ERDC/CHL CHETN-IV-33
June 2001
Jarrett (1976) compiled previous work and assembled additional data to establish an inlet cross-
sectional area - tidal prism relationship for 108 inlets on the Atlantic Ocean, Gulf of Mexico, and
Pacific Ocean coasts of the United States. Shigemura (1981) found relationships with
correlations between throat width and tidal prism at 231 natural bays on the major coasts of
Japan. Walton and Adams (1976) related the volume of sediment in the ebb shoal to the inlets
associated tidal prism and found increasing volumes of sediment with increasing tidal prisms.
Based on laboratory experiments, Hayter et al. (1988) developed relationships indicating that the
ebb jet flow governs ebb shoal size and shape. Gibeaut and Davis (1993) classified inlets based
on the statistical analysis of ebb shoal outlines along the barrier island coast of west central
Florida including Dunedin Pass, Longboat Pass, New Pass, Big Sarasota Pass, Midnight Pass,
Stump Pass, Gasparilla Pass, Captiva Pass, and Redfish Pass. Relationships similar to those
found for the ebb shoal, increasing shoal volume with increasing tidal prism, were found for the
sediment volume contained within the flood shoal (Carr 1999). This previous work shows that
the tidal prism is a decisive factor determining the morphology of a coastal tidal inlet, and it
enters discussion of asymmetries given in the following paragraphs.
Distances to Attachment Bars: Asymmetry indicators were determined by subtracting half
of the inlet critical width from the measurement of the distance from the channel center line to
the updrift or downdrift attachment point.
Figure 4 plots distance to the downdrift attachment point versus tidal prism. Jarrett (1976)
provides a listing of 108 inlets with known tidal prisms, as well as number of jetties. These
inlets were analyzed here. Data points are denoted by closed symbols for nautical charts and by
open symbols for aerial photographs. Consistency in notation is maintained through all plots of
asymmetry indicators in this Technical Note. The inlets were distinguished by the number of
jetties.
A trend of increasing distance to the downdrift attachment point with increasing tidal prism was
identified and quantified for each category of number of jetties as well as for the entire data set.
Regression lines were determined regardless of whether the measurement was taken from
nautical charts or from aerial photographs. The regression lines (as well as the data points) are
plotted in black for no jetties, in blue for one jetty, and in red for two jetties.
All trend lines in Figure 4 are governed by a power function as shown in Equation 2 with WA2
representing the distance to the downdrift attachment point as shown in Figure 2.
WA2 = a * Pb
(2)
The coefficients a and b, however, are distinct for each trend line associated with the individual
sets of data points (Table 1). The table also provides the correlation coefficient (R2) value for
each regression line shown in Figure 4. The coefficients of Equation 2 differ depending upon the
number of jetties at an inlet. If an inlet is to be modified, such as construction of a new jetty or
alteration of an existing jetty, it can be expected that a change in morphologic symmetry will
occur. Seabergh, Cialone, and Stauble (1996) and Stauble (1998b) document change in entrance
channel location at Barnegat Inlet, NJ, in response to modification of the jetties there. More
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