total shoreline undulation "height" on Fire Island is about 26 m with a standard deviation
of about 6 m. The seaward and landward average rms amplitudes are both about 14 m.
Significant amplitudes were found to be approximately 40 percent larger than the rms
amplitudes and the highest one tenth amplitudes are nearly 100 percent larger than the rms
amplitudes.
Spatial Analysis
This section describes an analysis aimed at identifying some of the spatial distribution
and propagation characteristics of the shoreline undulations. The procedure involved
comparing the shoreline undulation data to the computed landward and seaward rms
amplitude and recording those baseline stations where the shoreline undulation exceeded
either the landward or seaward rms amplitude. The results provide an indicator of the
location of the seaward and landward bulges associated with the shoreline undulations.
Consequently, if the shoreline undulations propagate along the shoreline one would expect
to find both the landward and seaward rms amplitude exceeded at most baseline stations
as the shoreline undulation moved as a unit along the shore. Likewise, if the shoreline
undulations do not propagate, one would expect to find specific baseline stations where
the landward rms amplitude is exceeded frequently and other baseline stations where the
seaward rms amplitude is exceeded frequently.
Example results from the spatial analysis are illustrated in Figs. 7 and 8. Plotted in the
top portion of the figures are the locations where the seaward rms amplitude was
exceeded. Plotted in the bottom portion of the figures are the locations where the
landward rms amplitude was exceeded. The individual shoreline undulation data sets are
segregated from each other by distance from the horizontal line in the middle of the figure.
The "trend line" plotted in Figs. 7 and 8 indicates the tendency for the presence of either
the seaward bulge of a shoreline undulation (accretion cusp) or the landward bulge of a
shoreline undulation (erosion cusp). The trend line was calculated by summing the
number of times an accretion cusp was found at that station and subtracting the number
times an erosion cusp was found at that station. Because each data set represents the
shoreline condition at a specific instant in time, plotting all data sets in one figure (as in
Figs. 7 and 8) provides an indication of the temporal characteristics of the shoreline
undulations. For example, at baseline station 24.5 km in Fig 7, the shoreline undulation
exceeded the seaward rms amplitude in 3 of the 11 data sets (May 1995, August 1993,
and September 1994), the shoreline undulation did not exceed the seaward rms amplitude
in any of the data sets, hence three tick marks are plotted in the top portion of the figure
and the trend line is given a value of 3.
The 8-km-long shoreline coverage in Fig. 7 corresponds to the shoreline reach from
Fire Island Pines to Davis Park (central Fire Island). In this figure there is notable
segregation between the locations where the accretion and erosion cusps were found
suggesting that the shoreline undulations do not propagate in the alongshore direction as
a collective unit. Furthermore, the shoreline undulations seem to be present only
intermittently, indicating that environmental conditions prior to or during the survey or
photography may be important for the formation and prominence of the shoreline
undulations. The interpretation of Fig 7 is that accretion cusps can be anticipated in two
Gravens
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