ERDC/CHL CHETN- IX-7
December 2001
hSF
= safety factor allowance for bottom type.
From Figure 14, values of hWA,Bow range from 0.20 to 0.88 m. The worst-case hWA occurred for
the slow-speed case at hWA,Bow = 0.88 m (2.9 ft). Most of the ship transits occurred on flood tide
cycles, so the effective channel depth increased for these runs up to 0.7 m (2.3 ft). In the case of
ebb tides, however, the channel depth would need to be reduced. The actual tide during the
outbound World Utility run on May 30, 1999 was hTI = + 0.20 m (0.66 ft). Squat was not
measured in either the field or laboratory measurements. Field measurements from the low pass
filtering of the data (see "Prototype Measurements" section) indicated that it was of the order of
0.3 m (1 ft). Several empirical estimates of squat were presented in Demirbilek and Sargent
(1999). In general, it depends on the vessel speed and the channel blockage, which is a ratio of
the ship cross-sectional area to the channel area. According to Figures 5-3 and 6-4 in EM 1110-
2-1613 (HQUSACE 1995) for a trench-type channel like Barbers Point, hSQ should be of the
order of 0.2 to 0.3 m (0.66 to 1.0 ft) for ship speeds during the field measurements. Finally, the
hSF accounts for the type of seabed. For hard bottoms (as at Barbers Point), an hSF = 0.91 m (3 ft)
is recommended.
Inserting these values for the allowances into Equation 2 for the worst-case hUKC that occurred
for the slow-speed case on flood tide
hUKC = 0.88 - 0.20 + 0.20 + 0.91 = 1.79 m
(3)
Comparing this value to the maximum hUKC at Barbers Point from Equation 1, there is only a
reserve of 0.11 m (i.e., 1.90 m - 1.79 m) (0.36 ft) in underkeel clearance available for use for
larger waves and low tides. No advance maintenance or dredging tolerance allowance hAM was
included, but because there is generally no silting problem due to the hard coral bottom, this is
probably not necessary at Barbers Point. There is some clearance in the hSF that can be used for
larger wave heights, so the existing channel depth is adequate. Based on this analysis with a
very limited data set, it does not appear that the existing channel depth is overly conservative.
How does the measured hWA compare to the existing guidance? Again, the point is to show that
the laboratory data are reasonable relative to existing guidance. Additional data for a variety of
ships and wave and channel conditions will be needed to make a recommendation in channel
design guidance. EM 1110-2-1613 recommends hWA = α HI, where α = 1.2. Figure 15 shows
the normalized ratio αmeas= hWA/HI for the laboratory data of Figure 14. The larger laboratory
values of HI = 0.75 m and 0.65 m for the DDU412 and DDU422, respectively, were used rather
than the field value of HI = 0.58 m. Thus, the αmeas ranges from a minimum of 0.3 to 1.3. The
EM 1110-2-1613 guidance is based on an assumed maximum value of pitch and roll angles in a
static environment without dynamic effects from the waves in combination. Thus, the hWA are
smaller than the guidance, except for the case of αmeas = 1.3.
The PIANC (1997) recommends βT ≤ hWA ≤ 2βT, where β = 0.2. The smaller value is for wave
heights less than 1 m. For the model ship draft T = 10.9 m, the corresponding values are 2.2 m ≤
hWA ≤ 4.4 m. These values are two and a half to five times larger (i.e., 2.2/0.88 = 2.5) than the
measured hWA largest value. Finally, the measured βmeas can be obtained by dividing the
measured hWA by T. The range of values for βmeas varied between 0.02 ≤ βmeas ≤ 0.08, a
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