VIMSWAVE - VIMS
DIRECTIONAL WAVE DATA
From the fall of 1988 until the
spring of 1995, the Virginia Institute of Marine Science, in cooperation with
the Virginia Department of Conservation and Recreation, operated a wave station
2.5 nautical miles northeast of the Thimble Shoal Light (TSL) in Lower
Chesapeake Bay to collect long-term directional wave observations. After the
initial season, a second wave station was briefly placed in operation north of
the Wolf Trap Light (WLT) and a third wave station was operated
simultaneously with the TSL station during the winter and spring of 1993 at the
eastern end of the Thimble Shoal Entrance (TSE) channel near the
Chesapeake Bay mouth. Although the program was terminated in 1995 for lack of
funding, these stations provide the only systematic wave observations known to
exist in Chesapeake Bay. The observations are made available here in html
format (Microsoft Excel).
Station Locations
Thimble
Shoal Light (TSL): latitude 37º 2.4’N, longitude 76º 11.9’W
Thimble Shoal
Entrance (TSE): latitude 36º 58.4’N, longitude 76º 2.3’W
Wolf Trap
Light (WTL): latitude 37º 24.8’N, longitude 76º 11.8’W
Station Instrumentation
Waves were measured and recorded
at the above stations using self-contained Sea Data Model 635-9RS directional
wave gages with Paro Scientific high precision quartz pressure transducer, KVH
digital compass, and Marsh-McBirney 2-axis electromagnetic flow sensor. The
gages were attached to weighted tripods mounted on the bay floor and retrieved
at monthly intervals for data recovery and servicing by scientists and crew of
the R/V Bay Eagle commanded by Captain L. D. Ward. Scientist in charge:
J.D. Boon (boon@vims.edu).
Wave Parameters
Both Sea Data wave gages were programmed
to conduct burst-sampling; i.e., to power on sensors and record a burst
sample of either 1024 (TSL, WTL) or 2048 (TSE) readings of pressure and current
flow at 1-second intervals once every three hours. Dynamic pressure was later
converted to fluctuating surface level using linear wave theory and corrected
for the effects of frequency-dependent attenuation at mean sensor depth. Wave
parameters determined during processing include the following international sea
state definitions adopted by the International Association for Hydraulic
Research:
·
Zero-moment wave height (Hm0) – The root mean square value of the
corrected surface level (CSL) times four; considered equivalent to the
significant wave height or average of the one-third highest waves in a sample
wave record.
·
Zero-crossing wave period (Tz) – The duration of a sample wave record divided by the
number of zero-up crossings of mean CSL in that record.
·
Spectral peak period (Tp) – The reciprocal of frequency corresponding to the
highest peak in power spectral density computed for a sample wave record.
Other parameters recorded:
- Mean
current speed (MC_SPD) – The vector-averaged bottom current speed per
burst.
- Mean
current direction (MC_DIR) – The vector-averaged bottom current
direction per burst referenced to true north using the digital compass
heading corrected for local magnetic variation.
- Principal
wave direction (PWD) – The mean wave direction determined by principal
component (eigenvector) analysis of the zero-mean vector (U,V) components of
the orbital current. The 180º ambiguity in principal direction was removed
through current-pressure correlation assuming progressive wave motion.
Reduction in variance achieved by the principal axis was initially
recorded as an index for directional spread but was later discontinued.
PWD, like Tz, is a whole-sample measure that ignores frequency-dependent
variations in spectral wave energy.
- Spectrum
analysis – Wave energy percentages are reported per burst in seven
discrete frequency bands delineated by the following wave periods in
seconds:
|
>20 |
20-16 |
16-12 |
12-8 |
8-6 |
6-4 |
<4 |
WAVE DATA BASE
Statistical summary data - Directional wave
data for the Thimble Shoal Light (TSL) wave station are organized in six files
between 1988 and 1995. Two additional files contain observations for Wolf Trap
Light (WTL) and the Thimble Shoal Entrance (TSE). Each file contains an array
of wave parameters in columns and bursts in rows identified by date and time.
Click on the files listed below to open (use Windows ‘copy’ and ‘paste’ to copy
data to an Excel workbook).
Wave records during storms – Individual waves can be viewed in simultaneous records
from station TSL and TSE during three northeasters in 1993 and a brief but
intense storm on September 22, 1994, that produced the largest individual wave
observed at TSL (3.4 meters from crest to trough). Click on the files listed
below to open (use Windows ‘copy’ and ‘paste’ to copy data to an Excel
workbook).
Energy-weighted joint
distribution of wave period and wave direction – The color-coded plots listed below show the periods of
the most energetic waves and their direction of travel during a typical winter
season. Westerly-directed Atlantic Ocean waves of 8-11 second period are
characteristic of station TSE and south-southeasterly moving Chesapeake Bay
waves of 4-5 second period are the norm at station WTL. Sea states at Station
TSL contain a mix of these waves.
Acknowledgements – Field operations in
all types of weather conditions, laboratory work maintaining instruments and
tripods (including parts of one recovered in Virginia Beach) and unending data
processing chores made collection of the data listed here anything but simple.
Special thanks go to Bob Gammisch, Frank Farmer, Dan Hepworth, Todd Nelson, and
Captain Duran Ward of the R/V Bay Eagle.