Characteristics of storms driving wave-induced seafloor mobility on the U.S. East Coast continental shelf

This paper is not subject to U.S. copyright. The definitive version was published in Continental Shelf Research 104 (2015): 1-14, doi:10.1016/j.csr.2015.05.003.This study investigates the relationship between spatial and temporal patterns of wave-driven sediment mobility events on the U.S. East Coast continental shelf and the characteristics of the storms responsible for them. Mobility events, defined as seafloor wave stress exceedance of the critical stress of 0.35 mm diameter sand (0.2160 Pa) for 12 or more hours, were identified from surface wave observations at National Data Buoy Center buoys in the Middle Atlantic Bight (MAB) and South Atlantic Bight (SAB) over the period of 1997–2007. In water depths ranging from 36–48 m, there were 4–9 mobility events/year of 1–2 days duration. Integrated wave stress during events (IWAVES) was used as a combined metric of wave-driven mobility intensity and duration. In the MAB, over 67% of IWAVES was caused by extratropical storms, while in the SAB, greater than 66% of IWAVES was caused by tropical storms. On average, mobility events were caused by waves generated by storms located 800+ km away. Far-field hurricanes generated swell 2–4 days before the waves caused mobility on the shelf. Throughout most of the SAB, mobility events were driven by storms to the south, east, and west. In the MAB and near Cape Hatteras, winds from more northerly storms and low-pressure extratropical systems in the mid-western U.S. also drove mobility events. Waves generated by storms off the SAB generated mobility events along the entire U.S. East Coast shelf north to Cape Cod, while Cape Hatteras shielded the SAB area from swell originating to the north offshore of the MAB.This work was funded by the U.S. Geological Survey

On the dynamics of shallow water currents in Massachusetts Bay and on the New England continental shelf.

Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences.Vita.Bibliography: p.160-163.Ph.D

On the dynamics of shallow water currents in Massachusetts Bay and on the New England continental shelf

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April, 1975Massachusetts Bay is a coastal Bay 100 km long and 40 km wide located in the western Gulf of Maine. The Bay is closed by land to the north, west and south, but is open to the Gulf to the east; the opening is partially blocked by a shallow bank. The bottom sediment distribution in the Bay is complex; fine grained material is found in the deep basin, sand and gravel on the shallow bank, and mixtures of sand, gravel and fine material nearshore. Richardson current meters were moored 1 m from the bottom over a one year period at several locations in the Bay to study the bottom currents and the equilibrium between current and sediments. The current measurements suggest that the bottom sediments can be expected to move only occasionally in certain areas. The maximum bottom speeds are principally determined by the strong tidal currents in the basin. In winter, the near bottom currents are dominated by wind stress associated with strong storms. Bottom currents in the shallow areas are generally in the direction of the wind while currents in the deep portion of the basin are often opposite to the direction of the wind. Sea surface setup in the direction of the wind is observed, as well as absolute changes in sea level as the Bay adjusts to changes in the level of the adjacent Gulf of Maine. Adjustment of the bottom currents to wind events requires approximately 12 hours. Moored current meter measurements and synoptic hydrographic observations made in Massachusetts Bay show that freshening from the spring runoff dominates the low frequency currents and the hydrography of the Bay in the spring months. The major freshening is attributed to the Merrimack River which empties into the Gulf of Maine 30 km to the north of the Bay; discharge of the Merrimack increases by at least a factor of two in spring. Flow directly into the basin from several smaller rivers is not important. Two major features are found: a fresh surface plume confined to the upper 10 m of the water column which becomes more distinct as the seasonal thermocline develops, and a large deep fresh lens. Flow is clockwise around the deep lens and is consistent with the thermal wind relation. Sustained currents of 10-20 cm sec -1 with time scales of 5-10 days were observed as the deep lens (or lenses) slowly advected through the basin. Current observations made in the previous spring show similar low frequency behavior. Two simple linear models of the semidiurnal tide on the continental shelf are used to estimate the vertical turbulent eddy viscosity, a linear bottom drag coefficient, and the change in the bottom drag coefficient during storms. The analytic solution for the response of a homogeneous water column with constant eddy viscosity to a sinusoidal body force with a slip bottom boundary condition is presented. with measurements of the tidal current at two depths, four parameters are shown to be independent of the body force: the ratio of the clockwise current at two depths, the ratio of counterclockwise current at two depths, the change in the tidal ellipse orientation, and the change in phase of the tidal ellipse. Observations of the semidiurnal tidal current on the New England continental shelf are consistent with a vertical eddy viscosity of 20-50 cm2 sec -1 and a bottom drag coefficient of .02-.05 cm sec -1. The Ekman depth is thus 10 m and the integrated adjustment time is approximately 28 hours. An integrated linear model with linear damping of the semidiurnal tide on the continental shelf, forced uniformly at the shelf edge, shows an increasing phase lag of the tide at the coast with increased damping; amplitude remains relatively constant over a wide range of damping coefficient. Observations of the tide at the coast during storms shows a phase lag of as much as 10 degrees for the semidiurnal tide. For approximate dimensions of the New England shelf, this implies an increase by a factor of 3-5 of the bottom drag coefficient and an integrated motion adjustment time of 6-9 hours. Waves may be an important contribution to the increased bottom stress.Prepared under National Science Foundation Grants GA 30729X, GA 41075, NSF Graduate Fellowship, M.I.T. Sea Grant Program No. 43-72, U. S. Geological Survey, Mass. Dept. of Public Works, Office of Naval Research Contract N00014-74-C-0262; NR 083-004

