Sediment Transport in the Middle Atlantic Bight of North America: Synopsis of Recent Observations
- S.-D. Nio,
- R. T. E. Shüttenhelm and
- Tj. C. E. Van Weering
Published Online: 29 JUN 2009
Copyright © 1981 The International Association of Sedimentologists
Holocene Marine Sedimentation in the North Sea Basin
How to Cite
Swift, D. J. P., Young, R. A., Clarke, T. L., Vincent, C. E., Niedoroda, A. and Lesht, B. (2009) Sediment Transport in the Middle Atlantic Bight of North America: Synopsis of Recent Observations, in Holocene Marine Sedimentation in the North Sea Basin (eds S.-D. Nio, R. T. E. Shüttenhelm and Tj. C. E. Van Weering), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303759.ch26
- Published Online: 29 JUN 2009
- Published Print: 23 DEC 1981
Print ISBN: 9780632008582
Online ISBN: 9781444303759
- sediment transport by storm flows - highly episodic and seasonal in nature;
- presence of two contrasting shoreface provinces;
- storm- and tide-dominated regimes;
- ‘scale matching’ storm;
- sediment resuspension by storm waves
The Middle Atlantic Bight of the North American continental shelf is a north-east-trending arcuate water body measuring approximately 600 by 180 km. The most energetic water motions are stormdriven flows, except near estuary mouths and in the Georges Bank–Nantucket Shoals sector, where tidal flows are dominant. The flow climate is complex, and a three-year time series is not sufficient to develop stable statistics. The shelf surface is a gently sloping plain, traversed by shelf valleys and associated landforms. Intervalley surfaces bear a sand-ridge topography formed during erosional retreat of the shoreface in the course of the Holocene transgression. Erosional shoreface retreat has veneered the shelf surface with a sand sheet up to 10 m thick. The ridges are moulded into the sand sheet, whose basal gravel is exposed in swales between the ridges. To the north, fine sediment is accumulating down-current from the eroding high of Nantucket Shoals–Georges Bank.
Sediment transport by storm flows is highly episodic and is seasonal in nature. In a 135-day observation of near-bottom flow, over 95% of the computed transport occurred during a single winter storm. Wave energy is best predictor for concentration in the boundary layer. Concentration–wave energy curves at different sites have similar patterns: after a threshold value has been exceeded, the concentration increases linearly with increasing wave energy.
Studies of sediment transport on the Long Island shoreface reveal the presence of two contrasting shoreface provinces. An upper shoreface zone is subject to nearly continuous alongshore transport by the wave-driven littoral current, and to onshore transport in response to landward-oriented wave orbital currents. Below approximately 10 m, the lower shoreface is subjected to alongshore tide- and wind-driven currents. These are not normally capable of transporting sediment. However, during a north-easter storm the wind-driven component intensified and was associated with downwelling. If wave orbital velocity is intense enough during storms, these flows will entrain sediment all the way down the shoreface. Transport by downwelling storm currents is alongshore and offshore, since on the lower shoreface wave orbital currents lack landward asymmetry.
Studies of sand ridges on the Maryland coast indicate that there is an onshore–offshore continuum of morphologic parameters. Along a shore-normal transect, grain size and topography are systematically related: grain size becomes finer across the ridge and is finest on the seaward flank. The relationship is believed to be indicative of a phase lag between bottom shear stress during storm flows and bottom morphology. Shear stress is greatest on the upcurrent flank rather than at the crest, inducing ridge growth. Tide-built sand ridges on Nantucket Shoals are morphologically similar to the ridges built by storm currents on the Maryland coast, and like the latter they are oriented obliquely to flow.