considered preliminary. Data from three events at each site are summarized in Figure 7. Incident
wave directions are rough approximations. The results indicated that about 20-50 percent of the
wave energy passed the pocket at the Lake Michigan sites and about 60-80 percent at Ontonagon. No
strong dependence on incident wave direction is evident.
Dir e ction (de g r e lative to e ntr ance )
Wave energy fraction passing pocket for three field sites; relative direction is wave approach
direction minus direction aligned with entrance channel (Carpenter 2001)
These field results provide some information about the effectiveness of a double versus single pocket
absorber. The Lake Michigan results compare reasonably well with corresponding physical model
data for configuration C in Figure 5. However, the pocket effect on waves does not appear to vary
with incident wave direction as much as in the physical model studies (Figure 4). This difference in
behavior can be attributed to the unidirectional, monochromatic waves used in the physical model.
Distance between the pockets and the jetty entrance may also affect comparability of field and
physical model data, though this effect cannot be evaluated with existing data. The Ontonagon
results, which were selected to represent waves coming straight into the entrance, compare reason-
ably well with corresponding physical model data for configurations A and B, 0-deg direction, in
Figure 4. It was noted, however, that these were single-point field measurements in a system with
potentially significant cross-channel variation.
MONITORING PROGRAM: As part of the Monitoring Completed Navigation Projects (MCNP)
program, pocket wave absorbers at Pentwater Harbor entrance, MI, were selected for monitoring. An
aerial view of the site is shown in Figure 6. The objective of the monitoring program was to
determine the effectiveness of pocket wave absorbers in reducing wave heights in entrance channels
and harbor areas where they are utilized in parallel steel sheet-pile jetty configurations. Additional
prototype wave data would be obtained and a physical model would be constructed. After validation
of the physical model with prototype data, it was anticipated that design guidance relative to pocket
wave absorber parameters would be developed.