Field Research Facility
Coastal & Hydraulics Laboratory

Experiment at Duck, N.C., Beach Explores Nearshore Processes
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The following article was published in EOS, Transactions of the American Geophysical Union, 76:49, December 5, 1995. It summarizes post-DUCK94 research activities.

Last year's first northeaster yielded a few surprises about the physics of nearshore oceanographic processes along a well-studied patch of the North Carolina coast. The storm winds and incident waves were typical, but nearshore currents and bathymetric response took an unexpected turn. An immense rip channel cut through the nearshore bar. Previous storm observations at this site had suggested that mean currents and bathymetry become uniform in the alongshore direction, but the rip channel challenged this assumption. Mapping after the storm showed that the bar crest in the vicinity of the rip had migrated up to 100 m seaward over an alongshore distance of 0 m.

This northeaster and other observations were discussed at a post-experiment meeting of participants in DUCK94, a field investigation of nearshore and adjacent shelf processes near Duck, N.C. (See Eos, October 25, 1994) As part of a continuing series of experiments conducted at this site, DUCK94 studied sediment transport and the effects of severe storms on the beach during summer and fall of 1994 and helped to develop and test instruments and techniques that will be used during SandyDuck, a larger experiment planned for 1997. The meeting was held April 10-12 in St. Petersburg, Fla.

Nearshore Morphology

Bottom morphologic features are central to nearshore sediment transport processes. Bottom cores taken during DUCK94 indicated the usual dominantly coarse, poorly sorted grain size distributions at the backwash toe of the beach, grading to finer, well-sorted distributions seaward of the bar crest. Acoustic imaging systems revealed that centimeter-scale sand ripples commonly observed in low-energy conditions formed infrequently and often for short durations during storms.

For the first time, details of meter-scale morphologic features called megaripples were detected. Megaripples result from important sediment transport mechanisms and act as major surface roughness elements affecting fluid flow through turbulence-induced friction. During high mean flow, megaripples were found to be ubiquitous and quite dynamic, migrating at rates of up to 150 cm/hr. In less energetic conditions, megaripples often coexisted with sand ripples.

More fluid-sediment interactions were seen in beach cusps. In DUCK94, daily contour maps of the beach were created using a GPS-based surveying system that, coupled with video imaging of the beach, revealed much about cusp development. During the October northeaster, the beach became relatively flat. As the storm waned, a cusp field formed rapidly in the presence of long-period, nearly shore-normal swell waves. Surprisingly, an abrupt decrease in cusp wavelength occurred over a single tidal cycle, and coincided with a modest increase in incident wave period. This observation suggests that cusp evolution is sensitive both to wave conditions and the shapes of pre-existing cusps.

Wave and Current Processes

In the surf zone, sediment transport is associated with the dynamics of waves and currents. As discussed at the meeting, mean currents on the barred system at Duck continue to defy conventional models of wave-driven flow on beaches. When these models are applied at Duck, longshore current jets are expected near the bar crest and shoreline. Observations showed that a single dominant jet is more likely to occur in the nearshore. It is not clear why this happens. One hypothesis, supported by tower-mounted video observations of the surf zone, is that the energy-conversion process is not localized at the point of breaking, but evolves over a considerable distance shoreward. If momentum transfer from waves to mean flow is delayed in this way, a longshore jet would form in the deeper trough in the lee of the bar. Though details of wave breaking processes are still under investigation, evidence was obtained for verification of two models of nonlinear, shallow water wave transformation. One model represented the dissipation of sea energy and reflection of swell energy from the beach in a case of mixed sea and swell. In a case of nearly pure swell waves crossing the bar, the other model predicted proper growth of higher harmonics and sea surface elevations of shoaling waves.

Turbulence and Sediment Transport

Several investigations addressed turbulence processes associated with the bottom boundary layer and breaking waves. During the northeaster, vertical turbulent momentum fluxes were consistently related to mean velocity shear following conventional boundary layer theory. Such findings are important because turbulent fluctuations are often imbedded in much stronger wave motions, yet are primary mechanisms for sediment suspension and vertical diffusion. In a creative application of existing technology, an array of hot film sensors indicated rapid boundary layer response to individual waves, consistent with a flow-adaptive eddy coefficient closure of the governing equations.

Several investigators observed sediment transport processes with acoustic and optical backscatter devices. Suspension was observed as the usual plume-like events associated with individual passing waves, which lead to mean sediment concentrations that decay rapidly away from the bed, even in modest storm conditions. Video images of the benthos indicated a substantial bed load, which is commonly overlooked in nearshore transport models.

Bubbles are important in breaking-wave conditions because they affect mass and momentum exchange at the sea surface and turbulent fluxes within the water column. Presented at the meeting were vertical profiles of bubble-induced void fractions observed in 2-m water depth in modest storm conditions. As much as 5% of the upper cm of the water column was entrained air. Though mean concentrations decayed dramatically with depth, bubbles reached the bottom in more intense waves.

Several speakers described the behavior of acoustic signals generated by fluid and biological activity. In the nearshore trough about 50 m offshore, an omnidirectional hydrophone detected the sound of breaking waves passing overhead with low signal levels between waves. Similar behavior was observed 500 m offshore. A maximum signal induced by many sources occurred 2 km offshore, with successively quieter regions at 4 and 7 km offshore, though all sites were far louder than the deep ocean.

Shelf Processes

In the first 24 hours following a wind shift from southerly to northerly in August, water temperatures changed from 18 to 22 C throughout the water column. This could have resulted from downwelling of well-mixed nearshore waters, or alongshore advection of warm Chesapeake Bay discharge. Concurrent with this event were south-tending wind- and wave-driven mean surface currents of 70 cm/s outside the surf zone, and near-bottom currents of 30-40 cm/s within the surf zone. In a similar wind shift in October, wind forcing from the north opposed swell-derived forcing from the southeast. Currents were strong and southward in the outer surf zone, strong and northward in the nearshore trough, and weak at an intermediate site. This suggests that direct wind forcing, usually neglected in nearshore models, may be important well into the surf zone.

Many investigators discussed observations during Hurricane Gordon, the most intense event during DUCK94. It produced waves with characteristic heights exceeding 10 m at the shelf break, 90 km east of Duck. Hurricane waves propagated landward relatively unattenuated to about 5 km from shore, where substantial dissipation and transformation began. Consequent sediment transport resulted in as much as cm of erosion 2 km from shore where the depth was 12 m. Such an exceptional bottom change at this depth and distance offshore provides intriguing data for testing and developing models that conserve total nearshore sediment mass.

The following agencies supported the experiments: the Office of Naval Research, U.S. Army Corps of Engineers, U.S. Geological Survey, National Science Foundation, Naval Research Laboratory, and Natural Sciences and Engineering Research Council of Canada. The meeting was hosted by the U.S. Geological Survey. More information is available at on the World Wide Web.

Acknowledgments: We thank W. A. Birkemeier, A. J. Bowen, S. Elgar, R. T. Guza, J. Haines, A. E. Hay, T. Holland, R. A. Holman, B. Raubenheimer, and E. B. Thornton for their comments on earlier drafts of this article. --C. E. Long [U.S. Army Engineer Waterways Experiment Station, Field Research Facility, Duck, NC] and A. H. Sallenger, Jr. [U.S. Geological Survey, St. Petersburg, FL]

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