require adding approximately 15 m to the berm width and is expected to significantly
increase the cost of the project because of the large volume of sand needed to construct
the wider beach berm. A design concept that relies more on the dune structure and less
on a wide berm to provide the desired level of protection may prove to be an effective and
cost efficient alternative to the traditional design on Fire Island. It is noted however, that
the dune must be protected from erosion through the construction of a berm with
sufficient width to allow for the occurrence of shoreline undulations and the expected
long-term shoreline recession between renourishment intervals.
CONCLUDING DISCUSSION
Shoreline undulations are a natural part of the Fire Island shoreline morphology and the
presence of shoreline undulations should be anticipated after the construction of any beach
erosion control project on Fire Island. The analyses discussed herein have shown that the
wavelength of the shoreline undulations generally ranges between 1 and 2 km. The total
rms shoreline undulation height was determined to be about 26 m. The landward and
seaward rms amplitudes were both quantified at about 14 m. A spatial analysis indicated
that the shoreline undulations on Fire Island do not appear to propagate from one end of
the barrier to the other. Propagation of the shoreline undulations with in a limited (1 to
2 km) domain is possible. An important finding of the spatial analysis was that the
seaward and landward bulges of the shoreline undulations were preferentially positioned
along the shoreline. That is, based on the data sets examined in this study, certain
locations along the shoreline can be expected to periodically develop large erosion or
accretion cusps but not likely both. This finding indicates that the shoreline undulations
may be excited by specific environmental forcing conditions (waves from a particular
direction) and their location controlled by irregularities in the offshore bathymetry. In
support of the assertion that specific environmental forcing excites the shoreline
undulations is the finding from the spatial analysis that the shoreline undulations are
intermittent features that are more prominent in some data sets than in others. Further
study is required to determine the validity of this speculation.
The impact of shoreline undulations on a typical beach fill design configuration was
shown to be significant and could lead to greater than anticipated maintenance costs or
a reduced level of protection. Explicit consideration of the presence of shoreline
undulations in the development of alternative design configurations was found to be
essential for a successful project. Alternative design concepts include construction of a
"shoreline undulation buffer" or constructing a substantial dune section that provides the
desired storm protection without relying on a wide beach berm for protection. Regardless
of how the design is modified, the geometric characteristics of the shoreline undulations
on Fire Island must enter into the development of the design concept for a successful
project.
ACKNOWLEDGEMENTS
Permission to publish this paper was granted by the Headquarters, U.S. Army Corps
of Engineers. Funding for the studies that lead to development this paper was provided
through the Fire Island to Montauk Point Reformulation Study by U.S. Army Engineer
District, New York. The thoughtful review of this paper provided by Ms. Julie Rosati and
Mr. Bruce Ebersole of the U.S. Army Engineer Waterways Experiment Station, Coastal
Gravens
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