A NEARSHORE PROCESSES FIELD EXPERIMENT AT CAPE

HATTERAS, NORTH CAROLINA, U.S.A.

JEFFREY H. LIST1, JOHN C. WARNER1, E. ROBERT THIELER1, KEVIN HAAS2,

GEORGE VOULGARIS3, JESSE E. MCNINCH4, AND KATHERINE L. BRODIE4

1. U. S. Geological Survey 384 Woods Hole Rd., Woods Hole, MA, 02543,

jlist@usgs.gov, jcwarner@usgs.gov, rthieler@usgs.gov

2. Georgia Tech Savannah, 210 Technology Circle, Savannah, GA 31407,

kevin.haas@gtsav.gatech.edu

3. Department of Earth and Ocean Sciences, University of South Carolina, Columbia,

SC 29208, gvoulgaris@geol.sc.edu

4. USACE-Coastal Hydraulic Lab, Field Research Facility, 1261 Duck Rd, Duck, NC

27949, Jesse.Mcninch@usace.army.mil, Katherine.L.Brodie@usace.army.mil

Abstract: A month-long field experiment focused on the nearshore

hydrodynamics of Diamond Shoals adjacent to Cape Hatteras Point, North Carolina, was conducted in February 2010. The objectives of this multi-

institutional experiment were to test hypotheses related to Diamond Shoals as a

sink in the regional sediment budget and to provide data for evaluating numerical models. The experiment included in-situ instrumentation for measuring waves and

currents; a video camera system for measuring surface currents at a nearshore

transect; a radar system for measuring regional surface currents over Diamond Shoals and the adjacent coast; a vehicle-based scanning lidar and radar system for

mapping beach topography, nearshore wave breaking intensity, bathymetry

(through wave celerity inversion), and wave direction; and an amphibious vehicle

system for surveying single-beam bathymetry. Preliminary results from wave and

current measurements suggest that shoal-building processes were active during the

experiment.

Introduction

Diamond Shoals is a complex of sand shoals extending 40 km seaward from

Cape Hatteras Point, North Carolina. Previous studies on the sediment budget of

the Outer Banks have hypothesized that Diamond Shoals represents a major

regional sink for sand derived from the coast to the north of Cape Hatteras and

transported via a prevailing southward littoral drift (e.g., Inman and Dolan,

1989; Moore et al., 2010). The existence of the shoal complex as a quasi-linear

bathymetric feature extending 10s of km seaward also suggests that: 1. processes

removing sand from the shoal are weaker than processes delivering sand; 2.

there are processes acting to contain the sand in a coherent body rather than

allowing it to become dispersed over the shelf; and 3. if the main source of the

shoal sand is the adjacent littoral zones, there must be processes transporting

sand from the proximal (near coast) part of the shoal to the more distal parts of

the shoal (or the proximal part would not remain subaqueous). However, despite

the likely importance of Diamond Shoals in the regional sediment budget and

2

the many unanswered questions about the formation and maintenance of

Diamonds Shoals, there have been very few hydrodynamic measurements due to

the extreme difficulty of deploying instruments in the area.

An initial 2009 U.S. Geological Survey experiment focused on processes on the

distal part of the shoal, with three instrumented tripods placed 10-20 km from

Cape Hatteras Point (see Martini et al., 2009, for the experiment design). These

data were collected to address the potential transport of sediment by wind-driven

currents near the tip of Diamond Shoals. Here, we describe a second field

experiment, conducted in February 2010, focusing on the hydrodynamic

processes on the proximal part Diamond Shoals and adjacent coasts (Fig. 1),

where wave-driven flows are expected to be the dominate sediment-transporting

process. We describe the data collected during this experiment, the conditions

that occurred during the deployment period, and provide some preliminary

results relevant to the net delivery of sand to Diamond Shoals.

TRDI ADCP

Fig. 1. Cape Hatteras experiment instrument locations. The WERA coverage area is shown by the

shaded region extending from the location labeled WERA Radar.

