SandyDuck '97 Experiment Descriptions

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Layout of the "Minigrid" surf zone experiments.

Layout of experiments outside the Minigrid zone.
 

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Table of Contents

Beach, Holman, Sternberg, Ogston, Conley Fluid-Sediment Interactions in the Surf Zone
Drake, Snyder Side-Scan Sonar Studies of Nearshore Morphology in the Vicinity of Duck, NC
Dugan Nearshore Measurements for Long-Range Remote Sensing
Edson Application of a Marine Surface Layer Model to the Coastal Environment
Elgar, Herbers, O'Reilly, Guza Surf zone Waves, Currents, and Morphology
Friedrichs, Brubaker, Wright, Vincent Cross-Shoreface Suspended Sediment: A Response to the Intersection of Nearshore and Shelf Processes
Haines, Gelfenbaum, Wilson Vertical Structure, Bedforms, and Turbulence
Hanes, Vincent Near Bed Intermittent Suspension
Hay, Bowen, Doering, Zedel Nearshore Sediment Dynamics: Suspension, Bedforms, and Bubbles
Heitmeyer Surf-Noise Experiment
Herbers, O'Reilly, Guza Wave Propagation Across the Continental Shelf
Holland, Sallenger Swash Zone Morphology
Holman Large Scale Response
Howd, Beavers Geologic Signature of Storm Events on the Inner Continental Shelf and Outer Surf Zone
Howd, Hathaway Shoreface Processes and Resulting Bed Response
Jensen Evolution of Wave Spectra in Shallow Water Part II
Jol Ground Penetrating Radar of the Beachface/ Shoreface, SandyDuck Experiment
Lippmann Observations of Nearshore Wave Breaking, Whitecapping, and Large Scale Sand Bar Morphology
List Regional Shoreline Change
Long Directional Wave Observations
Miller, Resio "Bulk" Sediment Transport Rates During Storms
Sallenger Coastal Applications of Scanning Airborne Laser (LIDAR)
Smith Observations of Waves and Currents Near the Surf Zone
Su, Teague Coastal Breaking Wave and Bubble Measurements
Svendsen, Grosskopf Field Measurements and Nearshore Modeling at SandyDuck
Thornton, Stanton Nearshore Wave and Sediment Processes
Trizna, Kirby Experiment Tests of Boussinesq Wave Models in the Near Shore Zone
Trizna Marine Radar Remote Sensing of Bar and Rip Morphology
Trowbridge Measurement of Bottom Stress in the Wind- and Wave-forced Nearshore Environment
Wu, Shih, Kobayashi Nearshore Water Level Profiles During Storms

FLUID-SEDIMENT INTERACTIONS IN THE SURF ZONE

Scientific Objective

This investigation focused on the horizontal spatial variations of bottom boundary layer processes and sediment transport associated with large-scale fluid forcing within the surf zone, e.g., edge waves, shear waves and mean currents.

Experiment Plan

Cross-shore and longshore arrays of instrumentation were deployed to investigate bottom boundary layer fluid-sediment interactions within the active surf zone. At each location, vertical profiles of horizontal velocities and suspended sediment, in addition to sea surface fluctuations, were recorded continuously (16 Hz sampling rate) during the high energy portion of the experiment. A typical pipe mount with VEMA (Vertical ElectroMagnetic Array) current meter and FOBS (Fiber-Optic Backscatter Sensor) is shown in the figure.

The instrument layout is shown in the following figure which also shows the position of all other sensors deployed within the surf zone. Note that this map shows only instrument locations not the individual sensors. Refer to the descriptions of the individual experiments to determine what instruments were deployed.

Funding

Funding was provided by the Coastal Sciences Program of the Office of Naval Research, Dr. T. Kinder, Program Manager.

Click here to view Beach and Holman's 1997 ONR Year End Report (205 kb) and here for Sternberg and Ogston's (10 kb) on this project in Adobe PDF format (requires free Adobe Acrobat Reader).

Links to collaborative experiments:

• Go to Haines/Gelfenbaum
• Go to Hanes/Vincent
• Go to Hay/Bowen/Doering/Zedel
• Go to Thornton/Stanton

SIDE-SCAN SONAR STUDIES OF NEARSHORE MORPHOLOGY IN THE VICINITY OF DUCK, NC

Scientific Objectives

Digital side-scan sonar equipment was used to map nearshore morphology and estimate surficial sediment size in the vicinity of Duck, North Carolina. The study had two objectives:

• To obtain kilometers-long side-scan sonar images of surf zone and nearshore bathymetry including the DUCK94 and SandyDuck field experiment areas at the FRF; to provide an expanded morphological and geological context for SandyDuck and future nearshore experiments at the FRF. After SandyDuck we will conduct high-resolution seismic surveys and seasonal side-scan sonar surveys in the Duck area through 1999. Seismic mapping will help delineate areas in which sediment supply is limited; these so-called hardbottom areas are outcrops of cohesive or lithified substrate on the shoreface. They exert a controlling influence on the gross nearshore morphology of the N.C. barrier islands, but their presence and effect on sediment transport processes has not been incorporated into any models for nearshore sediment transport processes and the resultant nearshore morphology. This objective was funded by the Army Research Office, Terrestrial Sciences Program.
  
• To obtain detailed side-scan sonar images of the SandyDuck study area throughout the intensive phase of the experiment, using the FRF's CRAB as the sonar platform. Side-scan sonar images provide a synoptic picture of the seafloor morphology that directly complements other SandyDuck bathymetric measurements, in particular, those of Thornton/Stanton/Gallagher's CRAB-mounted acoustic altimeters. This objective was funded by the Office of Naval Research, Coastal Dynamics Program.

Experiment Plan

Digital side-scan sonar and high-resolution seismic mapping for large-scale surveys using a four-wheel drive amphibious truck (LARC) from the Army Coastal Engineering Research Center's Field Research Facility (FRF) conducted surveys extending several kilometers on either side of the FRF pier. Surveys were conducted four times each year, and additional surveys of selected areas were performed after storms and for bed perturbation studies. These observations primarily addressed morphological questions having time scales of order several months to years.

CRAB-based surveys were conducted during every mini-grid survey, conditions permitting. Energetic conditions created significant acoustic noise that could have prevented observations in the surf zone.

Instruments and Logistics

EdgeTech DF-1000 digital side-scan sonar towfish operating at nominal frequencies of 105 kHz and 390 kHz was rigidly mounted on the CRAB, or towed in close proximity to the LARC (see figure). While mounted on the CRAB, position and orientation was obtained from CRAB DPGS and ancillary instruments (see Thornton et al.). Similar DGPS hardware provided locational information for LARC surveys. Our work provided supporting information for several SandyDuck investigations of large-scale beach and nearshore processes, notably Holman,  List, and Howd and Beavers among others.

Funding

This work was funded by the Army Research Office, Terrestrial Sciences Program and Office of Naval Research, Coastal Dynamics Program.

Click here to view the 1997 ONR Year End Report (235 kb) on this project in Adobe PDF format (requires free Adobe Acrobat Reader).

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NEARSHORE MEASUREMENTS OF LONG-RANGE REMOTE SENSING

 

John Dugan Arete Associates 1725 Jeff Davis Hwy, Suite 703 Arlington VA 22202 PH: (703) 413 0290 FAX: (703) 413 0295 E-mail: dugan@arete-dc.com

Scientific Objective

The objectives were:

• to provide in situ data on waves, currents, tide level, wind, and bathymetry for interpretation of remote sensing imagery and for comparison with these parameters as extracted from imagery; and

• to test an instrumented jet ski and compare measured waves, currents and bathymetry with other in situ instrumentation.

Experiment Plan

The approach was twofold. First, to collaborate with FRF and other SandyDuck investigators to provide the ONR Littoral Remote Sensing program with data that was collected during SandyDuck, specifically directional wave spectra, tide level, currents, small-scale bathymetry, wind vector, surf characteristics, and beach slopes.

