The DUCK94 Coastal Field Experiment - Overview
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IntroductionSince 1979, the Coastal and Hydraulics Laboratory (formerly the Coastal Engineering Research Center) of the US Army Engineer Waterways Experiment Station has hosted a series of increasingly complex, multi-investigator, multi-agency nearshore field experiments at its Field Research Facility (FRF) located in Duck, North Carolina, USA. Three experiments, DELILAH and DUCK94, and SandyDuck, all evolved from scientific and pragmatic successes of prior work at this site, 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 have added components to resolve sediment transport and morphologic evolution at bedform scales from ripples to nearshore bars. DUCK94 was designed as a pilot effort to test instruments and procedures required for the more comprehensive SandyDuck experiment. This document summarizes DUCK94 including participants, environmental conditions, data collected, and data availability.
DUCK94Success of DELILAH, and the evident need for more detailed information about sediment transport and morphologic evolution that results from hydrodynamic forcing, initiated interest in further field work to be supported by the US Army Corps of Engineers, Office of Naval Research, the Naval Research Laboratory and the US Geological Survey. A plan for two additional field experiments developed. The first, DUCK94, was intended as a test run for new instrumentation, a more formal experiment organization, and more complicated logistics in preparation for SandyDuck, the second experiment. DUCK94 was scheduled for August and October 1994 to take advantage of the synergy offered by the National Science Foundation's Coastal Ocean Processes (CoOP) experiment (Butman, 1994), being conducted at the FRF during that time. The following focus topics were established as fundamental to improved understanding of surf zone sediment transport:
a. small and medium scale sediment transport and morphology;
Considerable interest was expressed for DUCK94. Table 1 lists the 19
organizations that participated, involving more than 100 scientists, students,
and technicians. Instrument measurements were complemented by observations
from ground- and aircraft-based radar and video systems. Table 2 lists
the 31 basic studies, along with the principal investigators, their primary
focus areas, and experiment durations. The extensive instrumentation resulted
from consideration of relevant measurement scales required to address SandyDuck
science objectives. Guidance was provided by using measured velocity data
from DELILAH and sediment transport modeling. Based on this analysis, a
general nearshore instrumentation array was designed (Birkemeier &
Thornton, 1994). The full array, shown in Figure 1, was used during the
October phase of DUCK94. An abbreviated form of this array was used in
the August segment of the experiment. Formal dates for DUCK94 were 8-24
August and 1-24 October, though some investigations (Table 2) of various
durations were underway between June and November.
A wide variety of instrumentation was used in DUCK94. Conventional total-station surveying techniques were used in subaerial morphology studies (29, referring to investigations by experiment number in Table 2), minigrid surveys (15), and positioning of all stationary instruments. Central to the main layout were cross-shore arrays of instrument clusters (11), each containing an electromagnetic current meter, a pressure gauge, an acoustic altimeter, and a thermometer (Fedderson, et al.,1997). The altimeters permitted the first comprehensive real-time measurements of bottom changes (8) (Gallagher, Elgar & Guza, 1997). A large number of suspended sediment concentration gauges were deployed, including optical backscattering sensors (16, 22, 26, 30), and less intrusive fiber- optic backscattering sensors (1). The Coherent Acoustic Sediment Probe (Stanton & Thornton, 1997) was mounted on a mobile sled along with current meters, pressure gauges, scanning sonars, and void fraction sensors (20, 25, 26). The Sensor Insertion System,
located on the FRF pier, provided a stable, mobile platform for sediment
transport measurements during high-energy conditions (1, 22). In situ (16)
and CRAB-mounted (25) side-scan sonars provided observations of bottom
bedforms, including megaripples. Most array positions included one or more
current meters (1, 3, 11, 12, 13, 15, 16, 18, 22, 26, 30). Incident wave
conditions were monitored with directional wave buoys (6, 19), and a direction-sensing
array of pressure gauges (21).
