**Dynamics of Ripples on the Sandy Inner Shelf off Martha's Vineyard:**

**Surveys, Field Measurements, and Models**

Christopher R. Sherwood

U.S. Geological Survey, Woods Hole Science Center

384 Woods Hole Road

Woods Hole, MA 02543-1598

Phone: (508) 457-2269 Fax: (508) 457-2310 Email: csherwood@usgs.gov

Richard P. Signell

U.S. Geological Survey, Woods Hole Science Center

384 Woods Hole Road

Woods Hole, MA 02543-1598

Phone: (508) 457-2229 Fax: (508) 457-2310 Email: rsignell@usgs.gov

Document Number: N0001407IP20087

### LONG-TERM GOALS

The long-term goal of our research is to improve fundamental
understanding and numerical representation of coastal sediment-transport
processes. We participated in EuroSTRATAFORM project, where our goal has
been to improve quantitative models describing the relationships among
meteorological and oceanographic forcing, freshwater influx, particle
resuspension, and transport and accumulation of sediment in the coastal
ocean. We helped obtain data for the Optics Acoustics and Stress In Situ
(OASIS) project with many of the same goals and additional focus on the
interaction between bed and suspended sediments and the influence of
fine sediments on optical properties in the water column. We are funded
through the U. S. Geological Survey (USGS) Coastal and Marine Geology
Program to help a National Oceanographic Partnership Program
(NOPP)-funded partnership develop a community sediment-transport model
(CSTM). Quantitative understanding of sedimentary processes is important
to the Navy because they define environmental conditions in coastal
regions, including current speeds, turbulence, water-column turbidity,
and bottom acoustic properties. They are also of great interest to
geologists and coastal resource managers.

**OBJECTIVES**

Our objectives are to support the Office of Naval Research (ONR) Ripples
Directed-Research Initiative (DRI) studies at MVCO with data collection
and modeling. We have three tasks, all related to the ongoing NOPP CSTM
funded by ONR and to the OASIS project, and closely aligned with
long-term USGS science objectives. They are as follows.

• Mapping: Conduct a high-resolution bathymetric survey and make a
gridded bathymetric chart for use as initial conditions in wave and
circulation models for the Ripples DRI study area.

• Tripod measurements: Measure ripple morphology and evolution, along
with wave-orbital motions, currents, bottom stress, suspended-sediment
concentrations, and sediment fluxes during the Ripples DRI experiment.

• Modeling: Implement a regional wave, circulation, and
sediment-transport model for the Ripples DRI and OASIS study areas using
code developed in the NOPP CSTM.

**APPROACH**

Task 1. Mapping -- We have conducted a high-resolution bathymetric
survey of the Ripples DRI experiment site south of Martha's Vineyard
using USGS survey instrumentation from a vessel of opportunity leased by
WHOI.

Task 2. Tripod Measurements -- We have deployed a pair of autonomous
tripods to measure ripple morphology, waves, currents, bottom stress,
suspended-sediment concentrations, and temperature at a site on the
inner shelf. The exact location of the site was chosen after discussions
with researchers from both ONR-funded experiments, the Ripples DRI and
the OASIS project (Traykovski, Hay, Herbers, Boss, Hill, and Milligan).
We hope to map ripple morphology on both sides of the boundary and
measure changes in bottom stress, near-bottom turbulence, apparent
roughness, and suspended sediment as the observed boundary-layer
properties change with alternating tidal flow, advecting to our sensors
properties acquired over either coarse sand or fine sand. We also hope
to measure bedform transport associated with the migration of the
fine-sand boundary, and compare it with bedload transport rates inferred
from ripple geometry and calculated from wave and current measurements.