Second, to field a jet ski equipped with real-time-kinematic DGPS, fathometer, and current meter to collect bathymetry and current data that otherwise would not be available. The jet ski (shown at right) was operated outside the SandyDuck surf zone array to make fine scale bathymetry surveys and collect rip current data on a long stretch of beach mostly outside the FRF property lines. The bathymetry data were supplemented with several all-terrain-vehicle surveys of beach topography to provide continuous profile data from above the high water line through the surf to about 8 m depth. The currents were collected by running alongshore lines outside the breaker line.

In addition, a remotely controlled video camera installed on the research tower was operated to collect data on the surf, the shoreline, and the waves outside the breaker line.

Funding

The equipment was available, and funding for data collection and analysis was provided by the Littoral Remote Sensing Program of the Office of Naval Research.

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APPLICATION OF A MARINE SURFACE LAYER MODEL TO THE COASTAL ENVIRONMENT

Scientific Objective

The objectives were:

1. to adapt an existing Eulerian model of turbulent flow to investigate the structure of the coastal atmosphere over shoaling waves;

2. to use oceanic measurements to determine appropriate boundary conditions for the model at the sea surface; and

3. to test model predictions by comparing them with atmospheric turbulence measurements. The model will ultimately be used to examine the production and transport of sea-spray in the coastal environment using a Lagrangian approach.

Experiment Plan

One sonic anemometers was deployed to measure the wind stress and any horizontal or vertical flux divergence downwind of the surf zone. The anemometer was deployed at the end of the research pier on a mast near the FRF's existing K-Gill anemometer.

Funding

Funding for equipment, deployment and recovery was obtained from a number of different sources. Funding for analysis was requested from ONR.

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SURF ZONE WAVES, CURRENTS, AND MORPHOLOGY

Scientific Objective

The long-term goal is to understand the interactions between complex and changing bathymetry, waves, and the quasi-steady nearshore circulation. Specific goals for the SandyDuck experiment included observing and modeling in the nearshore and surf zone:

• the evolution of directionally-spread swell and sea
• the near-bottom, quasi-steady circulation
• the evolution of bar-scale morphology
• the mean water level across the surf zone and within the foreshore

Experiment Plan

33 Collocated sonar altimeters, 69 pressure gages, and 33 velocity sensors were deployed in a 2D grid, 200 (longshore) X 420 (cross-shore) m extending from about 1 to 5 m water depth using the mounting frame shown right. The array encompassed the region of strongest bar motion and morphological change and was large enough to include a significant bathymetric inhomogeneity (based on DUCK94 bathymetry) so that effects of irregular bathymetry on waves and circulation could be investigated. The array was also long enough to resolve directional properties of incident, infragravity, and shear waves (based on SUPERDUCK and DELILAH results). 'Compact' arrays of PUV gages embedded in the main cross-shore transect provided independent estimates of wave direction and may have resolved narrow rip currents and the (high) wavenumbers of bar-intensified edge waves at incident wave frequencies. The mean water level was measured with a single cross-shore array of buried pressure sensors. The instrument array is illustrated in the figure below.

A plan view of the array is in the figure below. Sensor types, indicated in the legend, are P = pressure, SPUV = collocated sonar altimeter (S), pressure gage (P), and biaxial electromagnetic flowmeter (UV), PUV = pressure gage and flowmeter, X = buried Paroscientific pressure gage. Click for the SandyDuck surf zone layout including this experiment. 

Funding

Funded by the Office of Naval Research

Results

Click here to view the 1997 ONR Year End Report (90 kb) on this project in Adobe PDF format (requires free Adobe Acrobat Reader). A summary of data collected is provided in four text files. A list of instruments, their numbering and location is included in spuv_sensors.txt., for summaries by month of instrument operation select spuv_aug.txt, spuv_sep.txt, or spuv_oct.txt

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CROSS-SHOREFACE SUSPENDED SEDIMENT TRANSPORT: A RESPONSE TO THE INTERSECTION OF NEARSHORE AND SHELF PROCESSES

Scientific Objective

This study observationally and theoretically examined sediment suspension, fluid movement, and sediment transport across the lower shoreface (h = 8 - 20 m). The shoreface is the pathway for exchange of fluid and sediment between the surf-zone and the shelf, and physical understanding of long term erosion or deposition in either environment requires an understanding of processes in this transition zone. Specific motivation for improved analytic models and observations of cross-shore suspended sediment transport is provided by extreme inconsistencies in predictions of net transport by commonly applied models and limitations in the ability of typically deployed instrumentation to accurately represent suspended sand transport over the lower shoreface.

In this investigation new analytic models for cross-shoreface sediment and fluid motion will be developed which better recognize the simultaneous contributions of wave and wind forcing. Acoustic instrumentats, more capable of accurately quantifying cross-shore sediment and fluid motion than in previous studies were deployed. Analysis of these data will better constrain analytic models by providing insight into such issues as: the nature of eddy diffusivity and viscosity; orbital phase-dependent sediment concentration; wind- and wave-forced cross-shore mean currents; and cross-shore suspended sediment transport as a function of along-shore current strength, relative roughness, wave period and grain-size.

Experiment Plan

Two instrumented benthic boundary layer tripods, each based around a stainless steel frame 2.5 m high and 3 m across at the base, were deployed on the lower shoreface. for a month in the summer near the beginning SandyDuck and again for a month in the fall near the end of SandyDuck.

Experiment Layout

VIMS benthic boundary layer tripods typically support 5 Marsh-McBirney electromagnetic current meters and 5 Downing optical backscatter sensors at 0.1, 0.4, 0.7, 1.0 m and 1.3 m above the base; a Paroscientific pressure sensor at 2.5 m; and a thermistor at 0.7 m. To supplement this standard tripod configuration, sand concentration was also measured at each tripod using vertically profiling acoustic backscatter sensors. In addition, all three velocity components plus acoustic backscatter were provided at each tripod by one or two Sontek Acoustic Doppler Velocimeters (ADV) mounted near the bed. More complete density information was provided by the addition of salinity sensors. Finally, velocity throughout the water column was measured at each pod site by an Acoustic Doppler Current Profiler (ADCP) deployed in an upward looking mode. The water depths were about 8 m and 12 m for the lower energy summer deployment, and about 12 m and 20 m for the higher energy fall deployment. Deployment locations were coordinated with Peter Howd and Kent Hathaway who also investigated shoreface processes. The positions are shown on the map below.

Funding

Funding was provided by the National Science Foundation, Oceanography Division, jointly between the Marine Geology & Geophysics Program and the Physical Oceanography Program, NSF Grant No. OCE-9504198.

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VERTICAL STRUCTURE, BEDFORMS, AND TURBULENCE

Scientific Objective

Our objectives were to observe and to test existing models of the vertical structure of mean currents within and outside the surf-zone. In particular we addressed:

1. the temporal and vertical variability in the turbulent flow field;

2. the potential for surface enhancement of mean flows and turbulent fluctuations associated with breaking waves;

3. the evolution of sedimentary bedforms and the influence on turbulence generation and parameterizations of bed roughness.

Our primary approach was to deploy a vertical stack of Acoustic Doppler Velocimeters (ADV) while making simultaneous observations of breaking waves (Video monitoring with Tom Lippman) and bedform development (rotary side-scan, with Alex Hay).

Experiment Plan

A vertical stack of Acoustic Doppler velocimeters (shown at right) was deployed outside the primary sandbar in 3-4 m water depth. A supplementary deployment in close proximity provided a platform for rotary and pencil-beam side-scan deployment. Ancillary instrumentation included pressure, speed of sound, and suspended sediment (Acoustic). An in situ data acquisition system was networked (fiber optic) to acquisition control and storage on the beach. In collaboration with Peter Howd, deployed 2 Acoustic Doppler Current Profilers (ADCP) in intermediate depths to permit a secondary focus on the role of wind-forced flows.

