Date of Award

Summer 1971

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean & Earth Sciences

Program/Concentration

Oceanography

Committee Director

Donald Swift

Committee Member

Ronald E. Johnson

Committee Member

Robert Cheng

Call Number for Print

Special Collections LD4331.O35H65

Abstract

The Virginia inner shelf is dominated by southwest-trending sand ridges. Wavelengths are 2 to 4 km; amplitudes are up to 10 m; crests can be traced for up to 10 cm, Nearshore ridges trend southwest into the shore face and merge with it at depths as shoal as 5 m. The ridges have been examined by bathymetric mapping, grab sampling, coring, seismic profiling, SCUBA diving, and current monitoring. They are hemi-cylindrical sand bodies of recent age, resting on a pre-recent substrate. In troughs a thin, medium- to coarse-gained, pebbly lag veneers the substrate. Crests consist of a better sorted, medium- to fine-grained sand, while the flank sands are well sorted and fine- to very fine-grained.

The fair-weather hydraul1c regime has been assessed by means of a direct-readout, orthogonal, current meter system. Five, 12- or 24-hour bottom stations were monitored. Wave surge and long per1od, roast-wise, residual currents were generally sub-equal in intensity, ranging from 0 to 20 cm/sec. Four stations yielded records of weak currents that could not be readily related to ridge building. SCUBA dives indicate that main fair-weather bottom activity is the slow migrat1on of wave-generated ripples up to 5 cm high, obliquely shoreward across ridge crests. However, a station in the most landward trough recorded a current that appeared to be actively scouring the trough floor. During the station the wind built up from a calm to 25 knots from the northeast. Sea rose to 2 m, and began to break over the ridge seaward of the trough. A south trending bottom current of 20 cm/sec developed and continued despite the turn of the tide. Circumstantial evidence suggests that such strong south-trending currents dominate the inner shelf during storms, and are the ridge-building currents. Shortly after a storm, sand waves with a wavelength of 25 - 30 m and amplitudes of 1 - 2 m were observed on a ridge crest. Cores reveal cross-bedded horizons up to l m thick within ridge crests. Second-order ridges on the flanks of some major ridges are asymmetrical towards the main crest suggesting that during storms, south-trending bottom currents may diverge from trough axe and converge toward ridge crests.

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DOI

10.25777/kbr1-xv29

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