Geology topics

Research about Georges Bank, Mid-Atlantic Bight

Source-linked reports with geographic coverage including Georges Bank, Mid-Atlantic Bight.

3 recordsLinked to original sources

USGS1990

Sediment movement along the U.S. east coast continental shelf-I. Estimates of bottom stress using the Grant-Madsen model and near-bottom wave and current measurements

Bottom stress is calculated for several long-term time-series observations, made on the U.S. east coast continental shelf during winter, using the wave-current interaction and moveable bed models of Grant and Madsen (1979, Journal of Geophysical Research, 84, 1797-1808; 1982, Journal of Geophysical Research, 87, 469-482). The wave and current measurements were obtained by means of a bottom tripod system which measured current using a Savonius rotor and vane and waves by means of a pressure sensor. The variables were burst sampled about 10% of the time. Wave energy was reasonably resolved, although aliased by wave groupiness, and wave period was accurate to 1-2 s during large storms. Errors in current speed and direction depend on the speed of the mean current relative to the wave current. In general, errors in bottom stress caused by uncertainties in measured current speed and wave characteristics were 10-20%. During storms, the bottom stress calculated using the Grant-Madsen models exceeded stress computed from conventional drag laws by a factor of about 1.5 on average and 3 or more during storm peaks. Thus, even in water as deep as 80 m, oscillatory near-bottom currents associated with surface gravity waves of period 12 s or longer will contribute substantially to bottom stress. Given that the Grant-Madsen model is correct, parameterizations of bottom stress that do not incorporate wave effects will substantially underestimate stress and sediment transport in this region of the continental shelf.

Georges Bank, Mid-Atlantic Bight↗

USGS1990

Sediment movement along the U.S. east coast continental shelf-II. Modelling suspended sediment concentration and transport rate during storms

Long-term near-bottom wave and current observations and a one-dimensional sediment transport model are used to calculate the concentration and transport of sediment during winter storms at 60-80 m water depth along the southern flank of Georges Bank and in the Mid-Atlantic Bight. Calculations are presented for five stations, separated by more than 600 km alongshelf, that have different bottom sediment texture, bedforms and current conditions. A modified version of the sediment transport model presented by Grant and Glenn (1983, Technical Report to the American Gas Association), Glenn (1983, D.Sc. Thesis, M.I.T.), and Glenn and Grant (1987, Journal of Geophysical Research, 92, 8244-8264) is used to examine the influence of wave-current interaction, sediment stratification, and limitations on the erodibility of the bottom sediments on the concentration of sediment in the water column and on transport. Predicted suspended sediment concentrations are higher than observed, based on beam transmissometer measurements, unless an erosion limit of order a few millimeters for sediments finer than 94 ??m is imposed. The agreement between predicted and measured beam attenuation is better at stations that have significant amounts of silt plus clay in the surficial sediments than for stations with sandy sediments. Sediment concentrations during storms estimated by Moody et al. (1987, Continental Shelf Research, 7, 609-628) are within 50% of the model predictions. Sediment transport rates for sediments 94 ??m and finer are determined largely by the concentrations in the surficial sediment and the erosion depth limit. Large alongshelf transports in the direction of storm-driven currents are inferred for stations in the Mid-Atlantic Bight. During a 115-day period in winter 1979-1980, the net transport of sediment along the shelf was westward; benthic storms (defined as periods when the bottom wave stress exceeded the current stress by 2 dyn cm-2) occurred between 23 and 73% of the time, and greater than 91% of the net alongshelf transport was during storms. ?? 1990.

Georges Bank, Mid-Atlantic Bight↗

USGS1980

Semidiurnal bottom pressure and tidal currents on Georges Bank and in the Mid-Atlantic Bight

Observations of the semidiurnal (M 2 ) tidal current, bottom pressure, and coastal sea level from Nova Scotia to Cape May, New Jersey) show that: 1) the surface pressure at the edge of the Continental Shelf in the study area has a relatively uniform amplitude (40 cm) and phase (12 hr Greenwich); 2) in the Mid-Atlantic Bight, the semidiurnal M 2 tide is co-oscillating, and the onshore M 2 tidal current reaches a maximum 2 approximately 90° or 3.1 hr before high water; 3) in the Georges Bank region of the shelf, the M tide is more progressive than it is in the 2 Mid-Atlantic Bight, and the onshore M 2 tidal current in the Georges Bank 2 region of the shelf reaches a maximum about 7 ° or 14 min before high water; 4) the strongest M 2 tidal currents observed in the study area are 2 near the crest of Georges Bank (typical amplitudes are 74 cm/s near the surface); 5) the M 2 tidal currents are weaker in the Mid-Atlantic Bight 2 (typical amplitudes are 16 cm/s near the surface) than on Georges Bank; and 6) the tidal currents are weakest near the bottom south of Cape Cod in a region of transition between progressive and co-oscillating tides.

Georges Bank, Mid-Atlantic Bight↗