ERDC/CHL CHETN-IV-33
June 2001
Distance Offshore vs. Tidal Prism
Nautical Chart (NC) and Aerial Photograph Data (AP)
20
No. of Jetties
0 - NC
1 - NC
2 - NC
0 - AP
15
2 - AP
0 trendline
1 trendline
2 trendline
10
5
0
106
107
108
109
1010
3
Tidal Prism, m
Figure 7. Distance from shoreline intersection with channel center line
to most offshore point of the ebb-tidal shoal (L)
Table 3
Coefficients of Equation 2 for Trend Lines in Figure 7 (most
offshore distance of ebb shoal)
2
a
b
R
Number of Jetties
-3
0
2.5x10
0.404
0.839
-3
1
1.5x10
0.399
0.536
-4
2
5.0x10
0.483
0.692
slightly with changes in impressed forces). The morphology of some inlets may undergo semi-
periodic cycles or changes irregularly spaced in time (episodic changes, as triggered by a storm).
At such inlets, it is difficult to predict, for example, the location of the natural channel that might
be maintained by dredging. The temporal behavior of the ebb shoals is also of interest for the
development of bypassing and causative relationships between dredging and process responses.
Time-varying behavior of inlets is illustrated in Figures 8 and 9 for St. Augustine Inlet, located
on the northeast coast of Florida. The data were obtained through analysis of aerial photographs
available from the mid 1940s to the present. The net direction of longshore transport on this
coast is from north to south. St. Augustine Inlet was originally a natural inlet that migrated
between two well-defined locations prior to stabilization in 1941, when the north jetty was
constructed. By 1957, the old secondary inlet had closed. By 1970, a massive spit (named
Conch Island) and located directly to the south of St. Augustine Inlet merged with surrounding
Anastasia Island and Bird Island. Construction on the south jetty was completed in 1975
(Marino and Mehta 1986).
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