ERDC/CHL CHETN-IV-33
June 2001
inlet to obtain comparisons independent of the width of the inlet itself (inlet width varied from
0.25 to 1.7 km). Nearly symmetrical shapes of the ebb shoal complex outline as well as
asymmetric shapes can be identified, showing wide diversity.
Relationships among forcing variables at tidal inlets and inlet asymmetry indicators were
developed through examination of 108 tidal inlets in the United States to aid in the analysis of
ebb shoal outlines as shown in Figure 3. The values for three indicators of ebb shoal asymmetry
as determined from nautical charts and aerial photographs are presented. The asymmetry
indicators are as follows:
a. Distance to the updrift point where the ebb shoal complex attaches to the shoreline
(labeled WA1 in Figure 2).
b. Distance to the downdrift point where the ebb shoal complex attaches to the shoreline
(labeled WA2 in Figure 2).
c. Distance of the offshore extent of the ebb shoal measured from the shoreline (labeled
L in Figure 2).
Estimates of error can be made for the process of interpretation of the asymmetry indicators.
Stauble (1998a) discusses more accurate procedures. The distance to the farthest offshore extent
of the ebb shoal complex was measured from the water-beach interface. The location of the
shoreline depends on the tide level at the time the photograph was taken (see Kraus and Rosati
(1998) for discussion of interpretation of shoreline position). An additional source of error is
associated with the method of identification of ebb shoal outlines. On aerial photographs, the
ebb shoal complex is best identified by the pattern of breaking waves. On a calm day, a minimal
number of waves will break, and the aerial photograph would either show no shoal or the shoal
would not be as easily identified, thus increasing the possibility for error. Such photographs
were eliminated from analysis.
For each inlet, many aerial photographs of various years and various maturity stages were
examined, and asymmetry indicator measurements made. Because the morphology of mature
inlets varies through time about an assumed dynamic equilibrium, the asymmetry indicator
measurements for each individual inlet were averaged. These average values were plotted in
Figures 4, 5, and 7.
Identification of asymmetry indicators from nautical charts eliminates visual error in
distinguishing the ebb shoal. National Ocean Service (NOS) nautical charts are comprised of
measurement made at different times; however, they were considered acceptable to obtain
asymmetry indicators. Only one set of indicators for each inlet examined was interpreted from
the most recent nautical charts. For some inlets, both aerial photographs and nautical charts were
available. In such case, measurements were taken from both. Therefore, only a single value of
the three asymmetry indicators was measured and plotted. Distances to the offshore extent of the
ebb shoal were identified on NOS charts through examination of the point at which the contour
lines were oriented similar to offshore contours far from the inlet. This distance was visually
clear and easily identified by assessment of the slopes of the contour lines.
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