evolved to a substantially wider berm condition in the bottom image. Table 1 lists
computed rates of shoreline change since 1979 over the entire Fire Island barrier together
with the standard deviation associated with the listed shoreline change rate. From Table
1 it is seen that on average, the Fire Island shoreline is retreating landward at rates ranging
from 0.04 m/year to 2.5 m/year depending on the time interval. However, the large
standard deviation (equal to or exceeding 2.5 times the mean rate of shoreline change)
indicates that notable shoreline accretion occurred at some locations during each of the
time intervals. The approximate 15-year (December 1979 to April 1995) average rate of
shoreline change was calculated at 0.66 m/year. The rate of shoreline change plays a
central role in the development of sediment budgets and estimating design quantities such
as beach fill volumes and renourishment intervals. Because the large spatial and temporal
variability in the shoreline rate of change is largely influenced by the presence of shoreline
undulations, a detailed examination of these morphologic shoreline features was initiated.
The goal of these analyses was to quantify the space and time scales associated with the
shoreline undulations in order to develop design concepts compatible with the presence
of shoreline undulations. Another goal was to develop a better understanding of the
shoreline undulation scale of influence in the context of the larger-scale morphology and
evolution of the Fire Island barrier.
Table 1. Average Shoreline Change Rates on Fire Island Since 1979
1
Time Interval
Rate of Shoreline Change (m/year)
Standard Deviation (m/year)
"6.7
1979 1983
2.5
"2.5
1979 1988
1.0
"1.9
1979 1995
0.66
"5.2
1983 1988
0.04
"2.9
1983 1995
0.14
"3.6
1988 1995
0.22
1
Adjusted to account for beach fill placement.
Spectral Analysis
This section discusses a spectral analysis of the shoreline position data aimed at
quantifying the length scale of the shoreline undulations observed and measured on Fire
Island (Fig. 4). An estimate of the power spectrum was computed for each of the
shoreline position data sets using the Fast Fourier Transform (FFT) technique, which
transforms the data (in this case) from the spatial domain to a cycles per meter (inverse
of the wavelength) domain. Peaks in the energy spectrum define those shoreline
undulation wavelengths that contain the most energy and consequently represent the
dominate wavelengths present in the input data. The first step in this analysis involved
developing a method to isolate and extract the shoreline undulations from the shoreline
position data. This step is necessary in order to remove the large-scale curvature
associated with the ocean shoreline of the Fire Island barrier. This procedure is analogous
to removing the tidal signal from data recorded by a wave gauge prior to analyzing the
data for wind wave information and is referred to as de-trending the signal. The procedure
used to isolate the shoreline undulation signal from the shoreline position data sets
involved computing a running average shoreline position using a 1.525-km averaging
window and then subtracting the average shoreline from the original shoreline. An
example of the procedure is illustrated in Fig. 5 for a 10-km segment located in
Gravens
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