With the arrival of researchers from the Center for Coastal Studies at Scripps Institution of Oceanography (SIO) in June 1997,
the Field Research Facility (FRF) began hosting SandyDuck, an ambitious nearshore field experiment that culminated seven
years of planning. Their early start was required to insure that the over 400 sensors being deployed were installed and operational
by the time the six-week primary measurement period began 22 September 1997.
SandyDuck capped a recent series of increasingly complex, multi-investigator,
multi-agency field measurement efforts held at the Duck, NC facility. This
series began in 1990 with DELILAH, followed in 1994 by DUCK94. These
experiments evolved from scientific and pragmatic successes of prior work at
the FRF (back to 1981), and have the basic objectives of improving
fundamental understanding and modeling of surf zone physics. The emphasis in
DELILAH was surf zone hydrodynamics in the presence of a changing barred
bathymetry. DUCK94 and SandyDuck had added components to resolve
sediment transport and morphologic evolution at scales from bedform ripples to
nearshore sandbars. DUCK94 was designed as a pilot effort to test new
instruments and procedures required for the more comprehensive SandyDuck
experiment. The experiments were cooperatively supported by the Coastal Research and Development Program of the US Army
Corps of Engineers, the Coastal Dynamics Program of the Office of Naval Research, and the Coastal and Marine Geologic
Surveys Program of the United States Geological Survey.
This article provides a general overview of SandyDuck '97 including participants, experiments and measurements. It's a good
place to start learning about this interesting experiment!
Following the success of DELILAH in 1990, and motivated by a desire to promote research into coastal sediment transport, the
program managers of the sponsoring agencies initiated interest among the coastal research community for further field work. The
following focus topics evolved and were established as fundamental to improved understanding of surf zone sediment transport:
- small and medium scale sediment transport and morphology (sediment grains to 100 m scale);
- wave shoaling, wave breaking, and nearshore circulation;
- swash processes including sediment motion.
Between continuing DUCK94 investigators and additional ones, there were 26 participating organizations (Table 1) conducting
30 experiments involving more than 250 scientists, students, and technicians.
The FRF staff provided the infrastructure support. Eleven trailers were brought in to serve as offices and data collection centers.
The trailers were linked to each other and to the Internet through fiber optic cables. Over 100 computers were added to the FRF's
network. The pipes, cables, boats, and divers required to deploy and maintain the sensors fully utilized the resources of the FRF
including the CRAB, amphibious LARC-V, and several all-terrain forklifts. The CRAB was operated daily for five months --
equaling nearly four years of typical use. Because these experiments were planned in anticipation of early nor'easters and a
possible hurricane (or two), FRF expertise was called upon to help insure the survival of the deployed instruments.
Most of the core DUCK94 experiments were repeated, with improvements to take advantage of two major changes in the basic
experiment design. DUCK94 revealed that nearshore dynamics are far less uniform alongshore than had previously been
assumed. Consequently, instruments were added to expand longshore coverage of currents, bottom changes, and sediment
transport. Missing from all prior Duck experiments has been accurate, spatially detailed measurements of sea surface elevation,
the gradient of which is a primary force in the surf zone. As the second change in the experiment plan, new instruments were
deployed to attempt to resolve this very important component of nearshore dynamics.
Table 2 lists the 30 basic studies along with the principal investigators and their primary focus areas.
Central to the experiment was the surf zone array of in situ instruments. This extensive array of
instrumentation is shown in the layout figure shown below. The design of this array was lead by Dr. Edward Thornton from the
Naval Postgraduate School. The design results from discussion with the different investigators and consideration of relevant
scales required to address SandyDuck objectives and guidance gained using previously measured velocity data and sediment
transport modeling. Note that most of the positions shown in the figure included multiple sensors.
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Central to the surf zone array are instrument frames (5, numbers refer to
investigations by experiment number in Table 2), each containing an
electromagnetic current meter, a pressure gauge, an acoustic altimeter, and a
thermometer. In DUCK94, a single cross-shore array of the altimeters permitted
the first comprehensive real-time measurements of bottom changes including
monitoring the offshore movement of the nearshore bar. During SandyDuck, Drs.
Elgar, Herbers, O'Reilly, and Guza deployed the frames (small "+" signs in the
layout figure) in multiple lines, and at varying spacing, in order to measure
nearshore dynamics and bed level changes in both cross-shore and longshore
Drs. Thornton and Stanton (16) deployed a complementary spatial array of
manometers (indicated by the small solid circles) to provide precise measurement
of the water surface slope, critical to understanding the mechanisms driving the
longshore currents. In addition, they also deployed an updated version of their DUCK94 instrumented sled (see above photo).
