ERDC/CHL CHETN-IX-14
March 2004
d. Both Huuska and Barrass equations are compared to the maximum of the observed bow or
stern squat.
DATA ANALYSIS: The purpose of this CHETN is to calculate ship squat from the GPS measure-
ments and compare to the three empirical equations. Several steps are involved in this process and
are discussed in the paragraphs that follow.
a. Convert the GPS antenna measurements using the appropriate datum conversions discussed
in the previous section.
b. Pre-process the ships according to ship trim considerations.
c. Accurately determine surface-water elevations using measurements and/or numerical model
predictions.
d. Correct ship speeds during the transits.
e. Correct ship vertical elevations during the transits.
Select sites based on required channel cross section and ship speeds.
f.
Ship Trim Considerations. According to PIANC (1997), large initial trim can affect the squat
characteristics of a ship. A reasonable criterion of ships having differences in at-rest bow and stern
draft of less than or equal to 1/200 (i.e., 0.5 percent) of the ship length was selected to insure that a
sufficient number of ships would be available for analysis. Therefore, the database of 12 ships
(Table 1) was limited to six ships: Alligator Liberty, Ever Diamond, Golden Farmer, Munkebo
Maersk, Newark Bay, and MSC Sabrina. Two of the vessels, the inbound Munkebo Maersk and the
outbound Alligator Liberty, are used as examples of inbound and outbound transits in this CHETN.
Water-Surface Elevation (WSE) Determination. One of the formidable challenges at
Charleston Harbor is the relatively large tidal variation that makes an accurate determination of the
water-surface elevation (WSE) along the path of the ship very difficult. Tidal variation during the
field study was semidiurnal and up to 1.8 m (6 ft). A two-dimensional (2-D) numerical model
(RMA10-2D) existed for the Charleston Harbor reach from an earlier study. The RMA10-2D model
is part of the TABS-MD Numerical Modeling System within the Surface-Water Modeling System
(SMS). RMA10-2D is a 2-D depth-averaged hydrodynamic, finite element model for free surface
flows.
This model was calibrated using observed tides during the field study at the NOAA Customs House
gauge (Figure 4) and a reasonable match was obtained for the observed gauge data at Fort Sumter,
North Charleston, and Wando River. When available, actual gauge data were used to define the
WSE along the ship track because differences between computed water levels and observed water
levels were sometimes up to 30 cm (1 ft). The North Charleston (NC), Customs House (CH), and
Wando River (WR) gauge data were used for the at-rest locations of the ships, depending on their
berthing location.
referenced to NGVD29. To obtain the ship's actual speed relative to the water and the antenna height
above the water, the tide-generated velocity and water surface at the ship's location for each point of
evaluation were determined.
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