ERDC/CHL CHETN-I-67
June 2003
differences are of interest to the local residents of Tedious Creek since they imply that the authorized
configuration would have reduced the wave energy in the harbor.
In general, the maximum difference wave heights HDMax are not very large and occurred for
southwest waves for both the PP and CD area boxes. For the PP boxes, the HDMax = 0.08 m in box 6
for T = 6 sec; HDMax = 0.08 m in boxes 2, 5, and 6 for T = 10 sec; and HDMax = 0.09 m in box 6 for
T = 16 sec. Averaged over all boxes, the maximum difference wave heights H∆ = 0.04, 0.06, and
0.06 m for T = 6, 10, and 16 sec, respectively. For the PP boxes, all values are for waves traveling to
the southwest except for T = 6 sec (NW). For the CD boxes, the HDMax = 0.21 m in box 10 for
T = 6 sec; HDMax = 0.13 m in box 11 for T=10 sec; and HDMax = 0.10 m in box 11 for T = 16 sec.
Averaged over all boxes, the H∆ = 0.18, 0.10, and 0.08 m for T = 6, 10, and 16 sec, respectively.
All values are for waves traveling to the southwest for the CD boxes. Finally, the 95 percent
confidence interval for average difference wave height inside all boxes for all wave conditions is H∆
= 0.03 0.01 m. In summary, the wave height reduction that would have been afforded by the
smaller entrance gap had the authorized configuration been constructed is insignificant.
COMPARISON TO PREVIOUS STUDIES: The Offshore and Coastal Technologies, Inc. (OCTI
2001) performed a numerical model study of Tedious Creek, MD, for the Baltimore District in 2001.
They used the Army Corps STWAVE shallow-water directional spectral model to investigate wave
conditions in the vicinity of the CD for (a) pre-project or no-breakwater, (b) existing as-built project
with 122-m (400-ft) gap, and (c) authorized project with 91-m (300-ft) gap. The STWAVE finite
difference model had a grid spacing of 15.2 m (50 ft), which is much larger than the CGWAVE
spacing. The STWAVE model is appropriate for open coast and deepwater applications, and may
not be suitable for shallow depths where diffractions, reflection, and nonlinear dispersion could be
important.
A JONSWAP spectrum with 30 frequencies and 3.3 peak enhancement factor and Cosine-4th
directional spreading with thirty-five 5deg direction bins was used. They looked at waves from
nine directions from southwest to southeast, three wave heights from 0.5 to 2 m (1.6 to 6.6 ft), and
five water levels from 0 to 1.8 m (0 to 6 ft). Not all combinations were run. Most of the runs were
for waves traveling to the southwest, wave height of 1 m (3.3 ft), and water level of 0.73 m (2.4 ft).
The extreme storm case had a wave direction of southwest, height of 2 m (6.6 ft), and water level of
1.8 m (6 ft).
As a base case, OCTI ran a "no-breakwater" case of the original harbor before the breakwater was
constructed. They found that the existing jetties reduce the incoming wave heights by as much as
70 percent versus a no-breakwater condition. For the location near the CD that OCTI selected, this
is equivalent to a 0.3-m (1.0 ft) wave height from an incident 1-m (3.3-ft) wave offshore of the
breakwaters. This is comparable to the results obtained with CGWAVE for the existing gap. A "no-
breakwater" case was not run using CGWAVE since the breakwater exists and this was not an
alternative to be considered.
OCTI found that wave heights were increased as much as 50 percent at high tide, but negligibly at
low tide. This is why CHL selected only the worst-case high-tide level in this study. Wave breaking
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