3

Data Collected

The 2010 experiment consisted of hydrodynamic instrumentation mounted on

poles and tripods for measuring waves, currents, and suspended sediment; a very

high frequency (VHF) radar system for mapping regional surface currents and

wave characteristics; a lighthouse-mounted video camera system for nearshore

currents; the vehicle-based Coastal Lidar and Radar Imaging System (CLARIS)

for measuring topography, breaking wave intensity, bathymetry (from wave

celerity inversion), and wave direction; and the Light Amphibious Resupply

Cargo (LARC) vessel used to deploy instrumentation and survey bathymetry.

Fig. 1 and Table 1 summarize the data collected during the experiment.

Fig. 2. Jet Pipes used for instrument mounts

Hydrodynamic Instrumentation

The instrument deployment design consisted of cross-shore arrays of sensors on

the north and west sides of Diamond Shoals and an array across the axis of

Diamond Shoals (Fig. 1). Instrumentation consisted primarily of Acoustic

Doppler Current Profilers (ADCP), including Nortek Aquadopps and Teledyne

RDI Workhorse ADCPs, which were mounted on 4 m long aluminum poles

(Fig. 2) and jetted into the seafloor using the LARC (Fig. 3). These instruments

recorded near-bed pressure and vertical structure of the 3D currents in 40 cm

4

bins from 40 cm above the seafloor to near the surface, and were recorded

continuously at 1 Hz. At site N13 (Fig. 1), sediment concentration was measured

using an Aquatec AQUAscat acoustic backscatter sensor (ABS; 1 cm resolution)

and currents were measured at a single elevation within the water column using

a Sontek-YSI Triton Acoustic Doppler Velocimeter (ADV). Additional

Teledyne RDI Workhorse ADCPs were deployed on bottom tripods at sites O1,

O2, and O3 (Fig. 1).

Due to the difficulty of deploying instruments on or adjacent to Diamond Shoals

during February conditions, no sensors could be deployed in water shallower

than 3.5 m. In particular, a pole-mounted Aquadopp was intended for a location

midway between N6 and N8 on the crest of Diamond Shoals, but could not be

safely deployed due to crossing swell approaching from multiple directions.

Fig. 3. The LARC amphibious vehicle

WERA Radar

Our in-situ instrumentation provides information throughout the water column,

but only at a limited set of discrete locations. Radar systems, on the other hand,

can quantify the regional field of currents and wave characteristics, but only at

the surface. Radar data aid in the interpretation of instrument data and help test

numerical models for simulating waves and currents at similar temporal and

spatial scales.

5

Table 1. Summary of Data Collected

Site Name Instrument Dates

(2010)

Nominal

Water

Depth (m)

Quantities Measured

N1 Nortek Aquadopp Feb. 9-21 3.9 p; u,v,w vertical profile

N2 Nortek Aquadopp Feb. 9-21 6.0 p; u,v,w vertical profile

N3 Nortek Aquadopp Feb. 9-22 4.9 p; u,v,w vertical profile

N4 TRDI Workhorse

ADCP

Feb. 9-22 7.8 p; u,v,w vertical profile

N5 TRDI Workhorse ADCP

Feb. 2-21 7.1 p; u,v,w vertical profile

N6 Nortek Aquadopp Feb. 2-21 4.7 p; u,v,w vertical profile

N8 Nortek Aquadopp Feb. 4-22 4.8 p; u,v,w vertical profile

N9 TRDI Workhorse

ADCP

Feb. 4-22 7.0 p; u,v,w vertical profile

N11 Nortek Aquadopp Feb. 3-22 4.9 p; u,v,w vertical profile

N12 Nortek Aquadopp Feb. 3-22 2.9 p; u,v,w vertical profile

N13 Triton ADV Feb. 8-22 3.5 p; u,v,w

N13 AQUAscat ABS Feb. 8-22 3.5 Sediment concentration

O1 TRDI Workhorse

ADCP

Feb. 4-March

20

9.4 p; u,v,w vertical profile

O2 TRDI Workhorse

ADCP

Feb. 4- March

20

9.8 p; u,v,w vertical profile

O3 TRDI Workhorse ADCP

Feb. 4- March 20

9.4 p; u,v,w vertical profile

WERA WERA Radar Feb. 3-25 Remote

Sensing

Surface current field up to

10-15 km from transmitter

Lighthouse Video camera Feb. 3-22 Remote

Sensing

Cross-shore transect of

alongshore surface

currents near site N1

Mobile CLARIS Feb. 4, Feb. 6,

Mar. 3

Remote

Sensing

Beach topography; wave

breaking intensity,

bathymetry, and wave direction over Diamond

Shoals and the adjacent

coast to approx. 1 km from shore.