Instruments and Logistics

Primary - 8 SonTek Acoustic Doppler Velocimeters, 8 Compass/Tiltmeter Packages, 2 Pressure sensors, 2 Speed of sound sensors, 1 Rotary Side-scan Sensor, 1 Acoustic Altimeter

Secondary - 2-3 Marsh-McBirney Electromagnetic Current Meters, 2-3 Pressure Sensors, 1 Pencil Beam Acoustic Sounder, 1 Acoustic Altimeter, 2 ADCP's

Experiment Layout

Instruments were deployed on cross-shore transect along with Hay/Bowen/Doering/Zedel and Beach/Holman/Sternberg/Ogston/Conley.. Research interests located stack on outer flank of bar. Secondary instrumentation (PUV, ADCPs) was utilized to fill-in cross-shore transect. Click here to view the SandyDuck surf zone experiment layout.

Results

For a listing of instruements, locations, and collected data: haines.txt

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NEAR BED INTERMITTENT SUSPENSION

Scientific Objective

The phenomena investigated were the interactions between fluid and sediment near the seabed in regions significantly influenced by surface gravity waves. Emphasis was on the interactions resulting in momentum and energy transfer between fluid and grains near the seabed, and the influence of those interactions upon the local transport of sediment. The ultimate objective being to develop the capability to model, predict, and control coastal sediment transport and associated bathymetric change. Achievement of this objective required significant improvement of our understanding of the physical interactions between fluid and grains near the seabed, as well as the development of models derived from our understanding of the relevant physical processes. Collected measurements also provided a useful tool for determining the accuracy of theoretical models of the time averaged concentration profile and time averaged suspended sediment flux.

Experiment Plan

Our field observations focused on measurement of small scale sediment processes with emphasis on the measurement of suspended sediment concentration, and bedform geometry. An instrument array (see figures below and at right) consisting of an acoustic concentration profiler (ACP), pressure transducer, Acoustic Doppler Velocimeter (ADV) , underwater video camera, rotating side-scan sonar, multiple transducer array (MTA) ripple profiler, optical backscatter sensor (OBS) and a data acquisition package was deployed in approximately 4 meters water depth (see layout figure). A similar array of instruments was also deployed off of the FRF pier using the Sensor Insertion System (SIS) at a variety of cross-shore locations with depths between 1 and 6 meters. The data permit examination of wave and current induced sediment suspension processes over time scales ranging from approximately one second to hourly. The influence of bedforms upon the local suspension of sand was the focus of some of the experiments. In particular, we documented the transition from a rippled bed to plane bed conditions, and the effects of this transition upon the associated suspended sediment field.

Experiment Layout

This experiment was part of the surf zone array. Click here to view the SandyDuck surf zone experiment layout.

Funding

Funding for this study was provided by the Coastal Dynamics Program, ONR.

Click here to view the 1997 ONR Year End Report (258 kb) on this project in Adobe PDF format (requires free Adobe Acrobat Reader).

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NEARSHORE SEDIMENT DYNAMICS: SUSPENSION, BEDFORMS, AND BUBBLES

Scientific Objective

We conducted three inter-related studies of nearshore sediment dynamics, with emphasis on the processes active within the surf zone:

1. sediment suspension mechanisms;
2. bedform scales, migration velocities, development rates; and
3. ambient noise generation and propagation in the surf zone.

The project addressed sediment dynamics issues mainly within the surf zone -- specifically, in the nearshore trough and in the immediate vicinity of the sandbar crest. Central objectives were:

1. To investigate the processes contributing to sediment suspension under different bedform and fluid forcing conditions; and to test our understanding of the net local suspended sediment flux balance;
2. To determine the time and space scales of bedform development and migration as a function of wave and current forcing parameters, at different points within and immediately outside the surf zone, simultaneously;
3. To investigate the roles of long waves and low frequency currents in nearshore sediment dynamics;
4. To investigate wave breaking in the surf zone; and the effects of wave breaking intensity and bubble plume penetration on sediment suspension, bedform development, and sound propagation;
5. To test models of bedform growth and bedload transport;
6. To estimate the relative importance of bedform and suspended load transport in the natural surf zone.

Experiment Plan

The approach involved a 6-node, L-shaped array, with 20- to 50-m horizontal spacing between adjacent nodes. At each node, rotary pencil-beam and fan-beam sonars imaged the bedform field. Combination pressure gauge and current meter (PUV) sensors monitored the local hydrodynamics. Fixed upward- and downward-looking sonars monitored sediment suspension and bubble cloud penetration. The central node in the array had additional sensors, including a newly developed 3-component coherent Doppler profiler, for velocity and suspension measurements. Near four of the nodes, broadband hydrophones were deployed on separate single pipes for ambient noise measurements.

The instruments deployed were: rotary fan and pencil beam sonars (5 and 6 ea, respectively), 3-component coherent Doppler profiler (1), fixed up- and down-looking sonars (4), broadband hydrophones (4), narrowband (50 kHz) projector (1), Marsh-McBirney flowmeters (12), pressure and temperature sensors (6), dual axis tilt sensors (6). Click here to see the full SandyDuck surf zone layout including the layout of this experiment.

Much of this work was collaborative. Dr. Len Zedel (Memorial University), participated in the coherent Doppler profiler work. Dr. Diane Foster (Dalhousie University), worked on bottom boundary layer models. Other collaborations included:

• R. Beach (Oregon State),low frequency variations in sediment suspension;
• J. Haines and G. Gelfenbaum (USGS),vertical structure and bottom roughness;
• D. Hanes (Florida),sediment suspension processes;
• T. Lippmann (Scripps), wave breaking statistics;
• T. Stanton and E. Thornton (NPS), sediment suspension mechanisms and bedforms, and bubble injection by breaking waves;
• P. Howd and R. Beavers (USF and Duke), bedform migration and resulting stratigraphy;
• S. Elgar (Washington) and R. Guza (Scripps), on surf zone bathymetry and bottom roughness; and
• R. Heitmeyer (NRL), on surf zone ambient noise.

Funding

Natural Sciences and Engineering Research Council of Canada and U.S. Office of Naval Research, Coastal Sciences Program.

Click here to view the 1997 ONR Year End Report (115 kb) on this project in Adobe PDF format (requires free Adobe Acrobat Reader).

Web Site

For additional information about this experiment including research progress since DUCK94 and a publication list, connect to: CANUCKDUCK '97

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SURF-NOISE EXPERIMENT

Scientific Objective

The surf-noise component of the SandyDuck experiment was designed to provide the experimental basis for development of a quantitative model for describing the space- time properties of surf-generated noise. The scientific objectives of the experiment measurements were three-fold:

1. identify the dominant sound-generation mechanisms (e.g., oscillations of bubbles generated in breaking waves, momentum transfer at the air-sea and the water-sediment interfaces during wave breaking, etc.)

2. determine an acoustic source model that represents those mechanisms in terms of observable parameters of the underlying physical processes.

3. determine the impact of the acoustic environment (sound speed, bathymetry, geoacoustic parameters) on the propagation of surf-generated noise to regions outside of the surf-zone.

To achieve these objectives, systems were deployed to measure:

• surface characteristics of individual breaking waves within a control volume (space-time occurrences and size);

• acoustic time-frequency radiation pattern of the sound generated by the individual breaking waves within that control volume;

• surf noise contribution generated from within the control volume and by the total breaking wave field;

• broadband acoustic propagation characteristics from the control volume.

The results of these measurements, together with those from other SandyDuck experiments (e.g. surf strength dependence on bottom morphology, wave-height directional spectra, etc.) are being used to establish the physical parameters for the surf noise model.

Experiment Plan

The instruments deployed for the four measurement systems were:

1. Breaking wave characteristics.- two video cameras mounted on masts on the FRF tower;

2. Acoustic radiation pattern. - a 300 m long, 31-element, linear hydrophone array deployed in the surf zone;

3. Surf noise characteristics: - a 40 m, 64 element, linear hydrophone array deployed outside of the surf zone;

4. Acoustic propagation characteristics: - a broadband acoustic source (J9) also deployed outside the surf zone.

The deeper hydrophone array is shown in the figure at right, ready for deployment from the FRF's LARC. Data on these systems was acquired over a seven week period during both daytime and nighttime hours with specific acquisition periods selected to provide a statistically significant sample over a full range of surf conditions.

