ERDC/CHL CHETN-IX-14
March 2004
channel in 36.6-m water depth. Although wave transformation occurs as the waves travel from the
deeper NOAA location to the shallow-water CHL directional gauge, the comparisons are useful to
demonstrate possible ranges of the wave parameters.
During the transit times for the 12 ships from 21-26 April, wave heights at the CHL gauge varied
from 0.5 to 1.7 m, with an average of 1 m. Wave periods ranged from 3.5 to 9.2 sec, with an average
of 5.4 sec. Wave directions varied from 83 to 211 deg (direction toward which waves travel,
measured clockwise from north), with an average of 156 deg. In general, wave conditions were
relatively "benign" during this study.
Additional water level information was available at Charleston Harbor from NOAA and the
U.S. Geological Survey (USGS). The main NOAA gauge is on the downtown waterfront at the
Custom's House. An additional gauge is on the Highway 17 bridge. There was also a gauge where
the Interstate crossed both the Cooper River and the Wando River. The USGS did not have
elevations on these gauges. However, CHL used GPS to establish elevations on docks at North
Charleston and Wando River. Leveling was then used to get water surface elevations at the dock.
The data from the water level was used to adjust the elevation of the gauges on the interstate bridges.
A gauge at Fort Sumter and one outside the jetties at Folly Island were also used in the analysis.
GLOBAL POSITIONING SYSTEM DATA COLLECTION:
Global Positioning System. Ship motions were measured with GPS technology and post-
processed using dual-frequency, kinematic survey techniques to obtain the highest possible accuracy
of 1-3 cm. GPS surveys of this type have been successfully performed in several ports and rivers
over the last few years with little inconvenience to either the pilots or the vessels.
post-processing kinematic survey techniques. The technique requires two receivers recording
observations simultaneously. The receivers synchronize the data collection, depending on the speci-
fied rate. The reference receiver (base station) remains fixed on a known control point while the
roving receiver (onboard the ship) collects data. The observation data are post-processed by com-
puter to calculate relative vector or coordinate differences. The technique requires that four or more
satellites must maintain a constant lock at both receivers to process the data. The processing software
outputs the horizontal positions in South Carolina 1927 grid coordinates.
A temporary GPS base station (fixed) was located in a secure area at Fort Johnson during the ship
track study. The Fort Johnson site was selected so that data could be collected from the offshore end
of the channel to any dock within the harbor. Static GPS survey techniques were used to determine
the position of the Fort Johnson base station by using National Geodetic Survey (NGS) published
benchmarks in the area. A benchmark at Fort Moultrie was also used during the tracking of several
ships.
The procedure for obtaining these measurements required that a three-person survey crew board the
vessel and install three GPS antennas near the bow and the port and starboard bridge wings. This
process took approximately 10-15 min to complete. The bow antenna was positioned near the
forward edge of the bow along the center line of the ship. The bridge antennas were positioned as
near as possible to the outside port and starboard edges of the ship. Each antenna was positioned to
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