Task 3. Modeling -- We are developing of a regional circulation and wave
model, using the Regional Ocean Modeling System (ROMS) with the wave
model SWAN, as developed under the NOPP-funded CSTM. The model will
incorporate a coarse outer domain similar to the CBLAST model developed
by Wilkin (2006) and a finer inner domain centered on the MVCO region.
The objectives of the modeling studies are to critically evaluate the
performance of a regional wave, circulation, and sediment-transport
model with measurements made at several inner shelf locations occupied
as part of the Ripples DRI, OASIS, and ongoing MVCO data collection, and
to provide a regional sediment-transport context for to help interpret
the point measurements.

**WORK COMPLETED**

Mapping --The USGS conducted nearshore geophysical mapping off the south
coast of Martha's Vineyard, in the vicinity of the Martha's Vineyard
Coastal Observatory (MVCO) in August 2007. Jane F. Denny was the chief
scientist for the five-member survey team. The survey was conducted from
the *M/V Megan Miller* August 9-14, 2007. The study area covers 35 km^2^
from about 0.2 km to 5-km offshore, and ranges in depth from \~ 5 to 20
meters. The following high-resolution systems were used to map the
surficial sediment distribution, depth and sub-surface geology:
dual-frequency 100/500 KHz sidescan-sonar system, 234-KHz
interferometric sonar, and 500 Hz -12 KHz chirp sub-bottom profiler.

All geophysical data were collected concurrently at a variable line
spacing ranging from 40 to 70 m; ensuring overlap of the swath systems.
The sidescan-sonar and sub-bottom systems were towed off the stern, with
a digital cable-out display used to record sonar layback. The sub-bottom
system was surface towed and configured with a differential global
positioning system (DGPS) to acquire tow-fish position, yielding 1 -- 2
meter horizontal accuracy. Real-time kinematic (RTK) positioning was
acquired for the interferometric and sidescan-sonar systems, yielding
better than 10-cm horizontal accuracy.

A motion-reference unit mounted directly above the interferometric sonar
head recorded vessel motion (pitch, heave, roll, and yaw); attitude
information was used to rectify bathymetric soundings. Additionally, a
sound velocity profiler was used throughout the survey to measure the
speed of sound in the water column. Vertical resolution of the
bathymetric data is approximately 1% of water depth.

Sidescan-sonar and swath bathymetric data were acquired at a 50-m range
(100-m swath). Ping rates for both systems exceeded 10 pings/second,
providing cm-scale resolution for the sidescan-sonar in both along- and
across-track directions and dense swath soundings for the
interferometric sonar. Sub-bottom data were acquired at a 0.25-s fire
rate and a 0.5 -- 8.5 kHz sweep (swept frequency) and recorded in JSF
format. All data were processed in the field resulting in a composite
sidescan-sonar mosaic, preliminary bathymetric grid referenced to
relative water level heights at the MVCO, and SEG-Y files and JPG images
of all seismic data. Minimal post-cruise processing will be required.

![](media/image1.jpeg){width="6.892361111111111in"
height="5.329861111111111in"}

**Figure 1. Results of the bathymetric and geophysical survey conducted
August 9-14, 2007. a) Location of the survey at the Ripples DRI study
area near MVCO, south of Martha's Vineyard; b) high-resolution
sub-bottom profile along west-east track (A to A' in panel c) showing
buried channels and accumulation of fine sand; c) backscatter from
side-scan sonar draped on bathymetry; and e) shaded, illuminated
bathymetric map from survey.**

Tripods -- Two instrumented tripods were deployed near the MVCO 12-m
node from the *R/V Connecticut* on August 27, 2007. The tripods were
deployed at two sites approximately 40 m apart, both on the boundary
between fine sand and coarse sand. Prior to deployment, the sites were
surveyed by divers and short hand-cores were taken for grain-size
analysis. The southernmost tripod is designed to measure flow
parameters, and supports several pressure sensors, three
acoustic-Doppler velocimeters mounted \~0.4 mab, a downward-looking
pulse-coherent acoustic Doppler profiler, an upward-looking acoustic
Doppler profiler with wave-measurement capabilities, a three-frequency
acoustic backscatterance sensor, two conductivity-temperature sensors,
and several optical backscatterance sensors and transmissometers. The
second tripod is designed to measure suspended particle size and to
obtain sonar images of ripples and bottom topography. That tripod has a
LISST laser in-situ particle size device, a dual-axis pencil-beam sonar
for bathymetric mapping, a fan-beam imaging sonar, a camera for imaging
the bottom, and an upward-looking acoustic Doppler profiler. These
tripods are autonomous, and we hope to recover them in late October
2007.

