ERDC/CHL CHETN-IV-33
June 2001
will be analyzed as an inlet without jetties. Oregon Inlet has two small channel margin linear
bars. The property that is most likely limiting the distance from the inlet edge to the attachment
bars are the downcoast and upcoast spits encroaching into the inlet. At Oregon Inlet, the ebb
shoal-bypassing bar attaches to the southern spit near the inlet. Sediment from the spits becomes
entrained in the ebb jet and as the spits migrate into the inlet the attachment bars move toward
the inlet as well. Two jetties are proposed at Oregon Inlet (Miller, Dennis, and Wutkowski
1996) whose implementation will alter development of the ebb shoal, morphologic asymmetry,
and sediment bypassing.
Distance to updrift attachment points is plotted against tidal prism in Figure 5, which shows
patterns similar to those in Figure 4. Distance to the updrift attachment point from the inlet edge
increases with increasing tidal prism for any number of jetties, and a comparable scatter of prism
ranges under the 2-km distance to the attachment bar observed.
Figures 4 and 5 show that inlets with no jetties tend to form more distant attachment points than
those with jetties. The trend toward greater scale is attributed to the fact that unjettied navigable
inlets such as Willapa Bay, WA and San Francisco Bay, CA, are larger than typical jettied inlets.
For these large inlets, jetties are unnecessary or infeasible.
Regression lines were determined for the updrift attachment bars (Figure 5) similar to the method
for downdrift attachment bars (Figure 4). Table 2 gives the coefficients and R2 values associated
with Equation 2 with WA2 replaced by WA1, the distance to the updrift attachment bar shown in
Figure 2. There is reduced correlation between distance to the updrift attachment point and tidal
prism shown in Table 2 at inlets with one jetty. Examination of the data in Figure 5 shows that
at the inlets with only one jetty the jetty is consistently located on the updrift side of the inlet.
This is a typical configuration because one purpose of a jetty is to reduce the flow of sediment
into the inlet and to afford shelter from the predominant waves. This jetty placement may
interrupt the ebb shoal complex at the attachment point and cause a shortened distance.
From Tables 1 and 2 it is evident that the downdrift asymmetry indicator (Figure 4) has stronger
correlation with tidal prism than the updrift indicator. Available information was insufficient to
examine the dependence of inlet asymmetry on magnitude and direction of the longshore
sediment transport rate, expected to be a leading parameter and to be considered in future CIRP
research.
The asymmetry of the main navigational channel may contribute to the data spread in Figures 4
and 5. A straight channel is expected to promote morphological symmetry and a reduced
distance to the downdrift attachment point. Conversely, it is hypothesized that an asymmetrical
channel with a smaller angle α between the thalweg and the shoreline (Figure 6) will result in an
increased distance to the downdrift attachment point. This phenomenon is believed to act at
Shinnecock Inlet, NY, where the ebb shoal is attached to the updrift shoreline at the jetty but has
and extended distance to the downdrift attachment bar under conditions of a dominant direction
of longshore transport. At Shinnecock Inlet the net west-directed longshore drift maintains the
updrift attachment point close to the jetty. The strong net drift and, possibly, migration of the
ebb jet (Militello and Kraus 2001) promote an asymmetrical channel alignment and increase the
distance to the downdrift attachment point. Additionally, the longer length of the east jetty
compared to that of the west jetty may contribute to the inlet asymmetry.
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