Wind–Current Coupling on the Southern Flank of Georges Bank: Variation with Season and Frequency

Comparison of several years of current observations on the southern flank of Georges Bank with nearby wind data shows that the wind–current coupling is primarily between longshelf wind stress and longshelf current. The strongest wind–current coupling occurs in winter, when the water column is homogeneous. The weakest coupling is in late summer and early fall, when the water column is highly stratified. The coherence and transfer coefficient between longshelf wind and longshelf current is highest for periods between 4 and 12 days, decreasing both for longer periods (out to 56 days) and shorter periods (down to 2 days). Models of the wind–current coupling indicate that a highly damped resonance may exist on Georges Bank and that a smaller current response is expected when the water column is stratified. The observations also indicate that the wind-driven currents on Georges Bank are strongly controlled by friction. The near-surface current moves to the right of wind stress and there is a spring–neap modulation of the wind–current transfer coefficient caused by the modulation of the bottom stress associated with the spring–neap tidal cycle. The longshelf current is linearly related to wind stress and responds almost symmetrically to wind forcing

On the interpretation of energy and energy fluxes of nonlinear internal waves : an example from Massachusetts Bay

Author Posting. © Cambridge University Press, 2006. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 561 (2006):103–112, doi:10.1017/S0022112006000991A self-consistent formalism to estimate baroclinic energy densities and fluxes resulting from the propagation of internal waves of arbitrary amplitude is derived using the concept of available potential energy. The method can be applied to numerical, laboratory or field data.MBIWE98 was supported by the US Geological Survey and the Office of Naval Research. A.S. received support from the Office of Naval Research (N00014-05-1-0361), R.B. from the Walter A. and Hope Noyes Smith Chair on Coastal Oceanography and B.B. from the US Geological Survey

Long-term performance of Aanderaa optodes and Sea-Bird SBE-43 dissolved-oxygen sensors bottom mounted at 32 m in Massachusetts Bay