![](media/image2.jpeg){width="6.8902777777777775in"
height="5.383333333333334in"}

**Figure 2. Side-scan sonar returns (brown and white) and multibeam
bathymetry (green and blue) from autonomous underwater vehicle along
west-east track between USGS tripods at 12-m sites, Sept. 13, 2007.
Courtesy P. Traykovski, WHOI.**

Modeling -- We have updated regional bathymetry in the MVCO region for
our regional circulation, wave, and sediment-transport modeling efforts.
A new 1 arc-second digital bathymetric grid was constructed for the
region 41.0-42.4°N, 71.2-69.7°W using a variety of sources (Figure 3).
All digital sounding data was obtained from NGDC using the GEODAS web
interface and quality controlled. Remaining data gaps were filled by
digitizing smooth sheets from older non-digital surveys. The sounding
data were adjusted from mean low water or mean lower-low water datums to
mean sea level using the NOAA Vdatum model for the Gulf of Maine and
merged with 1 arc-second land topography. The soundings were gridded
using a technique similar to the one used to generate the 3 arc-second
NOAA Coastal Relief Model. We used the "blockmedian" and "surface"
routines from the Generic Mapping Tools (GMT) with slightly different
tension parameters judged to represent this data better and with grid
anisotropy to account for gridding in geographic coordinates. The
resulting topographic dataset is available in several common formats at
<http://stellwagen.er.usgs.gov/models/grids/>.

The following files are available.

32-bit GeoTIFF format vs_1sec_20070725.tif 117 MB

GMT NetCDF format vs_1sec_20070725.grd 109 MB

ArcGIS 9.2 NetCDF format vs_1sec_20070725.nc 109 MB

Google Earth KMZ (image)) vs_1sec_20070725.kmz 1.3 MB

![](media/image3.png){width="5.9944444444444445in"
height="4.291666666666667in"}

**Figure 3. Illuminated, shaded relief map of regional bathymetric
grid.**

**RESULTS**

The valuable results obtained in FY2007 are high-resolution bathymetric
datasets. Because bathymetry is one of the most important factors in
determining the evolution of incident waves and their influence on the
sea bed, these results are critical to any subsequent models for wave
and ripple evolution, and provide a fundamental description of field
conditions for the Ripples DRI project. Comparison of this survey with
previous surveys indicates that the fields of fine and coarse sand have
not moved substantially since the first detailed surveys were performed
despite well-documented sediment-transport events. Instead, the patches
have undergone slight modifications and subtle shifts at the boundaries.
The importance of this result is that it means grain-size variations can
produce relatively stable, self-perpetuating bottom features that can
retain important characteristics (elevation, ripple morphology,
hydrodynamic roughness) for many years, even in energetic inner shelf
environments.

**IMPACT/APPLICATIONS**

The bathymetric and geophysical data collected this summer will be used
in wave and circulation models. The near-bottom measurements and ripple
characteristics being recorded by the bottom tripods will be used to
validate models for waves, currents, sediment transport, and bottom
morphology.

**RELATED PROJECTS**

These studies are part of the Ripples DRI and closely related to the
OASIS studies and the NOPP Community Sediment Transport Project.

**REFERENCES**

Wilkin, J. L. (2006) The summertime head budget and circulation of
Southeast New England Shelf waters, *Journal of Physical Oceanography*,
36(11): 1997-2011.