Date of Award

Spring 1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Program/Concentration

Oceanography

Committee Director

J. C. Ludwick

Committee Member

G. F. Oertel

Committee Member

Chester Grosch

Committee Member

G. Richard Whittecar

Abstract

Rip currents have often been noted in physical model studies of groins and their effectiveness. Nevertheless, detailed field investigations of rip currents along a groin wall have not been heretofore conducted. A new digital current meter system was designed and built to study the groin-associated rip currents in a test groin compartment at Willoughby Spit, Norfolk, VA. Six ducted impeller current meters are controlled by a data acquisition system that utilizes an onboard microprocessor and solid state memory. The new system provides high quality data on horizontal water velocities associated with wave action. The velocities stored are time mean velocities averaged over one second. The system will make possible future studies of three dimensional flow in the nearshore and coastal zones at relatively low cost.

The present study verified the existence of a groin-associated rip current in the test compartment. Measured vertical velocity profiles showed that the near bottom time mean flow was often directed onshore while the time mean flow in the upper levels was nearly always directed offshore. The presence of the rip current reduces groin effectiveness by transporting sediment from the compartment seawards beyond the groin ends. The complex vertical structure of the time mean flow makes the use of depth-averaged velocities to estimate sediment transport in a groin compartment questionable. The flow intensity for both the near botton flow and the upper flow was directly proportional to the wave breaker height and reflects the low wave energy input to this estuarine beach site. Eddies that rotate clockwise were observed at the upstream groin wall relative to the tidal current. The action of these eddies may contribute significantly to sediment scour near the groin ends.

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DOI

10.25777/362r-7k58

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