Author Posting. © American Meteorological Society, 2007. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Atmospheric and Oceanic Technology 24 (2007): 1924-1935, doi:10.1175/JTECH2078.1.A field evaluation of two new dissolved-oxygen sensing technologies, the Aanderaa Instruments AS optode model 3830 and the Sea-Bird Electronics, Inc., model SBE43, was carried out at about 32-m water depth in western Massachusetts Bay. The optode is an optical sensor that measures fluorescence quenching by oxygen molecules, while the SBE43 is a Clark polarographic membrane sensor. Optodes were continuously deployed on bottom tripod frames by exchanging sensors every 4 months over a 19-month period. A Sea-Bird SBE43 was added during one 4-month deployment. These moored observations compared well with oxygen measurements from profiles collected during monthly shipboard surveys conducted by the Massachusetts Water Resources Authority. The mean correlation coefficient between the moored measurements and shipboard survey data was >0.9, the mean difference was 0.06 mL L−1, and the standard deviation of the difference was 0.15 mL L−1. The correlation coefficient between the optode and the SBE43 was >0.9 and the mean difference was 0.07 mL L−1. Optode measurements degraded when fouling was severe enough to block oxygen molecules from entering the sensing foil over a significant portion of the sensing window. Drift observed in two optodes beginning at about 225 and 390 days of deployment is attributed to degradation of the sensing foil. Flushing is necessary to equilibrate the Sea-Bird sensor. Power consumption by the SBE43 and required pump was 19.2 mWh per sample, and the optode consumed 0.9 mWh per sample, both within expected values based on manufacturers’ specifications.This work was funded by the MWRA and USGS

Water level response in back-barrier bays unchanged following Hurricane Sandy

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geophysical Research Letters 41 (2014): 3163–3171, doi:10.1002/2014GL059957.On 28–30 October 2012, Hurricane Sandy caused severe flooding along portions of the northeast coast of the United States and cut new inlets across barrier islands in New Jersey and New York. About 30% of the 20 highest daily maximum water levels observed between 2007 and 2013 in Barnegat and Great South Bay occurred in 5 months following Hurricane Sandy. Hurricane Sandy provided a rare opportunity to determine whether extreme events alter systems protected by barrier islands, leaving the mainland more vulnerable to flooding. Comparisons between water levels before and after Hurricane Sandy at bay stations and an offshore station show no significant differences in the transfer of sea level fluctuations from offshore to either bay following Sandy. The post-Hurricane Sandy bay high water levels reflected offshore sea levels caused by winter storms, not by barrier island breaching or geomorphic changes within the bays

Observations and a linear model of water level in an interconnected inlet-bay system

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 2760–2780, doi:10.1002/2016JC012318.A system of barrier islands and back-barrier bays occurs along southern Long Island, New York, and in many coastal areas worldwide. Characterizing the bay physical response to water level fluctuations is needed to understand flooding during extreme events and evaluate their relation to geomorphological changes. Offshore sea level is one of the main drivers of water level fluctuations in semienclosed back-barrier bays. We analyzed observed water levels (October 2007 to November 2015) and developed analytical models to better understand bay water level along southern Long Island. An increase (∼0.02 m change in 0.17 m amplitude) in the dominant M2 tidal amplitude (containing the largest fraction of the variability) was observed in Great South Bay during mid-2014. The observed changes in both tidal amplitude and bay water level transfer from offshore were related to the dredging of nearby inlets and possibly the changing size of a breach across Fire Island caused by Hurricane Sandy (after December 2012). The bay response was independent of the magnitude of the fluctuations (e.g., storms) at a specific frequency. An analytical model that incorporates bay and inlet dimensions reproduced the observed transfer function in Great South Bay and surrounding areas. The model predicts the transfer function in Moriches and Shinnecock bays where long-term observations were not available. The model is a simplified tool to investigate changes in bay water level and enables the evaluation of future conditions and alternative geomorphological settings.New York State Department of Environmental Conservation Grant Number: (NYS-DEC); U.S. Geological Survey (USGS

Surface circulation in Block Island Sound and adjacent coastal and shelf regions : a FVCOM-CODAR comparison

© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Progress in Oceanography 143 (2016): 26-45, doi:10.1016/j.pocean.2016.02.005.CODAR-derived surface currents in Block Island Sound over the period of June 2000 through September 2008 were compared to currents computed using the Northeast Coastal Ocean Forecast System (NECOFS). The measurement uncertainty of CODAR-derived currents, estimated using statistics of a screened nine-year time series of hourly-averaged flow field, ranged from 3-7 cm/s in speed and 4°-14° in direction. The CODAR-derived and model-computed kinetic energy spectrum densities were in good agreement at subtidal frequencies, but the NECOFS-derived currents were larger by about 28% at semi-diurnal and diurnal tidal frequencies. The short-term (hourly to daily) current variability was dominated by the semidiurnal tides (predominantly the M2 tide), which on average accounted for ~87% of the total kinetic energy. The diurnal tidal and subtidal variability accounted for ~4% and ~9% of the total kinetic energy, respectively. The monthly-averaged difference between the CODAR-derived and model-computed velocities over the study area was 6 cm/s or less in speed and 28° or less in direction over the study period. An EOF analysis for the low-frequency vertically-averaged model current field showed that the water transport in the Block Island Sound region was dominated by modes 1 and 2, which accounted for 89% and 7% of the total variance, respectively. Mode 1 represented a relatively stationary spatial and temporal flow pattern with a magnitude that varied with season. Mode 2 was characterized mainly by a secondary cross-shelf flow and a relatively strong along-shelf flow. Process-oriented model experiments indicated that the relatively stationary flow pattern found in mode 1 was a result of tidal rectification and its magnitude changed with seasonal stratification. Correlation analysis between the flow and wind stress suggested that the cross-shelf water transport and its temporal variability in mode 2 were highly correlated to the surface wind forcing. The mode 2 derived onshore and offshore water transport, and was consistent with wind-driven Ekman theory. The along-shelf water transport over the outer shelf, where a large portion of the water flowed from upstream Nantucket Shoals, was not highly correlated to the surface wind stress.This work was supported by the NSF grants OCE-1332207 and OCE-1332666, MIT Sea Grant College Program through grant 2012-R/RC-127, and the NOAA NERACOOS program funds for NECOFS. Operational funding for the CODAR systems used in this study was provided by the Mid-Atlantic Regional Association Coastal Ocean Observing System. The development of the Global-FVCOM system has been supported by NSF grants OCE-1203393. C. Chen’s contribution was also supported by the International Center for Marine Studies at Shanghai Ocean University through the “Shanghai Universities First-class Disciplines Project”.2017-03-0

Storm-driven sediment transport in Massachusetts Bay

This paper is not subject to U.S. copyright. The definitive version was published in Continental Shelf Research 28 (2008): 257-282, doi:10.1016/j.csr.2007.08.008.Massachusetts Bay is a semi-enclosed embayment in the western Gulf of Maine about 50 km wide and 100 km long. Bottom sediment resuspension is controlled predominately by storm-induced surface waves and transport by the tidal- and wind-driven circulation. Because the Bay is open to the northeast, winds from the northeast (‘Northeasters’) generate the largest surface waves and are thus the most effective in resuspending sediments. The three-dimensional oceanographic circulation model Regional Ocean Modeling System (ROMS) is used to explore the resuspension, transport, and deposition of sediment caused by Northeasters. The model transports multiple sediment classes and tracks the evolution of a multilevel sediment bed. The surficial sediment characteristics of the bed are coupled to one of several bottom-boundary layer modules that calculate enhanced bottom roughness due to wave–current interaction. The wave field is calculated from the model Simulating WAves Nearshore (SWAN). Two idealized simulations were carried out to explore the effects of Northeasters on the transport and fate of sediments. In one simulation, an initially spatially uniform bed of mixed sediments exposed to a series of Northeasters evolved to a pattern similar to the existing surficial sediment distribution. A second set of simulations explored sediment-transport pathways caused by storms with winds from the northeast quadrant by simulating release of sediment at selected locations. Storms with winds from the north cause transport southward along the western shore of Massachusetts Bay, while storms with winds from the east and southeast drive northerly nearshore flow. The simulations show that Northeasters can effectively transport sediments from Boston Harbor and the area offshore of the harbor to the southeast into Cape Cod Bay and offshore into Stellwagen Basin. This transport pattern is consistent with Boston Harbor as the source of silver found in the surficial sediments of Cape Cod Bay and Stellwagen Basin.We gratefully acknowledge support from the USGS Mendenhall Post-Doctoral Research Program for John C. Warner