Mobile LARC Bathymetry One

composite

survey: Feb.

12, 20, 21

Amphibious

surveying

Singlebeam bathymetry

over Diamond Shoals and

adjacent coast

6

High-Frequency (HF) radars have evolved considerably over the last 50 years

following the work of Crombie (1955). They utilize the guided propagation of

electromagnetic (EM) waves along the conductive sea surface well beyond the

horizon (Barrick, 1992). The radar signal is backscattered from the continuous

moving ocean surface by the surface ocean waves with wavelength equal to half

of the emitted EM wavelength (Bragg resonant waves).

Fig. 4. The 12 Antenna receiving array of the WERA radar system

Although HF radars provide oceanographic information over long ranges (e.g.,

Savidge et al., 2010), this occurs at the expense of spatial resolution. For

nearshore applications as in this study, VHF radar can provide high spatial

resolution at reduced ranges of the order of 10-15 km. WEllen RAdars (WERA)

are phased array systems that use EM waves between 6 and 48 MHz (49.9 to 5.7

m1 wavelength) to measure surface current velocities, ocean wave spectra, and

wind direction (Gurgel et al., 1999). For this study we deployed a single 48

MHz WERA station (transmit and receive arrays; Fig. 4) manufactured by

Helzel Messtechnik GmbH. It uses a direction-finding algorithm to estimate

radial currents with a spatial resolution of 150 m over an area extending 120

degrees from the radar installation site (Fig. 1), with a varying range of 10 to 15

km (depending on environmental conditions).

Mapping of 2D surface currents previously have required at least two radar

systems deployed at separate coastal locations. However, new methods are being

7

developed for generating 2D currents from a single station using assumptions

regarding the coherent structure of the ocean surface over sampling areas

significantly larger than those of the raw data (Voulgaris et al., in prep). In

addition to surface currents, surface wave signatures are being derived from the

second-order returns for both the significant wave heights (Hs) and the

directional wave spectrum following the technique of Gurgel et al. (2006).

Data were collected for the period February 3-25. Ocean backscatter data were

collected every 30 minutes (centered on the hour and 30 minutes past the hour)

continuously for a period of 17.7 min.

Lighthouse Camera

We mounted a camera system to the top railing of the Cape Hatteras Lighthouse

(Fig. 5) to measure the full cross-shore profile of alongshore currents adjacent to

instrument station N1 (Fig. 1).

Fig. 5. The lighthouse camera

We recorded digital images of the surf zone during daylight hours at 3.3 Hz,

imaging approximately 100 m of coast at a resolution of 1024x1024 pixels.

Control points were surveyed each day to facilitate rectification of the imagery.

The procedure used for processing the video imagery into a quantitative

measurement of the cross-shore profile of alongshore velocity closely follows

the methodology described by Chickadel et al. (2003). At each cross-shore

8

location where the alongshore current velocity is derived, a time stack of image

intensity values is constructed that reflects the alongshore advection of foam

from breaking waves. From this a 2D “foam intensity spectrum” in wave

number and frequency space is used to extract the speed of the surface foam,

which is inferred to be the speed of the alongshore current at the surface.

CLARIS

CLARIS (Fig. 6) is a mobile, coastal surveying tool that enables simultaneous

collection of X-band radar data of the nearshore and topography data of the

beach and dune from a terrestrial laser scanner (Brodie and McNinch, accepted).

The methodology and applications of radar to coastal surveying are similar to

that of video (e.g., Lippmann and Holman 1989), as the foamy rough surface of

breaking waves and swash results in high-intensity radar returns that can be

Fig. 6. CLARIS: Coastal Lidar and Radar Imaging System

used to map swash and sandbar morphology (Ruessink et al., 2002; McNinch,

2007) as well as infer bathymetry (Bell, 1999; Brodie and McNinch, accepted)

by inverting the linear dispersion relationship to solve for depth from wave

celerity observations (e.g., Plant et al., 2008). Wave direction of the most

coherent (peak) wave is calculated throughout the radar domain from the

alongshore and cross-shore wave celerity observations (e.g. Stockdon and

Holman, 2000). Topography data is collected with a Riegl 3D terrestrial lidar

scanner (Riegl Z-390i) that scans to starboard of the vehicle during transit along

the beach.

