North shore restoration of West Ship Island
Geosynthetics | February 2013
This project, submitted by Flint Industries, won an Award of Excellence in the 2012 International Achievement Awards(IAA) / Geosynthetics presented by the Industrial Fabrics Association International. Each project in its own special way, exhibits outstanding innovation, technical skill and design excellence.
Submissions of geosynthetics projects for the 2013 International Achievement Awards are being accepted now. Submit your projects by June 15, 2013.
Ship Island is a barrier island 12 miles off the U.S. Gulf Coast near Gulfport, Miss. It is approximately 7 miles long, 1/8 mile wide, and predominately barren of grass and foliage. It also offers the only deepwater harbor between Mobile Bay and the Mississippi River.
During Hurricane Camille in 1969, the island was severed, effectively forming two separate islands—East Ship Island and West Ship Island. No permanent inhabitants reside on the islands and the only permanent structure is historic Fort Massachusetts, built in the 1860s.
As part of a $300 million project to make Ship Island whole again, in August 2011 the U.S. Army Corps of Engineers began dredging sand from the Gulf of Mexico seafloor and placing it on the north shore beach of West Ship Island.
Since the dredging activity would create significant turbidity that could damage prized sea grass beds, the Corps identified a need for a turbidity barrier with a high ultraviolet (UV) resistance and with a vertical curtain having a small aperture size to sufficiently contain suspended sediment, yet allow water to pass through. A continuous and secure anchorage system to the seabed, in addition to high upper buoyancy, was also necessary due to frequent tropical storms, including possible hurricanes. Accurate and sustainable placement was paramount so the vegetation it was intended to protect was not damaged.
New turbidity barrier
The stringent Corps requirements necessitated a change in design from traditional turbidity barriers. Buoyancy tubes were previously available in only a few common diameters.
Water depth, current velocity, tidal range, and weather conditions necessitated infinite buoyancies to suit site-specific conditions. To solve this problem, a buoyancy tube sleeve was fabricated in the desired diameter from high-strength woven geotextile fabric with an ultra-high UV resistance and then filled with spray-injected foam. Once the foam expanded, it embedded into the fabric weave creating an extremely strong buoyancy unit.
A continuous and secure anchorage system was also required to prevent displacement of the barrier, which could damage the sea grass. Previously used chain ballasts and cables attached to anchors in the seafloor would not provide an adequate bottom seal and would allow suspended sediment to escape underneath.
This method of anchorage required expensive components as well as extensive time to install. To address this concern, an anchor tube sleeve—also constructed with heavy woven geotextile fabric—was attached to the bottom of the filtration curtain. Once the barrier was positioned in the desired location, the tube was then filled with sand hydraulically using a small pump, creating a secure anchorage system. As with the upper buoyancy tube, infinite anchor tube diameters were possible to provide the necessary ballast to suit site-specific conditions.
A total of 5,500 linear feet of the redesigned barrier boom was installed, and although it was intended to perform for five months per the contract requirements, it functioned as specified for the entire 12-month project duration. During this time, the turbidity barrier withstood the impact of a Category 1 hurricane as well as several other tropical storms.
Since there was no displacement of the barrier during the term of the contract, no damage to the sea grass beds occurred. This is also the only known instance where a turbidity barrier withstood a hurricane with no noticeable damage or displacement.
Once the barrier system was removed and brought ashore upon contract completion, the empty anchor tube was removed from the barrier and was discarded. The turbidity barrier was then shipped to the manufacturer for installation of a new anchor tube, in readiness for a future project.
Product cost, as well as both installation and removal times, was significantly less than previously used turbidity barriers. Additional savings were realized since no maintenance was required during the life span of the turbidity barrier.
For years, turbidity barriers have been classified in the industry as Type 1, Type 2, and Type 3. But the performance of the revolutionary boom devised and put into action at the Ship Island site set a new industry standard for turbidity barriers used in severe marine environments, resulting in what could be a new Type 4 category.
“We took the liberty to call it a category 4,” said Flint’s Joe Smallwood, explaining how it exceeds the requirements of a Type 3 boom.