All systems in the water were deployed along a line orthogonal to the shore about 250 m north of the pier (FRF longshore coordinate of 730 m) as shown in the following figure. The surf zone array (#2 above) extended from 200-500 m, with a denser phone spacing (24 phones 6 m apart) over the first 140 m and a sparse spacing (7 phones at 20 m) over the remainder. The transition zone array (#3 above) extended from 600-640 m about 300 m from the surf zone and was cabled to the pier. The acoustic source was located at 590 m offshore and also cabled to the pier. The two video cameras were mounted on the tower. One of these systems monitored breaking waves over the surf zone array, and the other monitored events over the transition zone array. Layout and cabling of the four systems and array design details are shown in the accompanying figures. Click here to view this experiment in the SandyDuck surf zone experiment layout.

Funding

Funded by ONR Base Funding 6.1 and 6.2 Research Programs.

Results

For a text file of instrument locations and collected data: Heitmeyer.txt

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WAVE PROPAGATION ACROSS THE CONTINENTAL SHELF

Scientific Objective

The long-term goal of our research is understanding the physical processes affecting surface gravity waves on the continental shelf. Specific goals for the SandyDuck experiment were to determine the effects of nonlinear wave-wave interactions on shoaling waves and estimating wave energy losses owing to wave breaking and bottom friction.

Experiment Plan

A coherent array of nine pressure sensors and a directional wave buoy were deployed on the inner shelf, 5 km offshore of the FRF in about 20 m depth. The measured frequency-directional spectra of wind waves and swell was used in conjunction with data from the FRF 8-m array (Long) and the surf zone arrays (Elgar/Herbers/O'Reilly/Guza) to investigate wave shoaling and breaking processes. Video measurements of wave breaking in the array vicinity were collected by Lippmann.

Instruments and Logistics

The battery powered, internal recording pressure sensors were attached to moored anchors on the seabed. The directional wave buoy was a surface-following Datawell Directional Waverider with a radio-link to the FRF. Differential GPS and an underwater acoustic navigation system was used to survey the instrument locations.

Experiment Layout

The pressure sensors P1-P9 were arranged in an equilateral triangle with dimension 500 m, centered at approximately 36 12.1 N, 75 41.9 W. The directional wave buoy W0 was located inside the pressure triangle array. Four guard buoys G10-G13 with lights and radar reflectors were deployed around the perimeter of the array. This layout is shown in the two figures below.

Funding

This experiment was funded by the ONR Coastal Dynamics Program.

Click here to view the 1997 ONR Year End Report (110 kb) on this project in Adobe PDF format (requires free Adobe Acrobat Reader).

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SWASH ZONE MORPHOLOGY

Scientific Objective

Our overall objective was to understand the evolution of three-dimensional (3-D) morphology in the foreshore region. The present understanding of basic sediment transport mechanisms in this region has been constrained by the complexity and richness of foreshore processes, which are typically nonlinear and have the potential for feedback between forcing and response. By quantifying and monitoring patterns of net sediment transport and swash characteristics over a longshore length scale of approximately 100 m (several cusp wavelengths), we increased our ability to model this interaction. Our primary approach was to utilize multi-camera stereo-video techniques to measure beach surface profiles in the swash zone and to determine spatial and temporal gradients in swash flows. Of particular interest was the relationship between swash excursion patterns and the resulting profile change. We collaborated with Elgar, Raubenheimer, and Thornton in an effort to tie our observations to other processes including setup, groundwater, and tidal influences.

Experiment Plan

We mounted cameras on the FRF tower, near the dune line and on the FRF pier overlooking a ~100 m section of foreshore to the north side of the pier. Two overlapping study regions were designed. Relatively coarse, but frequent measurements were made in the larger region centered almost directly offshore (slightly north) of the FRF tower and extending approximately 50 m in each direction. Temporary towers (see figure at right) near the dune line were installed to supplement the coverage and to more throughly resolve the smaller 30x30 m subregion. These video-based foreshore surveys occurred round the clock. A small number of ground truth surveys and sediment samples were collected. The below shows the general layout.

Funding

This research was sponsored by the US Geological Survey and through base funding to the Naval Research Laboratory from the Office of Naval Research.

Results

For a listing of collected data: Holland.txt

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LARGE SCALE RESPONSE

Scientific Objective

The overall objective of this work was understanding the dynamics of the fluid field over shoaling, complex bathymetry and the response of that bathymetry to those fluid motions. Our principle science goal was to provide a larger-scale context for the intensive experiment area. In space, we sampled morphology (by time exposure) over a 5 km region centered on the pier, and foreshore bathymetry (by Argo shown at right) over a 2 km region centered on the pier. In time, the intensive period of field work would be the culmination of a nested program that included analysis of the previous decade of Argus images and a three-month daily sampling of foreshore beach profiles from August-October, 1996.

To supplement in-situ instruments and to ground truth some of our video techniques, we collected extensive time stack and pixel time series video data over a superset of the intensive in-situ region.

Experiment Plan

Video data were collected from a plethora of video cameras of appropriate focal length mounted on the FRF tower and a new south tower (cooperative with Lippmann). . These fed the image processors needed to provide time exposure coverage of the area described above and pixel time series coverage of an appropriate number of locations. Dry beach profiles were measured by phase-differential GPS mounted on an amphibious ARGO, the all terrain vehicle shown at right. Argo surveys were independent. Video coverage spanned 2.5 km on either side of the pier, while Argo surveys covered 1 km either side of the pier. Click here for detailed description of the observations with sample images

Funding

Funded by Office of Naval Research.

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GEOLOGIC SIGNATURE OF STORM EVENTS ON THE INNER CONTINENTAL SHELF AND OUTER SURF ZONE

Scientific Objective

Shoreface studies have involved collection of sediment cores and measurements of nearshore wave activity, longshore and cross-shore currents, sediment transport and seabed morphology. Rarely are these measurements combined to investigate the genesis of near surface stratigraphy.

• By combining these measurements, this research tested two primary hypotheses, that a known sequence of bedforms and bed elevation changes would be uniquely reflected in preserved shoreface stratigraphy, and that knowing these relationships for 6 sites, stratigraphic features can be used to hindcast surficial bedform types at 3 sites (the Hathaway/Howd bipods at -5.5 m, -8 m and -13 m) where flow conditions and bed elevation changes are known (but the bedform evolution is not documented).

• Secondary objectives were to supplement other SandyDuck coring and sediment sampling experiments, and to provide larger scale sediment continuity for investigators. This work began in the summer of 1996 and constructively overlapped the efforts of most geological/morphological investigators including Drs. Drake and Snyder (NCSU).

Experiment Plan

Since 1994, 3 bipod instrument packages in 5.5, 8, and 13 m of water on the Duck, NC shoreface have monitored near bed flows and the resulting bed elevation changes. The locations supported multiple (3) Acoustic Doppler Velocimeters (ADV), a pressure sensor, and an acoustic altimeter. An upward looking broad band Acoustic Doppler Current Profiler (ADCP) was located at the 13 m site. In 1996, 31 near surface (30 cm deep) diver-collected boxcores were used to investigate the stratigraphy preserved near this deployed instrumentation. Since geologic cores are rarely collocated with measurements of the physical processes and observations of bed elevation changes, the agreement between erosional contacts and thickness of depositional units in cores taken adjacent to this instrumentation and bed elevation changes measured by an acoustic altimeter is remarkable and very encouraging. Processing of the cores are shown at right.

Collaboration with Drs. Alex Hay and Tony Bowen used measurements and images of the seabed from their high-resolution fan-beam and pencil-beam rotary sonars at 6 shoreface locations to track bedform genesis and migration. Resolution of their sensors enabled tracking the types of bedforms, migration velocity, and sediment elevation throughout storm conditions. Boxcores were collected near the sites of rotary sonars during sensor deployment, pre-storm, post-storm(s), and prior to instrument retrieval in November. Some coring locations consisted of 2 orthogonal cores to better define orientation of bedding structures. In other locations, arrays of multiple cores were used to define larger scale sedimentary structures.

