ERDC/CHL CHETN-IX-14
March 2004
antenna elevation data. General trends are for decreasing speeds on inbound transits and increasing
speeds on outbound transits. Another significant feature is the variation in speed and acceleration at
some locations during the transits. These trends make it difficult to fulfill the requirements of the
PIANC empirical models.
The ship's location for the bow antenna track is shown in Figure 6 for the Munkebo Maersk and
Alligator Liberty. Ship speeds and times of passage at several locations are shown on these plots for
reference.
Site Selection Criteria. Sites selected for the squat analysis had to have a relatively constant
channel cross section and ship speed for the empirical equations to be applicable. Antenna elevations
exhibit a lag in elevation change after a change in ship speed since the water level depression that the
ship travels in takes time to reach an equilibrium shape after the ship changes speed. The constant
speed criteria led to the requirement that the ship's speed was relatively constant for 5 min prior to
the squat evaluation. Three locations (Figure 1) were selected that satisfied the constant channel
cross-sectional criteria. They are as follows:
a. Outside the jetties (OJ) at (2388369,313181).
b. Inside the jetties (IJ) at (2360109,328625).
c. Rebellion Reach (RR) at (2342952,342367).
Cross sections at the three locations are shown in Figure 7. Water levels were determined at these
locations using the Customs House, Fort Sumter, and Folly Island gauges along with the numerical
model output.
Ship Elevations. At each of the analysis locations (plus the at-rest location where squat was zero),
the elevations of the three GPS receivers were determined based on 120-sec averages. The averaging
was done to eliminate any ship motions other than squat at the bow and stern on the ship's center
line. Figure 8 shows these corrected elevations for the port, starboard, and bow GPS sensors for the
Munkebo Maersk and Alligator Liberty. Because of small values, the bow curve is shown with a
12.2-m (40-ft) constant value added to improve readability.
OBSERVED AND PREDICTED SQUAT CALCULATIONS: The "Water Resources Annual
Hydrologic Data Report of South Carolina for WY 1999" Web site was used to determine values of
specific conductance to define density differences due to salinity differences at pertinent locations in
the Charleston Harbor reach. From these data, the North Charleston berthing location on the Cooper
River was the only area where specific conductance differences were significantly different from
other locations at Charleston Harbor. At North Charleston, specific conductance was about 50 per-
cent of values at the other Charleston Harbor locations during the 21-25 April 1999 field study.
Based on 30 parts per million (1,021 kg/cu m (63.77 lbs/cu ft)) being applicable to the all Charleston
Harbor areas except the North Charleston area, typical ship block coefficients, and North Charleston
having 50 percent of the specific conductance of the other areas; density effects on ship draft amount
to about a 1-percent increase in draft at North Charleston due to density effects alone. For average
draft of Charleston Harbor ships, this amounts to an increase of about 0.1 m (0.3 ft). This value was
added to the observed squat for the three ships (Ever Diamond, Golden Farmer, and MSC Sabrina)
berthing at North Charleston.
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