Dynamics measurements were complemented by a series of geologic studies that included surface sediment samples (24) (Stauble & Cialone, 1997), short cores, box cores, and vibracores (4). Several remote sensing systems were used. Surf zone and swash processes were observed with tower-mounted video systems (14, 17, 20). Observations were also made with land-based marine radar systems (27), coherent radar systems (10), airborne synthetic aperture radar, topographic lidar, visible and hyperspectral light imaging, and scanning radar altimetry (5, 7, 28). Three studies examined fundamental nearshore acoustic behavior (16, 23, 31).
Environmental conditions during the October phase of DUCK94 are illustrated in Figure 2. Two high-wave events occurred. The first was on 2-4 October, wherein wave heights exceeded 2.5 m. Wave heights reached 4.5 m during the second storm, an eight-day event beginning on 10 October. During the larger storm, large bottom changes were accompanied by a complex nearshore circulation pattern wherein wave-driven currents in the surf zone were opposed by strong wind-driven longshore flows offshore. As shown in Figure 2, currents in the nearshore trough changed from about 1 m/s to the south at the beginning of the storm on 10 October to about 1 m/s to the north just prior to the peak of the storm on 15 October.
Figure 3 illustrates four of the 12 minigrid surveys collected during October. Following a pattern similar to that observed in DELILAH, the bar moved offshore and became more linear in the initial part of the 10 October storm. High waves prevented daily surveys until 21 October, when the survey revealed that a very large rip channel had developed. Evolution of this channel is evident in video time exposure images depicted in Figure 4. Sequences of profile data through the region of the rip are shown in Figure 5, where it is seen that the bar crest moved 100 m seaward, causing 1.2 m of deposition at its most seaward observed location on 18 October. By 21 October, the bar crest had begun migrating landward.
DUCK94 data are being analyzed, and research results are beginning to appear in the literature. Preliminary findings were discussed at a post-experiment meeting (summarized by Long & Sallenger, 1995), where adequacy of the DUCK94 experiment plan was also evaluated in preparation for SandyDuck.
Data AvailabilityThe investigators agreed on a data sharing policy that offers protection of data by collecting investigators, encourages collaboration, and provides for eventual public release. This policy is:
a. global release of all data three years after the experiment;The DUCK94 data are now available from the DUCK94 Data Server. The server provides easy access to the data through a html browser or ftp. Much of the data are in the form of processed statistics with some "raw" time series data are available as well. The majority of the "raw" data will be included by summer 2001. A discussion of the individual DUCK94 experiments, including tables listing sensors, data sets, and a summary of results, findings and publications, are available through this web link.
ReferencesBirkemeier, W. A. and Thornton, E. B., 1994, "The DUCK94 Nearshore Field Experiment," Proceedings of the Conference on Coastal Dynamics '94, 815-821.
Butman, C. A., 1994, "CoOP: Coastal Ocean Processes Study," Sea Technology, 35:1, 44-49.
Fedderson, F., Guza, R. T., Elgar, S., and Herbers, T. H. C., 1997, " Cross-shore Structure of Longshore Currents during DUCK94", Proceedings of the 25th International Conference on Coastal Engineering, Orlando, FL, ASCE.
Gallagher, E. L., Elgar, S., and Guza, R. T., 1997, "Observations and Predictions of Sand Bar Motion," Proceedings of the 25th International Conference on Coastal Engineering, Orlando, FL, ASCE.
Long, C. E. and Sallenger, A., 1995, "Experiment at Duck, N.C. Explores Nearshore Processes," EOS Transactions of the American Geophysical Union, 76:49.
Stanton, T. P. and Thornton, E. B., 1997, "Reynolds Stress and Small-Scale Morphology Measurements during DUCK94," Proceedings of the 25th International Conference on Coastal Engineering, Orlando, FL, ASCE.
Stauble, D. K., and Cialone, M. A., 1997, "Sediment Dynamics and Profile
Interactions: DUCK94," Proceedings of the 25th International Conference
on Coastal Engineering, Orlando, FL, ASCE.
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