ERDC/CHL CETN-IV-29
September 2000
Ocean
Ebb
Jet
Bay
Figure 5. Redirection of ebb flow by jetty
JETTY CONTROL ON FLOW PATTERNS: The presence and configuration of jetties at
inlets plays a significant role in controlling the circulation pattern. Patterns of current flow
within inlets are different for equal-length and offset parallel jetties, as shown in Hughes (2000).
Equal-length jetties require water to enter the inlet at the same distance offshore, preventing bias
of the current on one side of the inlet as schematically shown in Figure 6a. Flow separation at
the jetty tips is approximately distributed equally on each side of the inlet. This distribution
directs the current along the channel center line. Thus, near the jetty tips, the central portion of
the inlet carries the strongest current with strength weakening laterally toward the jetties.
Further into the inlet, flow entrainment by the flood jet decreases current speed in the center of
the channel and spreads the flow distribution across the channel. Thus, distribution of the
current speed in the interior of the inlet is broadened as compared to that near the inlet entrance.
Offset jetties create a bias in the current in which water with the greatest speed enters the inlet
from the side with the shorter jetty, as shown in Figure 6b. The current is then directed at an
angle to the inlet center line. This situation results in the strongest current being near the shorter
jetty on the outer portion of the inlet, and shifting to the other side in the central portion of the
inlet.
EBB JET AND EDDIES: Jet flows at inlets extend from the interior of the inlet to some
distance seaward, depending on the strength of the current. The presence of jetties enhances the
jet by constraining the flow. Jet orientation is initiated parallel to the jetties, but that orientation
can change through the ebb cycle. Steep velocity gradients occur if jetties are present and these
gradients are located within and parallel to the jet as well as perpendicular to it. Because of these
velocity gradients, the jet is enhanced and eddies are formed. In particular, jetty tips are points
where velocity gradients perpendicular to the jet flow are strong. Eddy formation initiates near
jetty tips, but eddies can migrate with lengthening of the jet and changes in ocean tidal current.
Shinnecock Inlet, NY, serves as an example of ebb jet and eddy patterns at jettied inlets.
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