Our Triton Marine Mattress System completed a shore stabilization project on Amelia Island:

1. PUBLIC/PRIVATE PARTNERSHIP FUNDS SHORE STABILIZATION
PROJECT
Amelia Island is the southernmost of the string of Sea Islands that
stretch along the east coast of the United States from North Carolina to
Florida. About 200 acres of the island’s south end is preserved as a state
park. In 2002, the Florida Park Service and a consortium of private property
owners known as the South Amelia Island Shore Stabilization Association
began to explore proposals for stabilizing the shoreline in this area to
protect the nearby maritime forest and ecosystem.
“The construction of terminal structures was the second phase of a larger
project to stabilize the southern 3.5 miles of ocean shoreline,” says Erik J.
Olsen, P.E. of Olsen Associates, whose firm engineered the $4.8 million
project. “The first phase of work involved placing 1.9 million cubic yards of
sand as beach fill. In 2003, we proposed building two structures to improve
the performance of the beach nourishment project by reducing accelerated
sediment losses at the terminus of the island.”
LEAKY STRUCTURES PROVIDE LONG-TERM SOLUTION
Olsen recommended building two “leaky” rock structures, a 1,500-foot-
long terminal groin and a 300-foot-long detached breakwater. The partially
permeable structures were designed to reduce the alongshore transport
rate of sand without adversely affecting various land forms or the overall
sediment budget of nearby Nassau Sound.
“We used large rocks and constructed relatively low structures,” Olsen
says. “They are purposely leaky, which means sand can continue to pass
into the sound and along the downdrift shoreline.”
STRONG CURRENTS ARE A BIG CHALLENGE FOR
CONSTRUCTION
Constructing the structures in a high-energy environment with tidal
currents of 3-to-4 feet per second was one of the trickier issues facing the
project contractor.
“Such a current would normally scour out the front of a traditional stone
foundation as it was being placed,” Olsen notes. “We needed to fix the
grade so we wouldn’t suffer the probability of large stone volume and
cost overruns.”
The best way to address this issue and prevent long-term subsidence
was to specify foundations using the Triton Marine Mattress System. Olsen
proposed confining 2- to 4- inch aggregate within a Tensar UX geogrid
structural matrix. Each mattress was 12 inches thick with overall dimensions
of approximately 5-feet by 38-feet. Where the groin structure extended
above the high-water mark, the foundation was changed to a composite
geotextile/geogrid underlayment manufactured by TET.
Both structures were constructed to a crest elevation of +5.5 feet
(NGVD) using a double-layer of 4-to-5 foot armor stone. Each structure
was designed to be overtopped during nor’easters, which is the period of
greatest sediment transport in the region.
TRITON INSTALLATION DOES THE JOB
The construction contract was awarded to The Industrial Company’s
(TIC) Marine Division in mid-2004. A subcontractor to TIC fabricated the
mattresses offsite and transported them to the island by truck. A
temporary trestle and crane were then used to install the mattresses and
armor stone.
TIC completed the installations in January 2005 after approximately
9 months of work. State park officials have since expressed great satisfaction
with the ingenuity of the engineering team, the commitment of the project
contractor and the performance of the Triton Marine Mattress System. The
project co-sponsors were particularly impressed with how well the new groin
and breakwater blend into their surroundings.
“They look very trivial,” TET Marine Manager Jeff Fiske says. “With Olsen
Associates’ designs, you’re literally only seeing the tip of the iceberg. But
that’s a good thing in a state park designated as a conservation area.” <
GEOSYNTHETICS APPLICATIONS NEWSLETTER ISSUE 15.1 FALL | 2005
Triton System Just Beachy for
Amelia Island
Installation of the Triton System at Amelia Island.