9

CLARIS surveys were conducted on February 4, February 6, and March 3. Each

survey extended about 1 km offshore and covered about 8 km of coast, including

5 km to the north and 3 km to the west of Cape Hatteras Point. Each survey took

3-4 hours to complete, providing one composite map of each quantity measured.

These surveys provide a measure of subaerial and subaqueous coastal change

during the experiment.

LARC Bathymetry

A 200 kHz echo sounder mounted on the LARC was used to survey single-beam

bathymetry over Diamond Shoals and the adjacent coast. Tracklines at

approximately 300 m spacing covered about 4 km to the north of Cape Hatteras

Point, 2 km to the west of Cape Hatteras Point, and extended about 2 km

offshore. Due to the difficulty of surveying over Diamond Shoals, only one

complete survey was possible during the experiment, with data collected on

three separate days (Feb. 12, 20, and 21) to form a composite survey. These data

will be merged with other sources of bathymetric data (covering a wider region

than the LARC survey) to construct a bottom grid for circulation modeling, and

will be compared to a similar LARC survey conducted in October 2009.

Experiment Conditions and Preliminary Results

Although there were no major storms that resulting in coastal erosion while all

instruments were deployed, winds and waves varied widely in intensity and

direction during the experiment as summarized by Fig. 7 and Table 2. Wind

speed and direction in Fig. 7 are from NOAA Buoy 41025, located about 25 km

from Cape Hatteras Point on the southwest side of Diamond Shoals. Significant

wave height, Hs, and the north/south component of near-bottom current velocity,

v, are shown for sites N6 and N8 on the east and west sides of Diamond Shoals,

respectively.

During most of the events listed in Table 2, there is a clear dichotomy of

conditions between the east and west side of the shoal. When waves and currents

are energetic on the east side conditions are quiet on the west side and vice-

versa. For example, on February 7-8 swell was from the northeast with Hs >2.0

m and |v|>100 cm/s on the east side of the shoal at N6 while conditions were

calm on the west side at N8; conversely on February 10-11 strong westerly

winds resulted in Hs >2.0 m and |v|>50 cm/s on the west side at N8 while

conditions were calm on the east side at N6. In both cases currents were directed

south (the east/west component is minor), suggesting an extension of alongshore

currents (observed at N3 and N11; not shown here) beyond Cape Hatteras Point.

The weak flows on the sheltered side of the shoal suggests that sediment

bypassing of Cape Hatteras Point was not occurring to any significant degree;

10

i.e., when waves are from the northeast or southwest sediment would be

transported to and likely deposited on the shoal, not on the leeward side of Cape

Hatteras Point.

Conditions on February 6 represented an exception to this general pattern, with a

short period of 20 m/s wind from the southeast producing waves with Hs >2.0 m

on both sides of Diamond Shoals and flows at both N6 and N8 intermittently

directed towards the north. These conditions appear to have the potential to

transport sand from the shoal back towards the adjacent coasts.

Table 2. Wave and Current Events during the Experiment

Event Date(s) Event Description, based on Fig. 7 and other observations.

1 Feb. 6 Waves > 2.5 on both east and west sides of Diamond Shoals and currents northwards towards Cape Hatteras Point. Event driven by 20 m/s winds

from the southeast, switching to the west.

2 Feb. 7-8 Large swell event from the northeast observed on east side of shoal; west side of shoal is sheltered. Winds from the north diminishing from 15 m/s to

6 m/sec. Currents > 1 m/s toward south on the east side of shoal; weak

currents on the west side.

3 Feb. 10-11 Wind 20 m/s from west/northwest driving Hs > 2 m on west side of shoal,

while Hs < 1 m on sheltered east side. Currents > 0.5 m/sec towards south

on west side of the shoal; weak currents on the east side of the shoal.

