Plaquemines Barrier Island Restoration Projects
Planning and Coastal Engineering
Located in southeast Louisiana, west of the present-day Mississippi River Delta, the Plaquemines barrier island system extends approximately 30 miles (48 km), from Sandy Point westward to the Grand Terre Islands. The Plaquemines shoreline is the youngest of the barrier island systems, formed during the reworking of abandoned Mississippi River delta lobes over the past 7,000 years. These barrier islands are transgressive in nature, migrating inland as they are overtopped and reshaped by wave processes. Subsidence, global sea level rise, and anthropogenic factors (i.e. levees, jetties, pipeline canals) contribute to the significant loss of barrier islands and wetlands in the study area.
Employed by Brown & Root Civil in Plaquemines Parish in the early 1990s, Lally served as coastal engineering lead on a number of feasibility studies and concept designs under contracts with Louisiana Department of Natural Resources, and Plaquemines Parish. As the first line of defense against hurricane storm surge from the Gulf of Mexico for several communities, restoration of the Plaquemines barrier islands system was as priority. At the time the Plaquemines coastline was experiencing retreat rates up to 49 ft/yr (15 m/yr), with the most severe erosion rates up to 190 ft / yr (60 m/yr), centered on Shell Island.
Originally a continuous peninsula 5 miles long, installation of the Empire jetties in 1949, immediately updrift of Shell Island, interrupted the natural east to west littoral drift in the area and contributed to sediment depletion on the island. In 1979, Shell Island was overtopped and breached by wave forces during Hurricane Bob and thereafter continued to break up, with a gap several kilometers wide exposing the interior back bay marshes to the Gulf of Mexico. A highly valued estuary for shrimp production and oyster cultivation, the marshes too deteriorated due to increased wave forces and saltwater intrusion.
Lally’s early coastal engineering work on Shell Island and other Plaquemines barrier islands in the 1990’s involved assessing site conditions through topographic and hydrographic surveys, shoreline change analysis, recurrence interval storm and wave forecasting, littoral processes modeling, and development of alternative restoration and stabilization solutions.
Several alternatives including structural (i.e. armor stone) were postulated, however barrier island restoration and long-term nourishment through introduction of large volumes of sand was recommended as the preferred solution. With the assistance of the Louisiana Geological Survey (LGS) and U.S. Army Corps of Engineers – New Orleans District Lally determined there were several potentially suitable sources of beach-quality sand available within reasonable distance of the barrier island restoration sites. Of interest were borrow areas in the Lower Mississippi River, as this was a renewable resource of high-quality sand, and within reasonable pumping distance (+/- 15 miles) to the eastern Plaquemine Barrier Islands. The area around 60-Mile Point (River Mile 60) showed the greatest potential as a large source of course sand in an accretional area of the river.
With the assistance of LGS and the Bureau of Ocean Energy Management (formerly the U.S. Minerals Management Service), several offshore borrow areas were also investigated, including Ship Shoal, a particularly large deposit of beach-quality sand to the southwest of the study area. Due to sediment quality (high overfill ratios), or, in the case of Ship Shoal, long haul distances, these offshore borrow areas were determined to be more economically viable for the barrier islands further to the west (i.e. Timbalier Island and Isles Dernieres).
Preliminary beach fill templates for emergent and breached island sections were developed by Lally with 30H:1V foreshore slopes, minimum crest elevations of + 4 ft. NGVD and variable crest widths, to be determined in final design once funding was known.
Another element of the concept Plaquemines barrier island restoration plan was installation of a permanent discharge pipeline from the center of the Lower Mississippi River borrow areas, south along the Empire waterway corridor to the coast. From the Empire jetties, sand slurry could be pumped east and west for restoration along Scofield, Pelican and Shell Islands, and potentially marsh creation projects throughout the Barataria Basin. Regional dredge contractors were consulted to acquire an understanding of equipment capabilities, production rates and costs.
Lally assembled plans for three (3) phases of the Shell Island to Sand Point Barrier Island Restoration, and a comprehensive project, as candidates for submittal to the 2nd Priority Project List under Coastal Wetlands Planning Protection and Restoration Act (CWPPRA). The proposals contained project descriptions, anticipated benefits, engineering and construction timelines and costs, concept plan and pipeline routes, required permits, and potential funding sources.
The devastating impacts of Hurricane Katrina in 2005, and Deepwater Horizon Oil Spill in 2010, came with the silver lining of funding to be able to restore several segments of the Plaquemines barrier island system. The substantial funds available now allowed for greater protection, with higher beach fill sections to prevent overtopping, and backbay marsh creation. Final engineering and construction have successfully been carried out by a larger, and very qualified range of coastal engineering consultants, scientists, and dredging contractors under a number of different contracts, and the reconstructed Plaquemines barrier island system now provides coastal communities and wetlands greatly enhanced protection from storm surge and salt water intrusion while creating substantial new beach, dune and back bay marsh habitat.
Services Provided
Field Investigations
Hydrographic and Topographic surveys
Shoreline Change Analysis
Concept Design of Feasible Barrier Island Restoration Projects
Beach Fill Template Design
Riverine and Offshore Borrow Source Identification
Wave Runup and Wave Force Calculations
Dredge Technology Investigations
Production and Cost Estimation
Funding Proposals to CWPPRA
Location
Plaquemines Parish, Louisiana
Period
1990s
Under CWPPRA, CIAP and Deepwater Horizon Oil Spill NRDA settlement funding mechanisms, approximately 10 miles of Plaquemines Barrier Islands Restoration Projects have been completed through placement of over 15.3 million cy of sand, and creation of over 1,200 acres of new beach/dune and marsh habitat, applying the borrow sources, pipeline routes and beach fill templates postulated by Lally in the 1990s.