Funding

US Army Corps of Engineers, Coastal Research and Development Program

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SHOREFACE PROCESSES AND RESULTING BED RESPONSE

Scientific Objectives

The primary objective was to understand the dynamics of wind-driven currents, wave-driven currents and wave orbital velocities as a function of cross-shore position and forcing conditions across the surf-zone/inner shelf boundary (or shoreface). An allied objective was to combine the flow measurements with bed elevation data from the acoustic altimeters to examine bottom shear stresses and the resulting evolution of the shoreface at time scales much longer than the SandyDuck experiment. A long data set is necessary due to the qualitative observations of non-linear relationships between sequences of storms, existing morphology, and resulting profile response previously made at the FRF. A wide range of environmental and morphological conditions are required to begin to understand the nature of the non-linear relationship(s) involved. Secondary objectives were to supplement the SandyDuck cross-shore arrays, and to provide large scale continuity between the DUCK94/CoOP measurements and SandyDuck. This work constructively overlapped the efforts of most investigators as these were some of the few measurements of the flow field deeper than the -5 m contour.

Experiment Plan

We established three instrument locations in 5.5 m, 8 m, and 13 m water depths for the period spanning the DUCK94 and SandyDuck experiments (see photo at right of "bipod" frame). Each site included 3 Acoustic Doppler Velocimeters (SonTek ADV Ocean probes) located in the bottom 1.5 m of the water column, a pressure sensor, and an acoustic altimeter. Each site also had an upward looking Acoustic Doppler Current Profilers (ADCP), shown at right, for defining the flow profile as a function of depth in the mid and upper portions of the water column. The Bipod instruments remained in the water through the winter following SandyDuck. This work is closely allied with Howd and Beavers sediment coring experiment. The Bipods are extant and can be seen on the map of instruments outside the Minigrid zone.

Funding

US Army Corps of Engineers, Coastal Research and Development Program and the Office of Naval Research, Defense University Research Instrumentation Program

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EVOLUTION OF WAVE SPECTRA IN SHALLOW WATER PART II

Scientific Objective

Ocean wave spectra transforms significantly once it enters shallow water. These transformations are based on specific mechanisms: spatially-dependent, and time-dependent processes imbedded in the source/sink terms of the energy balance equation. These mechanisms (atmospheric input, nonlinear wave-wave interactions, high frequency breaking, wave-bottom effects, depth induced breaking as well as refraction/shoaling) simultaneously act on a spectrum when it enters shallow water. The relative magnitude of each mechanism and its resulting change in the spectrum as well as the time required for the change are not well understood.

Experiment Plan

Sufficient high resolution directional wave spectra are required to adequately investigate the problem. The experiment plan used simultaneous measurements at the FRF from a National Data Buoy Center offshore directional buoy (NDBC buoy 44014); a newly deployed Dateawell Directional Waverider buoy which is part of the FRF measurement program, and data obtained by Chuck Long from the FRF's 8-m Array. We decompose the processes from the directional estimates, test hypotheses using 3rd generation wave model technology assessing all terms in the energy balance equation and then apply what was obtained from the data and model tests to present numerical wave models.

Experiment Layout

The waverider buoy is FRF Gauge number 630 and is on the map of instruments outside the Minigrid zone.

Funding

Funding by the Coastal Navigation Hydrodynamics Program of the US Army Corps of Engineers.

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GROUND PENETRATING RADAR OF THE BEACHFACE / SHOREFACE, SANDYDUCK EXPERIMENT

Scientific Objectives

Ground penetrating radar (GPR) investigation of the beach and shoreface deposits. Two-dimensional and three-dimensional grid datasets were collected to compare with offshore data. The preservation of depositional features is important in understanding what processes offshore are dominate in forming onshore deposits. The results provided analogues to both oil and gas reservoir and hydrogeology models. A similar experiment was conducted in a high energy coastal area along the West Coast. A comparison is valuable to look at differences and similarities in depositional patterns.

Experiment Plan

A portable, digital ground penetrating radar (GPR) system with a variety of antennae and transmitter powers was used to survey the beach and dune as shown in the figure. Test surveys at several locations were conducted to test the feasibility of GPR at the site and decide on appropriate instrument configuration. Following the test phase, survey lines (parallel and perpendicular to the beach ) were run across the dune to the beach above the high tide line.

Funding

Funded by the University of Wisconsin-Eau Claire, University Research and Creative Activity grant and Faculty-Student Research collaboration.

Results

There are four images are all very much in the early interp/processing stage - they do show interesting results. The data were collected with a Sensors and Software pulse EKKO 100 ground penetrating radar system. The profiles have been corrected for topography. All profiles are presented from west to east (ocean). Data is collected with a 500 ns time window and 800 ps sampling rate.

The first three - are from the northern portion of the SandyDuck site going over the dune along the access road. Duck1 is shown at right, click the others for a larger image.

Duck1 (36k) - 100 MHz antennae - 0.5 m step, antennae separation 1.0 m
Duck2 (57k) - 200 MHz antennae - 0.25 m step, antennae separation 0.5 m
Duck3 (35k) - 50 MHz antennae - 0.5 m step, antennae separation

This dataset shows a comparison of different frequency. The 200 MHz showing higher resolution (good foredune stratigraphy) but with less depth of penetration while the 50 MHz antennae show greatest depth of penetration but with less resolution. In general, the profiles show good internal dune stratigraphy and indicate that the sediment below and behind the dune are inclined and dipping towards the ocean - a progradational beachline. Note: in the first several meters along Duck1 and Duck3; the dipping reflections are due to the metallic fence.

The last profile goes along the sand road north of the FRF fenced compound near the observation tower:

Duck4 (84k) - 100 MHz antennae - 0.5 m step, antennae separation 1.0 m

This longer line shows the internal foredune stratigraphy, the water table (confirmed by test holes) and inclined reflections dipping toward the east. Again a progradational beach profile (upper shoreface). The hyperbolic reflection centered at 110 m is the observation tower. Note at 125 m that the depth of penetration decreases dramatically as we approach the ocean. This would be interpreted as the salt water wedge - with fresh water being inland from this point. Similar reflection patterns can be seen on Duck1, Duck2 and Duck3.

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OBSERVATIONS OF NEARSHORE WAVE BREAKING, WHITECAPPING, AND LARGE SCALE SAND BAR MORPHOLOGY

Scientific Objective

1. Improved modeling of the spatial distribution of surface shear stress induced by wave breaking in the surf zone. (collaborative with Thornton/Stanton )

2. Examine the relationship between wave breaking, model predicted shear stresses and wave energy flux decay, and surface generated bubbles and turbulence. (collaborative with Thornton/Stanton)

3. Examine the temporal and spatial relationship of whitecaps to the local wave and wind field, and in particular examine the transition region of wave evolution between the inner shelf and surf zone. (collaborative with Herbers/O'Reilly/Guza )

4. Examine the behavior of sand bar morphology near the FRF on a daily basis over scales ranging from 1-5 km alongshore, and on a bi-monthly basis from Chesapeake Bay to Cape Hatteras over scales ranging 10-100 km. (collaborative with Holman , and Haines/Gelfenbaum )

Experiment Plan

Measurements of wave breaking distributions (from 2 daylight and 1 intensified low-light video cameras) were continuously examined along cross-shore transects extending from the shoreline to approximately 4-5 m depth at several alongshore distances within the minigrid area. Surf zone wave breaking observations will be used to calibrate a model for the wave stress gradients. Additional wave energy transformation measurements (obtained by Elgar/Herbers /O'Reilly /Guza and Thornton/Stanton along the same cross-shore transects) will be combined with the breaking observations to give estimates of the cross-shore variation in set-up. The model will be tested with set-up measurements from the array of manometer tubes (obtained by Thornton/Stanton). The wave breaking observations (obtained from video cameras) were made at the same location and coincident with collaborative (Thornton/Stanton; Hay/Bowen/Doering/Zedel) measurements of the vertical distribution of void fraction (air concentration from bubbles), turbulence (measured acoustically), ambient noise (from a passive hydrophone), and sediment concentration profiles (also measured acoustically). Whitecapping measurements outside the surf zone were made primarily from shipboard mounted cameras during the deployment and retrieval of the bottom mounted wave directional pressure array and several Waverider buoys deployed by Herbers/O'Reilly/Guza. Video observations of the local whitecapping were made in the vicinity of either free floating or moored Waverider buoys during the cruises.