4 Feb. 12-13 Swell from northeast on Feb. 12th (with low wind) changing to wind sea

from northeast on Feb. 13th (with wind from north 15 m/s). Currents on

east side of shoal 0.4-0.6 m/sec towards south; weak currents on the west side of the shoal.

5 Feb. 16-19 West/northwest wind averaging 10 m/sec; Currents on the west side of

shoal 0.2-0.4 m/s towards south; weak currents on the east side of the shoal.

However, southerly-directed flows were much more common, and stronger, than

northerly directed flows at N6 and N8 during the month-long experiment (Fig.

7). This suggests that during the experiment there would have been a net

deposition of sand on the shoal at the expense of the adjacent nearshore zones

(north and west of Cape Hatteras Point). Examination of the long-term wind and

wave direction climatology in the area (Ashton et al., 2006) indicates a strongly

bi-modal distribution, with major components from the southwest and northeast

and less frequent events from the east or southeast. Thus the long-term

climatology is heavily weighted with conditions that appear likely to favor

deposition on the shoal at the expense of the adjacent coast, while events

11

0

5

10

15

20

Speed (

m/s

)

Wind Speed; NOAA Buoy 41025 Diamond Shoals

0

100

200

300

400

Direction (

deg)

Wind Direction (from); NOAA Buoy 41025 Diamond Shoals

0

1

2

3

4Significant Wave Height

Hs (

m)

N8

N6

2 3 4 5 6 7 8 9 1011121314151617181920212223-200

-100

0

Day in February 2010 (UTC)

North/South Velocity Component (Positive: Northward Flow)

v (

cm

/sec)

N8

N6

Event 1 2 3 4 5

(A)

(B)

(C)

(D)

N5N6 N12

N11

N9O1O2O3

N8

N13

N1N2N3N4

Fig. 7. (A) Wind Speed, (B) Wind Direction (from; Nautical convention), (C) Significant wave

height (Hs), and (D) 17 minute average north/south velocity component (v) 0.4 m above the seafloor.

Events numbered in panel (C) are described in Table 2. Instrument deployment dates are annotated in panel (D).

12

with the potential to reverse this transport direction, such as during the February

6 event in our experiment, are relatively rare.

Conclusions

A field experiment designed to measure waves, currents, topography, and

bathymetry over the proximal (near coast) part of Diamond Shoals near Cape

Hatteras Point was successfully completed in February 2010. Here, we mainly

provide a description of the methodologies utilized, data collected, and the

conditions that occurred during the experiment. A preliminary examination of

the wave height and near-bottom current time series on either side of Diamond

Shoals suggests that processes favoring the deposition of sand in the shoal at the

expense of the adjacent coast were active during the experiment. These

conditions appear to be typical when considering the long-term wind and wave

climatology in the area.

Further work is underway that compares the in-situ measurements with the

broader fields of waves and currents measured with the remote sensing systems

deployed during the experiment. These broader fields will be used to evaluate

numerical model simulations of the hydrodynamics and sediment transport

patterns over Diamond Shoals and the adjacent coasts for both single wave

events and the longer-term climatology.

Acknowledgements

For assistance with the field experiment we thank Marinna Martini, Jonathan

Borden, Brandy Armstrong, Chuck Worley, Dann Blackwood, Sandy Baldwin,

Michael Casso, B.J. Reynolds, and Jordan Sanford of the U.S. Geological

Survey; Ray Townsend, Jason Pipes, and Mike Forte of the U.S. Army Corps;

Stephanie Smallegan, Adam Sapp, Thomas Gay, and Xiufeng Yang of Georgia

Tech; and Jeff Morin and Kumar Nirnimesh of the University of South Carolina.

We thank the National Park Service, Outer Banks Group, for facilitating this

experiment. Manuscript reviews by Chris Sherwood and Bruce Jaffe are thanked

for improving this paper.

Disclaimer

Any use of trade, product, or firm names is for descriptive purposes only and

does not imply endorsement by the U.S. Government.

13

References

Ashton A. D. and A. B. Murray (2006). High-angle wave instability and

emergent shoreline shapes: 1. Modeling of sand waves, flying spits, and

capes. Journal of Geophysical Research-Earth Surface, 111, F04011,

doi:10.1029/2005JF000422.

