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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 http://frf.wes.army.mil 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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