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SandyDuck '97 General Overview

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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!

Background

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.

SandyDuck Experiments

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.

Table 2.   SandyDuck Experiments
No.	Investigators links to data	Experiment Title links to descriptions
1 - RAB1	Beach, Holman, Sternberg, Ogston, Conley	Fluid-sediment interactions in the surf zone		X	X		X
2 - TGDZ	Drake, Snyder	Side-scan sonar studies of nearshore morphology in the vicinity of Duck, NC						X
3 - JPD	Dugan	Nearshore measurements for long-range remote sensing		X				X
4 - ED1	Edson	Application of a marine surface layer model to the Coastal Environment			X
5 - SPUV	Elgar , Herbers, O'Reilly, Guza	Surf zone waves currents and morphology	X	X		X		X
6 - FBWV	Friedrichs, Brubaker, Wright, Vincent	Cross-shoreface suspended sediment: a response to the intersection of nearshore and shelf processes		X	X		X
7 - JHGG	Haines, Gelfenbaum,Wilson	Vertical structure, bedforms, turbulence		X	X		X
8 - UFDH	Hanes,Vincent	Near bed intermittent suspension		X			X
9 - AJB	Hay, Bowen, Doering, Zedel	Nearshore sediment dynamics: suspension, bedforms, and bubbles		X	X		X		X
10 - HEIT	Heitmeyer	Surf-noise experiment							X
11 - HER	Herbers, O'Reilly, Guza	Wave propagation across the continental shelf	X
12 - THAS	Holland, Sallenger	Swash zone morphology				X
13 - ROBH	Holman	Large scale morphology						X
14 - PAH	Howd, Beavers	Geologic signature of storm events on the inner continental shelf and outer surf zone						X
15 - PAH2	Howd, Hathaway	Shoreface processes and bed response	X	X				X
16 - RJ	Jensen	Evolution of wave spectra in shallow water	X
17 - JOL	Jol	Ground penetrating radar of the beach						X
18 - TL	Lippmann	Observations of nearshore wave breaking, whitecapping, and large scale sand bar morphology	X		X
19 - JL	List	Regional shoreline change						X
20 - FRF	Long	Wind wave frequency‑direction spectral measurements	X
21 - HCM	Miller, Resio	Sediment transport rates during storms		X	X		X
22 - SAL	Sallenger	Coastal applications of scanning airborne laser (LIDAR)						X
23 - SMT	Smith	Observations of waves and currents near the surf zone	X	X
24 - SU	Su, Teague	Coastal breaking wave and bubble measurements							X
25 - SV	Svendsen, Grosskopf	Models of nearshore circulation	X	X
26 - EBT	Thornton, Stanton	Nearshore wave & sediment processes	X	X	X		X
27 - DT	Trizna, Kirby	Experimental tests of Boussinesq wave models in the near surf zone	X	X
28 - DT	Trizna	Marine radar remote sensing of bar & rip morphology						X
29 - DT	Trowbridge	Measurement of bottom stress in the wind- and wave-forced nearshore environment	X	X	X
30 - DTEW	Wu, Shih, Kobayashi	Nearshore water level profiles during storms	X						X

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 directions.

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)

RESULTS

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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