Digital time-exposure images of nearshore wave breaking patterns over a 5 km alongshore range, centered around the FRF pier, were obtained from northward and southward looking video cameras mounted atop the 44 m high FRF tower and atop a 20 m high aluminum tower at the southern end of the FRF property. Changes in the bathymetry were inferred from the average breaking patterns on a daily basis in the years prior to, during, and following the SandyDuck experiment. Additional very large scale morphology patterns from Chesapeake Bay to Cape Hatteras were inferred from time exposure images obtained from aerial over-flights conducted on a bi-monthly basis beginning about 1 year prior to SandyDuck.

Experiment Layout

Video cameras were mounted on the FRF tower near the top. Dune Ground Control Points (GCP's) were established to provide geometric control of the video images. GCP's on jetted pipes in the surf zone were positioned to avoid interference with instrumented arrays and CRAB/Sled profile lines. The 20 m high south tower was located approximately 50-100 m north of the FRF south property line on the dune crest. The location of the video towers is shown on the map of instruments outside the Minigrid zone.

Funding

Surf Zone Wavebreaking Funded (ONR); Whitecapping Funded (ONR); Large Scale Morphology Pending (USGS - Sept. 1996)

Click here to view the 1997 ONR Year End Report (16 kb) on this project in Adobe PDF format (requires free Adobe Acrobat Reader).

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REGIONAL SHORELINE CHANGE

Scientific Objective

The science objective was to understand the regional variability of shoreline change in response to varying wave conditions. The shoreline is defined here as the horizontal position of a selected elevation contour on the foreshore (e.g., 1 m above NAVD88). Regional refers to an along-coast length scale of 10s of kilometers. The scale and focus of this work match that of a traditional shoreline change analysis from aerial photography, while employing a highly repeatable means of measuring shoreline position.

Experiment Plan

The horizontal position of the target elevation contour on the foreshore was measured over a 50-60 km stretch of shoreline (from about 20 km north of the FRF to 30 km south) every low tide or as often as logistically feasible. This was accomplished by driving the all-terrain vehicle (ATV), shown in the figure, along the foreshore of the beach and making a post-survey calculation of the target elevation contour using data from vehicle-mounted differential GPS and attitude sensors. This technique allowed the horizontal position of the target elevation contour to be measured with a repeatability error of less than 1 m RMS. It will be possible to compare the results from this project, measuring one contour over 10s of km of beach, with the fully three-dimensional beach grids measured by Holman over several km to answer the question of how representative the "shoreline" is of overall subaerial beach volume changes. Check out this experiment's web site with preliminary results and animations at: http://chumley.er.usgs.gov/sandyduck/prelim.html

Funding

Funded by United States Geological Survey

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DIRECTIONAL WAVE OBSERVATIONS

Scientific Objective

Regular observations of wind wave frequency-direction spectra in the vicinity of the 8-m depth contour were collected to serve as background information for other SandyDuck investigations. Historically, such observations have been useful for climatological purposes, detailing the nature of the ambient wind wave field, serving as control or boundary conditions in the execution and testing of dynamic models, and as one kind of ground truth for various alternative directional wave measurement schemes.

Experiment Plan

Data from an extant 15-element spatial array of near-bottom pressure gauges was used to estimate wind wave frequency-direction spectra. Location of the 8-m array is shown on the map below. Raw data were collected and archived on the Field Research Facility (FRF) VAX computer, and are available via the web at: http://www.frf.usace.army.mil/fdspec.html. Spectra were processed and archived in unformatted form on a Sun workstation. Images of directional spectra can also be found at the link above. An example of one of these frequency-direction spectra plots is shown at right.

Funding

Funding provided by the Coastal Navigation Hydrodynamics Program of the US Army Corps of Engineers.

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SEDIMENT TRANSPORT RATES DURING STORMS

Scientific Objective

The Corps of Engineers must be able to model longshore sediment transport. Knowledge of the "bulk" transport rate and the distribution of longshore transport across the surf zone is particularly important to the design of inlet stabilization, beach

renourishment, dredging, and most other coastal projects the Corps undertakes. At present the "bulk" transport rate is estimated using some version of the sediment transport formula developed by the Coastal Engineering Research Center (CERC). These CERC-type relationships generally do not predict the cross-shore distribution of longshore transport. The appropriate value of the coefficients to use and the reliability of these formulations are still in question. This investigation was designed to provide direct measurement of the cross-shore distribution of "bulk" longshore transport during storms. These data were not available previously and will be used to enhance the CERC-type formulation and provide the Corps with an improved engineering tool.

The objective of the investigation is to use the Sensor Insertion System (SIS) to make bulk longshore transport measurements during storm conditions. The SIS (shown in the figure) is a diverless instrument deployment system that can operate in wave heights up to 5.6 m. It provides the capability for instruments to be repositioned both horizontally and vertically during a storm allowing the measurement scheme to evolve with the cross-shore profile. Secondary objectives include documenting cross-shore sediment transport processes and profile evolution during storms, attempting order of magnitude measurements of swash-zone longshore transport rates, and bedload transport rates.

Experiment Plan

Around the time of high/low tide the SIS was used to deploy an array of instruments at nominally, 12 cross-shore locations, 512-sec records from all of the instruments were obtained. The instruments (shown at right) included 12 OBS concentration sensors, 5 EM (electro magnetic) current meters, 1 FOBS (fiber optic backscattering system) concentration array, 1 OBS (optical backscattering sensor) array, 2 acoustic (300 kHz & 1 MHz) bottom sensors, 1 underwater video camera. To document cross-shore processes and profile variation, the measurements were repeated throughout the day. To investigate swash processes during a number of storms, streamer sediment traps were lowered into the swash to accumulate sand for a short time. In a similar manner bedload transport was also sampled. A successful test of the SandyDuck SIS operation was conducted in April 1997. Click here for more information about this experiment and the April test.

Funding

Funding provided by Coastal Research and Development Program of the US Army Corps of Engineers.

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COASTAL APPLICATIONS OF SCANNING AIRBORNE LASER (LIDAR)

Scientific Objective

Airborne laser mapping systems provide unprecedented potential for the

mapping of coastal topography and bathymetry. The data quality and density promises to revolutionize the assessment and quantification of societal problems like coastal erosion and hurricane impacts. Further, the technology may eventually replace far more time consuming and labor intensive operations such as shallow water bathymetric surveys with boat and fathometer. The amplitudes of reflected laser light from the bottom may also prove useful in interpreting bottom characteristics similar to what is derived from sidescan sonar records. The primary purpose of this project was to thoroughly evaluate and assess the application of airborne laser mapping systems to the solution of a variety of coastal problems.

Experiment Plan

Our primary plan evaluated and compared two laser mapping systems: SHOALS, operated by the US Army Corps of Engineers (above right) and measures bathymetry and topography, and a NASA LIDAR (ATM) that does not penetrate water and is restricted to high resolution measurement of topography . A reach of coast, approximately 70-km long, from Corolla to Oregon Inlet (which includes the SandyDuck experiment site) was surveyed by the two LIDAR systems. The potential advantage of the NASA system, which is mounted on a fixed wing aircraft, is the higher firing rate of the laser which yields greater data density for the same aircraft speed. The SHOALS system, which is operated from a helicopter has the obvious advantage of doing both subaerial and subaqueous surveying. During the course of the SandyDuck experiment, a great deal of background data consisting of surveys and images of coastal form and bathymetry/topography was gathered. One example of these additional data is the Coastal Carolina University survey sled operated by Paul Gayes and his team. Our overall strategy is to use these data, supplemented where necessary, to thoroughly evaluate the airborne laser systems for use in a variety of coastal applications.