This sled was equipped with current meters, pressure sensors, optical backscattering sensors, rotating pencil-beam acoustic
altimeters, a Bistatic Coherent Doppler Velocity/Sediment meter, along with other sensors. The sled was deployed daily by the
CRAB to provide mobile measurements of sediment movement. Drs. Thornton and Stanton also added digital sonar-altimeters to
the CRAB in order to map the presence of bottom bedforms as the CRAB conducted its surveys of the area. Also mounted on the
CRAB was the side-scan sonar of Drs. Drake and Snyder (2) to measure the spatial coverage of bedforms.
The sled measurements complement a large number of other suspended sediment concentration gauges which were deployed,
including additional optical backscattering sensors (6, 7, 21), the innovative and less intrusive fiber-optic backscattering sensors
(1, 21), and acoustic concentration profilers (8, 9). During two weeks in October that included the "SandyDuck Storm" Mr. Carl
Miller and Dr. Don Resio operated the mobile Sensor Insertion System on the FRF pier, to collect sediment transport
measurements during high-energy conditions (21).
Most array positions included one or more current meters (1, 5, 6, 7, 8, 9, 15, 23, 25, 29). New to
SandyDuck was the experiment of Dr. Smith (23) to deploy two Phased-Array Doppler Sonars
(PADS) . Used in combination and deployed along the -6.5 m contour, they looked toward the beach
and were able to provide two-dimensional (horizontal) maps of the velocity field measuring rip
currents and nearshore circulation. Dr. Edson monitored surface wind-stress (4). Incident wave
conditions were measured with directional wave buoys (11, 16) and a direction-sensing array of
pressure gauges (20). Measurements of the shoreface, seaward of the surf zone were made with
bottom mounted instruments (6, 15, 29), and through geologic investigations (2, 14). Drs Wu and
Shih made water level measurements along the FRF pier (30).
A number of investigations used advanced remote sensing techniques. Surf zone and swash
processes were observed with tower-mounted video systems (10, 12, 13, 18, 24). Observations were
also made with land-based marine radar systems (27, 28). Several studies examined fundamental
nearshore acoustic behavior (9, 10) and bubble production (9, 24, 26), both topics of critical interest
to the Navy.
As in the past, the CRAB was used to collect daily maps of the bathymetry surrounding
the instruments in an area known as the minigrid. These surveys were augmented by daily
beach surveys over multi-km reaches of shoreline to examine large and medium-scale
patterns of beach changes. These surveys were be collected with a GPS surveying
all-terrain vehicles (12, 19), instrumented jet skis (1,3) and airborne systems (22)
The design of the surf zone instrument layout and the timing of the six-week experiment
were based on previous studies of sandbar behavior at Duck and expectations that a wide
range of conditions would occur. Wave conditions measured by a Datawell waverider
buoy in 18-m of water are shown in the adjacent figure. Incident wave height varied from
calm (<0.5 m) to a short-lived peak of just over 3.5 m during the "SandyDuck storm"
occurring between 18 and 22 October.
However, although the nearshore sandbar moved throughout
the experiment, it did not respond as expected during the
storm. During earlier experiments such as DUCK85 in 1985
and DELILAH in 1990, the sandbar formed and moved
offshore creating a linear longshore bar with a deep trough
close to the beach. Highest observed longshore currents were
found in this trough. As the storm passed, the linear bar
developed rip channels and became highly three-dimensional.
In contrast the SandyDuck sandbar remained
three-dimensional the entire period and although it moved
offshore during the storm, a deep inner trough never
developed. The shape and evolution of the nearshore can be
seen in the four minigrid surveys shown in the Figure. One
possible hypothesis for this response was that the outer bar, a
low relief feature at offshore coordinate 350 m, caused
sufficient energy dissipation to "protect" the inner bar. A
second hypothesis is that the duration of the storm was
insufficient to effectively rearrange the near-shore
morphology. These ideas and many others will be the subject
of SandyDuck research.
SandyDuck was an ambitious undertaking with many
different experiments all benefitting from the synergy of
efforts and instrumentation. With the completion of the
experiment and the removal of all instruments and cables, the
field experiment phase of SandyDuck is finished. The data
analysis phase is well underway and participants are reporting
initial results at meetings and conferences including the 1998
International Conference on Coastal Engineering held in
Copenhagen. Many papers are also expected to be presented
at the 1998 Fall meeting of the American Geophysical Union.
updated Oct 12, 2004
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