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Grand Isle shoreline stabilization

Helping protect a vulnerable environment

Hartman Engineering / Grand Isle, Louisiana

Opportunity

First settled by Europeans in the 1780s, when Louisiana was under Spanish rule, Grand Isle is still the state’s only inhabited barrier island. A popular destination for sport fishermen, Grand Isle has more than 280 species of fish in its waters. More than 15,000 people attend the annual Grand Isle Tarpon Rodeo, which was established in 1928.

The island’s chenier hardwood forests (primarily oak and hackberry) attract many species of migratory birds. The three-day Grand Isle Migratory Bird Festival is held when migrating birds are returning from the south. Elmer’s Island, on the Gulf side of Grand Isle, is the site of a wildlife refuge.

Grand Isle is located at the mouth of Barataria Bay. The focus of an ambitious 1998 coastal restoration project called Coast 2050, Barataria Bay lost an estimated 11 square miles of wetland per year between 1978 and 1990. Grand Isle has been hard-hit by hurricanes such as Hurricane Katrina, which severely damaged the only bridge to the mainland.

Solution

In 2008, Mott MacDonald was retained to develop a comprehensive long-term engineered solution that would reduce storm damage and provide a usable recreational beach for the Gulf shoreline of Grand Isle, between Elmer’s Island and Barataria Pass.

We assembled existing data such as historical aerial photos, bathymetric data, and engineering reports, and collected new data including new bathymetric and topographic measurements, hydrodynamic data (water levels and currents), and geophysical and geotechnical data.

Numerical modeling was conducted of wind-wave growth and transformation, tide- and wind-generated currents, sediment transport pathways, and shoreline morphology. A coastal engineering analysis identified reasons for the success or failure of previous coastal projects constructed along the shoreline of Grand Isle and Elmer’s Island.

Outcome

As a result of the study, Mott MacDonald developed a 50-year comprehensive shoreline protection and restoration master plan that identified and prioritized alternatives to increase shoreline stability and included these components:

  • Barataria Pass jetty rehabilitation
  • Caminada Pass jetty rehabilitation
  • Breakwater modification
  • Breach protection
  • Sediment bypassing at Caminada Pass through pass modification

The town of Grand Isle is located at the mouth of Barataria Bay.

Circulation modeling of alternatives.

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