Funding

This was a cost sharing and collaborative experiment between U.S. Geological Survey, U.S. Army Corps of Engineers, NASA, and the Coastal Services Center of NOAA.

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CURRENTS NEAR THE SURF ZONE

Scientific Objective

A major objective of our program is to document horizontal circulation patterns quantitatively within an area of order 200 m by 500 m adjacent to the surf zone, over times long enough to experience several kinds of conditions. We deployed a pair of Phased-Array Doppler Sonars (PADS) shown at right. With each of these, one can obtain sequences of images of one component of the velocity field over a continuous sector 90 degrees wide by almost 400 m in radius. With the PADS systems operating near 195 kHz and 225 kHz acoustic frequency, an area 400 m by 500 m is feasible, with roughly 5 m resolution. This provided movies of the vertical component of vorticity within the region, for example. Accurate pictures of the time evolution of individual "rip currents" can be obtained, together with a description of the background flow and surface wave propagation over the insonified area. The data can then be used to develop relationships between the evolution of these fields and both the incident surface waves and the along-shore flow field.

Experiment Plan

We deployed two Duck Landers by helicopter approximately 300 and 600 m North of the FRF's research pier along the 6.5 m depth contour. Click to see the location of the "Smith" experiment. The cables supplying power and information ran along this contour to the end of the pier, and connected through a junction box to power and to an optical cable leading to our lab ashore. Each lander had one "Phased-Array Doppler Sonar" (PADS), plus miscellaneous instrumentation such as pressure sensors, tilt meters, compass, transmissometer, and CTD. The landers extended about 2 meters above the bottom, with 4 jetted-in legs in roughly a 3-m square configuration. We imaged an area inshore of the landers, extending over many of the other instruments fielded. In particular, we were interested in intersecting several fixed-point current meters and one or two acoustic Doppler profilers deployed inshore of our arrays.

Results

For a table of instruments and collected data: smith.txt You can also go directly to Jerry Smith's web page for the latest details.

Four QuickTime movies have been created showing the capability of PADS (requires free Apple QuickTime viewer)

EricaWaves.qt (1,887 kb): Horizontal wave orbital velocities under passing tropical storm waves, 10 September 1997, The movie can also be found at Jerry Smith's web site at: EricaWaves.mov;
Surge.mov (1,541 kb): Horizontal velocities associated with tidal surge, 8 October 1997;

The two movies EricaWaves.qt and Surge.mov are formed by combining the information over the intersecting areas of two phased-array Doppler acoustic sonars. Each sonar provides a radial component of velocity over a 90-degree by 350m pie-shaped area; they are located about 310 m apart, so the two pieces of velocity information are fairly independent over a sizable area.

EricaWaves.qt shows surface wave propagation (swell generated by the hurricane Erica some distance away). This is a good indicator of how well the system is working. In this movie, each frame represents 1.5 seconds (it plays at 12 fps).

Surge.mov shows a Southward surge along the coast which was also seen in some video measurement (by R. Holman). Here each frame represents 30 seconds of real time (again playing at up to 12 fps, depending on your connection & quicktime player). A little way into the movie, the surge appears most clearly in the alongshore component of flow; then the flow swings around toward the shore; finally, starting from the South (bottom) the outer portion of the flow becomes aligned with the shore (the shore is parallel to the y-axis in these coordinates, and somewhat to the left of the area shown). The x and y coordinates are the FRF local coordinates.

Vorticity 14 Oct 1997 (2,040 kb), Vorticity 18 Oct 1997 (3,338 kb): Horizontal velocity estimates (small black arrows) and vorticity field (color patches); the small circles locate SandyDuck point instruments. Note the long rip current that appears to originate near the gap in the nearshore bar at 1000 m alongshore, and the development of vortex pairs, as sometimes seen in models, that pass offshore from top to bottom. This feature appears to leave behind a trail of red (negative) vorticity along the -4.5 m contour. These movies can also be found with more explanation at Jerry Smith's web page at: Vorticity 14 Oct 1997.mov; Vorticity 18 Oct 1997.mov

Click here to view the 1997 ONR Year End Report (108 kb) on this project in Adobe PDF format (requires free Adobe Acrobat Reader).

Funding

This experiment was supported by the Office of Naval Research

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COASTAL BREAKING WAVE AND BUBBLE MEASUREMENTS

Scientific Objective

NRL measured and modeled the spatial and temporal distributions of nearshore wave breaking, bubble size spectra, and void fraction from about 8 m to 20 m water depth under various sea states and weather conditions. Bubble size spectra and void fraction at each location was made from near surface to a depth about 4 m.

Experiment Plan

1. For each water depth of approximately 8, 10, 12, and 14 m, linearly offshore (offshore distances from 0.9 to 2.3 km from the FRF baseline), a surface-following buoy of vertical length of 4 m was deployed and moored to the sea floor. The location of the longshore line is 612 m, about 100 m north of the pier. On each buoy, a series of void fraction meters and acoustic resonators for bubble size spectra, accelerometers for wave heights, and hydrophones will be mounted.

2. Three wave-following (swinging bar) void fraction staffs hinged on pipes jetted into the bottom were deployed. Their locations along-shore were at 612 m, and their locations cross-shore are at 180, 240, and 300 m, respectively.

3. A video camera mounted on the top of the FRF tower for observing near-shore breaking waves covering mainly the area where the three void fraction staffs were located.

4. Instrument characteristics:

• Void fraction meter - 0.001 to 0.7 void fraction

• Acoustic resonator - bubble radius from 15 to 1200 microns, and maximum void fraction of 0.0001.

• Accelerometer - on the moored bubble buoy for wave height.

• Hydrophone - 10 to 200 KHz

• Video camera - Sony SVHS, professional quality.

The figure below shows the original layout and sensors (locations not to scale). Click here to see the actual SandyDuck surf zone layout including this experiment. The locations of the surface following resonators are also shown on the map of instruments outside the Minigrid zone.

Results

For a listing of instruments and collected data: su.txt

Funding

The project is funded by Naval Research Laboratory for FY97-00.

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FIELD MEASUREMENTS AND NEARSHORE MODELING AT SANDYDUCK

Scientific Objective

The primary objectives were:

1. establish a cross-shore array of self-recording velocity and wave sensors far to the north of the primary field of instrument arrays at the Field Research Facility, Duck NC, to measure waves and currents entering the experimental region.

2. collect velocity, wave and other data from the sensors during the experiment,

3. conduct a preliminary analysis of data quality and data reduction.

These field data can be used as appropriate boundary conditions for computational models. Such models will compute the flow in the internal part of the instrumented region. The results of the computations can be compared with and assimilated to the data from the internal region and assist in interpolation of the measured data.

Experiment Plan

A cross-shore array of current/pressure sensors was installed approximately 125 m north of the primary array to augment the alongshore coverage of the Elgar, Herbers, O'Reilly, and Guza array, to measure advection into the model domain and provide a northern boundary condition for the dominant waves from the north quadrant. The cross-shore array consisted of 3 current meter/pressure sensors (PUVs) and extended from approximately the -1 m MLW contour to the -5 m MLW contour. The current meters were deployed so as to maximize the coverage of the longshore flow pattern at the location. The objective was obtaining the best possible assessment of the longshore velocities and their cross shore distribution over about a 5 week time period. Since the meters are self recording, no cable connections to the shore were needed. Flow measurements were made approximately two feet above the bottom with the goal of measuring flows in the water column (as opposed to the boundary layer).

Two (2) electromagnetic current meter/pressure gages (PUV). Two (2) acoustic current meter/pressure gages (PUVW) were deployed. A PUV and PUVW was co-located at our mid-depth site (about -3 MLW). Click here to see the full SandyDuck surf zone layout including this experiment.

Results

Click here to view the 1997 ONR Year End Report (8 kb) on this project in Adobe PDF format (requires free Adobe Acrobat Reader).

Funding

Funded by the Coastal Dynamics program of the Office of Naval Research.

