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,
[email protected], [email protected], [email protected]
2. Georgia Tech Savannah, 210 Technology Circle, Savannah, GA 31407,
3. Department of Earth and Ocean Sciences, University of South Carolina, Columbia,
SC 29208, [email protected]
4. USACE-Coastal Hydraulic Lab, Field Research Facility, 1261 Duck Rd, Duck, NC
27949, [email protected], [email protected]
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.
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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.
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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;
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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
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