ERDC/CHL CHETN-III-69
March 2004
Table 3
Strengths
Limitations
Quantitative data on actual performance of prototype pockets
Limited length of record and range of conditions (no fall or
in presence of real waves
winter storms)
Hourly data over a 7-week time period
No measurements of incident waves
Includes time series data and spectral analysis
Gauges adjacent to jetty walls
Based on cases with significant height from Gauge MI002, Hm0MI002, greater than 0.1 m (0.33 ft), the
average ratio of significant wave height from the landside of the absorber, Hm0MI004, to that on the
lakeside of the absorber is 0.621 m (2.0 ft). The corresponding energy ratio is 0.39 m (1.3 ft),
indicating that wave energy after the pocket absorbers was 39 percent of the energy level before the
absorbers. The percent energy passing the pocket exhibits a mild tendency to increase with
significant height, reaching 45 percent for cases with Hm0MI002 greater than 0.5 m (1.6 ft) (McKinney
and Sabol 2003). This field data set suggests that the Pentwater absorbers are slightly less effective
than indicated by the University of Michigan field data (Figure 7) and physical model data
(configuration C, Figure 5).
Although the MCNP field study has limitations, it provides a much more extensive suite of field data
than was previously available for pocket absorbers. Pocket absorber effectiveness as a function of
various wave parameters can be examined. As before, absorber effectiveness is expressed with a
parameter indicative of relative transmitted wave energy, (Hm0MI004/Hm0MI002)2. An indication of
incident wave direction can be obtained from National Data Buoy Center (NDBC) buoy 45007,
which operated through the time period of the MCNP study. The NDBC buoy is located in the
middle of the southern lobe of Lake Michigan, about 144.8 km (90 miles) south-southwest of
Pentwater.
The dependence of absorber effectiveness on significant wave height lakeward of the pocket is
shown in Figure 9. Only cases with dominant deepwater waves traveling toward the entrance are
included (cases for which wave direction from the NDBC buoy fell within the range 225-360 deg).
Similar plots for dependence of absorber effectiveness on peak wave period, TpMI002 , and incident
wave direction, as represented by the NDBC buoy, DNDBC , are given in Figures 10 and 11. The
fraction of wave energy passing the absorber appears to be independent of wave height, period, and
direction.
SUMMARY OF INTERIM RESULTS AND FUTURE PLANS: Preliminary results from the
generic physical model and the prototype data presented herein indicate that pocket wave absorbers
are effective in reducing wave heights in vertical-wall entrance channels. However, both the physical
model and prototype data collected are limited in their applicability, as summarized in Tables 1-3.
The prototype data obtained were at single points adjacent to the jetty walls and do not depict
variation across the channel. Also, prototype data were obtained for limited wave conditions and
only one pocket configuration. Physical model and prototype data give an incomplete, and somewhat
inconsistent, portrayal of the dependence of absorber effectiveness on incident wave parameters.
1
McKinney and Sabol, op cit., p. 9.
10
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