Geosynthetics October/November 2019 - 28

Reinforcing dunes and bluffs with geosynthetics

FIGURE 2 Nantucket, Mass., coastline after geotextile tube project completion (Courtesy SBPF)

The primary concern
was whether the
GSC revetment, if
exposed, would
act as a seawall,
narrowing the beach
until it disappeared.

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Geosynthetics | October November 2019

made to the GSC permit requirements
in North Carolina over the past 20 plus
years, and it illustrates the evolution of
regulations as the performance of these
systems continues to be assessed. The
current conditions of the permits also
support the effectiveness of GSCs to
protect imminently threatened structures and that they can be incorporated
into beach nourishment projects without degrading the performance of the
placed sand.
Detailed field-monitoring programs
are ongoing at the two sites (Montauk,
N.Y., and Nantucket, Mass.), where a
GSC-reinforced dune and a geotextiletube-reinforced coastal bluff have
been constructed. In New York, the
U.S. Army Corps of Engineers has been
working with Long Island coastal communities since the 1960s to study the
coastal processes along the southern
shore of Long Island as part of the
Fire Island to Montauk Point (FIMP)
study. Due to the severe erosion resulting from Hurricane Sandy in 2012 in
the downtown Montauk Beach area,
one of the recommendations from the
reformulation study following the hurricane was to implement a onetime,

stand-alone stabilization project with an
estimated 15-year project life to protect
the extremely vulnerable shorefront
commercial buildings while a longterm plan was developed (USACE-NAN
2014). Waves are the dominant forcing
mechanism for most coastal processes
in the downtown Montauk area and
offshore significant wave heights during extreme storm events may exceed
18 feet (5.5 m) (USACE-NAN 2014).
The alternatives considered for the stabilization project included (1) beach
restoration, (2) beach restoration and
a buried seawall, (3) a feeder beach,
(4) dune reinforcement and (5) dune
reinforcement and a feeder beach. Since
alternatives 1, 2, 3 and 5 assumed a periodic nourishment would be required
every four years, this increased the
annualized cost estimates for these
alternatives and violated the stipulation that this project be a onetime,
stand-alone project in advance of whatever long-term FIMP solution may be
implemented (USACE-NAN 2014). As
a result, a GSC-reinforced dune was
considered the only viable option.
This was the first federally funded
GSC-reinforced dune in the U.S., and
there were numerous protests and concerns voiced by a variety of groups and
individuals. The primary concern was
whether the GSC revetment, if exposed,
would act as a seawall, narrowing the
beach until it disappeared. The stabilization project site is more than 0.6-mile
(1-km) long and consisted of more than
11,000 GSCs to reinforce the dunes,
which were then reconstructed along
with placement of a protective berm.
The height and configuration of the
GSCs were designed to conform to the
existing dune height, to tie into the
existing undisturbed dunes to mitigate
the transfer of erosion to adjacent areas,
and to be stable up to a 50-year storm
event. The initial cost of the project was
Geosynthetics October/November 2019

Table of Contents for the Digital Edition of Geosynthetics October/November 2019