Barrick, D.E. (1992). First order theory and analysis of MF/HF/VHF scatter

from the sea. IEEE Trans. Antennas Propag.,AP-20, 2–10.

Bell, P. S. (1999), Shallow water bathymetry derived from an analysis of X-

band marine radar images of waves, Coast. Eng., 37(3-4), 513-527.

Brodie, K. L. and McNinch, J. E. (Accepted). Coastal Lidar and Radar Imaging

System (CLARIS): A mobile, integrated system for measuring nearshore

wave parameters, bathymetry, and beach topography during storms. Coast.

Eng., CENG-D-10-00072.

Chickadel, C.C., R. A. Holman and M.H. Freilich (2003), An optical technique

for the measurements of longshore currents. J Geophys. Res., 108(C11),

3364, doi:10.1029/2003JC001774.

Crombie, D.D. (1955). Doppler spectrum of sea echo at 13.56 Mc.s-1. Nature.

175: 681-682.

Gurgel, K.W., H.H. Essen and T. Schlick (2006). An Empirical Method to

Derive Ocean Waves from Second-Order Bragg Scattering: Prospects and

Limitations. IEEE Journal of Oceanic Engineering, 31(4): 804-811.

Gurgel, K.W., Antonischski, G., Essen, H.H., Schlick, T. (1999). Wellen radar

(WERA): a new ground wave radar for remote sensing. Coast. Eng. 37,

219–234.

Inman, D. L. and Dolan, R. (1989). The Outer Banks of North Carolina: budget

of sediment and inlet dynamics along a migrating barrier system. Journal of

Coastal Research, 5, 193-237.

Lippmann, T. C., and R. A. Holman (1989), Quantification of sand bar

morphology: A video technique based on wave dissipation, J. of Geophys.

Res., 94(C1), 995-1011.

Martini, M., Armstrong, B., and Warner, J. W. (2009). High resolution near-bed

14

observations in winter near Cape Hatteras, North Carolina. Proceed.

OCEANS 2009 Marine Tecnology for Our Future: Global and Local

Challenges, Biloxi, MS, pp. 1-10.

McNinch, J. E. (2007), Bar and Swash Imaging Radar (BASIR): A Mobile X-

band Radar Designed for Mapping Nearshore Sand Bars and Swash-

Defined Shorelines Over Large Distances, J. Coast. Res., 23(1), 59-74.

Moore, L. J., List, J. H., Williams, S. J., and Stolper, D. (2010). Complexities in

Barrier Island Response to Sea-Level Rise: Insights from Numerical Model

Experiments, North Carolina Outer Banks. Journal of Geophysical

Research-Earth Surface, 115, F03004, doi:10.1029/2009JF001299.

Plant, N. G., K. T. Holland, and M. C. Haller (2008), Ocean Wavenumber

Estimation from Wave-Resolving Time Series Imagery, IEEE Trans.

Geosci. Remote Sens., 46(9), 2644-2659.

Ruessink, B. G., P. S. Bell, I. M. J. van Enckevort, and S. G. J. Aarninkhof

(2002), Nearshore bar crest location quantified from time-averaged X-band

radar images, Coast. Eng., 45(1), 19-32.

Savidge, D. K., J. Norman, C. Smith, J. A. Amft, T. Moore, C. Edwards, and G.

Voulgaris (2010), Shelf edge tide correlated eddies along the southeastern

United States, Geophys. Res. Lett., 37, L22604,

doi:10.1029/2010GL045236.

Stockdon, H. F., and R. A. Holman (2000), Estimation of wave phase speed and

nearshore bathymetry from video imagery, J. Geophys. Res., 105, doi:

10.1029/1999JC000124.

Voulgaris, G., K.W. Gurgel, N. Kumar, J.C. Warner and J. List (in prep). 2-D

Inner-Shelf Current Observations from a Single VHF WEllen RAdar

(WERA) Station, Proceedings of the IEEE/OES 10th Current, Waves and

Turbulence Measurement Workshop (CWTM), 20-23 March, 2011,

Monterey CA.