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NEARSHORE WAVE AND SEDIMENT PROCESSES

Scientific Objective

The long term goals are to predict the wave-induced three dimensional velocity field and induced sediment transport over arbitrary bathymetry in the nearshore. Specific goals of the SandyDuck experiment include:

• observe and model the vertical structures of 3-components of mean, wave-induced and turbulent velocities, sediment flux, and bubbles across the surf zone.
• observe and model the small-scale morphology.
• observe and model the cross-shore set-up/down and alongshore pressure gradients.

Experiment Plan

Vertical structures of 3-components of mean, wave-induced and turbulent velocities, sediment flux, and bubbles across the surf zone were measured from the mobile sled shown at right. Instruments on the sled include a vertical array of 8 electromagnetic current meters, 8 void fraction sensors, 2 acoustic resonators, 6 sensor pressure-array to measure wave direction, vertical and horizontal arrays of 5 optical backscattering sensors, rotating pencil-beam acoustic altimeter and a Bistatic Coherent Doppler Velocity/Sediment meter (BCDVS). The sled was deployed by the CRAB daily early in the morning offshore of the bar (see photo at right) The sled was then towed sequentially shoreward using the four-wheel-drive fork lift stopping at 5-8 cross-shore locations for at least one hour. The sled was deployed at longshore line 935 m (FRF coordinates)..

1. A second BCDVS measuring 3-components of mean, wave-induced and turbulent velocities along with sediment flux was mounted on a stationary frame within the trough of the surf zone (collaboration with Hay/Bowen/Doering/Zedel).

2. A Coherent Acoustic Sediment Profiler to measure 3-components of mean, wave-induced and turbulent velocities, and sediment flux was mounted on a stationary frame within the trough of the surf zone.

3. Small-scale morphology was measured in the minigrid area during each CRAB survey using an array of 7, 1-MHZ acoustic altimeters mounted on the CRAB to make area surveys. GPS differential navigation was used for location and elevation, tilt, acceleration and rotational acceleration were measured to correct for motion.

4. Cross-shore variations of wave height and set-up/down and alongshore pressure gradients were measured using 1 cross-shore arrays composed of 8 pressure senors and 11 manometers located at alongshore distance of 915 m relative to FRF coordinates, and an alongshore array of 5 manometers located in the trough.

Click here to see the full SandyDuck surf zone layout including this experiment.

Funding

This work was funded by the Office of Naval Research, Coastal Sciences.

Results

Click here to view Thornton and Stanton's 1997 ONR Year End Report (10 kb) on and here to view Stantion and Thornton's (102 kb) this project in Adobe PDF format (requires free Adobe Acrobat Reader).

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EXPERIMENT TESTS OF BOUSSINESQ WAVE MODELS IN THE NEAR SHORE ZONE

Scientific Objective

Boussinesq models of shallow water wave propagation into the surf zone and their ability to predict cross and alongshore current flow is of interest to Naval littoral interests. Field testing and field validation of such models has been difficult because of the inability to establish boundary conditions for the input wave field. Along-track interferrometric synthetic aperture radar (INSAR), flying along the coast and imaging cross-shore radial velocity components allows one to image the surface radial velocities on scale sizes of the order of a meter pixel size that can provide data for model boundary conditions. Moreover, imaging radars onshore, such as a marine radar (~1.5 km radius image, 6-m pixel) can provide nearshore wave field imagery against which to compare the results of numerically propagating the INSAR wave pattern using the Boussinesq model. The marine radar can provide NRCS maps every 1.85 s with a few minute continuous record. In this experiment we test Boussinesq-like wave models with the new NRL INSAR and an NRL digital imaging marine radar.

Experiment Plan

The shore based imaging radars was atop the FRF laboratory building. The NRL Ultra-Wideband SAR (NUWSAR) was flown on the NRL P3 for several days across the area. University of Delaware personnel ran the Boussinesq model.

Funding

Funded by the Office of Naval Research

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MARINE RADAR REMOTE SENSING OF BAR AND RIP MORPHOLOGY

Scientific Objective

Suitable averaging of marine radar imagery collected over one minute, with collection time intervals on time scales of five minutes have shown what appears to be the occurrence of nearshore rip current structures inside the offshore bar. The sensing of such current features using a radar intensity measure held promise for Naval littoral operations using standard onboard radars. To validate the hypothesis that the observed surface effects are indeed due to enhanced wave-current interactions over a current rip, we collected radar imagery for comparison with the acoustic Doppler imaging system deployed by Jerry Smith. A comparison of the location of the currents measured below the surface could be made with the radar estimate of the surface currents. With knowledge of the subsurface current magnitudes, an empirical estimate can be determined of the relationship between the current strength and the normalized radar cross section contrast of the sea surface region imaged over the rip relative to the ambient.

Experiment Plan

A marine radar (shown at right) operated continuously during the SandyDuck '97 experiment, with intense periods during the acoustic subsurface imaging system. During periods of strong alongshore flow, rip currents are expected. These have been observed in the past at the FRF, after flooding rains saturated the Chesapeake Bay basin and its outflow drains southward alongshore. Mushroom vortical structures were observed under such forcing. During strong wind wave forcing at oblique angles to the shoreline, linear rip currents are expected. Both types of events were anticipated, and the conditions forcing them as described above were the focus of coordinated data collections by the acoustic and marine radar remote sensors. Click here to see the where the Trizna radar experiment was in the SandyDuck surf zone layout.

Funding

Funded through the Office of Naval Research

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MEASUREMENT OF BOTTOM STRESS IN THE WIND- AND WAVE-FORCED NEARSHORE ENVIRONMENT

Scientific Objective

The objectives were:

1. obtain measurements of bottom stress in the shallow nearshore environment, where both wind stress and breaking waves force energetic longshore flows;

2. place the measurements of bottom stress in context by comparing them with measurements of the other dominant terms in the longshore momentum balance (wind stress and cross-shore gradient of wave-induced radiation stress); and

3. use the measurements to determine the mechanisms by which drag is transmitted to the sea floor.

Experiment Plan

The approach measured the turbulent Reynolds stress (minus the density times the covariance between horizontal and vertical turbulent velocity fluctuations) a short distance (roughly one tenth of the water depth) above the sea floor. The measurements were based on a new technique that has been shown theoretically and in pilot experiments to remove contamination by surface waves from estimates of turbulent Reynolds stress. The measurements were obtained by an array of Sontek acoustic Doppler velocimeters, which were fixed to a bottom-mounted frame (see figure). The deployment occurred at a water depth of approximately 4 m, where forcing by wind stress and forcing by breaking waves were both likely to be important during the course of the experiment. Estimates of bottom stress were compared with estimates of wind stress obtained by Edson and with estimates of cross-shore gradient of wave-induced radiation stress obtained by other SandyDuck PIs. Click here to see the location of the Trowbridge experiment surf zone layout.

Funding

Funding for equipment, deployment and recovery was obtained from a number of different sources. Funding for analysis was requested by ONR.

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NEARSHORE WATER LEVEL PROFILES DURING STORMS

Scientific Objective

Breaking wave induced coastal water level set-up during storm waves approaching the shore are reported to have significant impact on the mean water level predictions. For the protection of human life and properties in the coastal zone, tidal level and storm surges have been of national concern. For SandyDuck experiment (Oct. 1997), we measured wave set-up profiles along the FRF Pier. The coupled data of water level and waves benefits our understanding of nearshore process. The field data of wave set-up will be used to verify wave models which will be integrated into the operational storm surge model employed by the NOAA National Weather Service.

Experiment Plan

NOAA National Ocean Service operates a water level monitoring station at the pier end (offshore distance of 591 m in FRF coordinates). This includes an air acoustic gauge (primary) and a pressure gauge (secondary). In addition to these, the FRF operates a Paroscientific pressure sensor for wave and water level measurements at 239 m on the pier. Under this study and in cooperation with the FRF and supported by NOAA Sea Grant, an additional Paroscientific pressure sensor was deployed at 183 m (the shoreline located is located at ~130 m). Water levels from the pressure sensors were sampled at 4 Hz along with other FRF instrumentation.

Funding

Funded by NOAA Sea Grant with support from NOAA and the Coastal Research and Development program of the US Army Corps of Engineers.

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