Current sediment budget estimates for the Grand Strand yield a deficit for the region. Volume calculations of Holocene deposits on the inner shelf suggest that there is sufficient sediment to balance the sediment budget and provide a source of sediment to the shoreline. Although the processes controlling cross-shelf sediment transport are not fully understood, in sediment-limited environments such as the Grand Strand, erosion of the inner shelf likely contributes significant sediment to the beach system.
Fluid mechanical measurements within the bottom boundary layer during coastal mixing and optics

(Woods Hole Oceanographic Institution, 2001-08) Fredericks, Janet J. ; Trowbridge, John H. ; Williams, Albert J. ; Voulgaris, George ; Shaw, William J.

To quantify and understand the role of vertical mixing processes in determining mid-shelf vertical structure of hydrographic and optical properties and particulate matter, the Office of Naval Research (ONR) funded a program called Coastal Mixing and Optics (CMO), which was conducted at a mid-shelf location in the Mid-Atlantic Bight, south of Martha's Vineyard, Massachusetts. As part of the CMO program, a tall tripod, called 'SuperBASS,' was equipped to collect a year-long, near-bottom time-series of velocity, temperature, salinity and pressure. The BASS sensors were modified to measure absolute as well as differential acoustic travel time, to provide sound speed (a surrogate for temperature) and velocity in a single sample volume. Seven BASS velocity and time travel sensors were placed between 0.4 and 7 meters above bottom. Three acoustic Doppler velocity (ADV) meters were mounted near the bottom-most BASS sensors at 0.3 meters above bottom. The sensors were used to obtain high-quality time-series measurements of velocity and temperature throughout a large fraction of the bottom bondary layer on the New England shelf. The measurements provide vertical structure of the Reynolds-averaged velocity and temperature fields, direct covariance estimates of turbulent Reynolds stress and turbulent heat flux, and indirect inertial range estimates of dissipation rate for turbulent kinetic energy and temperature variance. The purpose of this report is to describe the SuperBASS instrumentation and deployments, to provide summaries of the data collected, and to document the processing, preliminary analysis and archival of data collected for this component of the program.
Spatial variability of bottom turbulence over a linear sand ridge mooring deployment and AUTOSUB AUV survey cruise report R/V RRS Challenger, cruise number 146 Broken Bank, North Sea, U.K., 17 – 28 August 1999 cruise report

(Woods Hole Oceanographic Institution, 2001-08) Voulgaris, George ; Trowbridge, John H. ; Terray, Eugene A.

Two successful AUTOSUB deployments were carried out during August 1999 as part of the AUTOSUB Thematic Program project titled “Spatial Variability of Bottom Turbulence over a Linear Sand Ridge,” funded by the Natural Environment Research Council (NERC), U.K. The AUTOSUB Autonomous Underwater Vehicle (AUV) was deployed and used to survey flow patterns at a location near the Broken Bank, southern North Sea, U.K. The AUV was equipped with acoustic flow and turbulence sensors and its surveys aimed at mapping the spatial variation of flow and turbulence near the bed and over topographic features. Three instrumented bottom mounted frames were also deployed, around the AUV survey area, for a period of approximately 5 days. The purpose of this array was to gather information on the temporal variability of the flow and turbulence near the seabed and to identify the important terms that drive circulation around the bank. Additional data were gathered including CTD casts, seabed samples and acoustic images of the seabed (side-scan sonar). The purpose of this data collection was to help identify the flow patterns around ridges and to understand the mechanisms controlling the maintenance and evolution of such features. This report describes the operations carried out by researchers from the University of South Carolina, Woods Hole Oceanographic Institution, Southampton Oceanography Centre and the AUTOSUB Team on the R.V. RRS Challenger during the period 17th –28th August 1999.

Voulgaris George

Last Name

First Name

ORCID

Filters

Author

Subject

Date

Type

Has files

Settings

Sort By

Results per page

Search Results