ERDC/CHL CETN-IV-29
September 2000
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Figure 7. Jet and eddy formation and migration at Shinnecock Inlet, NY, offset jetties
For equal-length jetties, the process of jet and eddy formation occurs, but the patterns are
modified as compared to offset jetties. To illustrate the differences, Figure 8 shows calculated
vector plots at Shinnecock Inlet with the west jetty lengthened to the same distance offshore as
the east jetty. Comparison snapshots correspond to the same times as in Figure 7. Figure 8a
shows the beginning of the ebb tidal cycle. At this time, flow curvature appears at the jetty tips,
but eddies have not formed yet. After one hour (Figure 8b), the jet and eddy formation is similar
to that with the offset jetties (Figure 7b). However, close inspection reveals that the center of the
eddy west of the west jetty is further seaward with the equal-length jetties as compared to the
offset jetties. The circulation patterns two hours into the ebb cycle are shown in (Figure 8c).
The jet is narrower than with the offset jetties and the center of the west eddy is further seaward.
Thus, the lengthened west jetty allows for propagation of the west eddy further offshore, which
constrains the rotation and width of the ebb jet. By the third hour of ebb flow (Figure 8d), the
third eddy west of the jet has formed. The eddy is located southeast of the similar eddy that
forms for the offset jetties. The jet is constrained on its east and west sides by eddies that are at
an equal distance seaward. This situation keeps the jet nearly parallel to the inlet. Because the
jet exerts control over the location of the navigation channel, self-scouring of the channel would
tend to occur parallel to the inlet with equal-length jetties. Offset jetties provide a pivot point
that allows the jet to swing during the ebb tide, displacing the location of the entrance channel
from straight.
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