Engineers develop plans to save Louisiana's coastal beaches and marshes from natural and human impacts.
BY MICHAEL F. STEPHEN, PH.D., P.G., AND MICHAEL POFF, P.E.
The Louisiana shoreline faced significant challenges even before Hurricanes Katrina and Rita hit last September. Two areas in particular require immediate attention if they are to survive and continue to provide the Gulf Coast, as well as the rest of the nation, with the range of natural resource benefits we have come to depend upon.
The first critical area involves 3.2 miles of shoreline from Pass Chaland to Grand Bayou Pass Barrier that, with restoration, will provide significant storm buffering for New Orleans. Without attention, any protection will be lost, given the current land loss rate of 73 acres per year (roughly three football fields per day).
The second project, equally critical, is the preservation and fortification of the 13- mile-long Caminada Headland shoreline.
As the receiving area for 75 percent of Louisiana's oil and gas production, the loss of the Caminada Headland would prove disastrous to a nation already reeling from escalating fuel prices.
In the fall of 2002 and spring 2005, the state awarded the planning phases of the two projects, respectively, to a consulting team on which Coastal Engineering Consultants, Naples, Fla., is the coastal design lead. In business since 1977, Coastal Engineering has completed several award-winning engineering projects, including two major coastal restoration projects. The firm also has expertise in mapping, surveying, land appraisal, and real estate - all services that must dovetail effectively for the Louisiana shoreline projects to be sound and durable investments for the people of Louisiana.
Following is a history and detail of the two projects, the strategies originally proposed, and the revised strategies that Coastal Engineering and its partners are recommending in the wake of Hurricanes Katrina and Rita.
Pass Chaland to Grand Bayou Pass Barrier
This project's objective is to restore approximately 3 miles of barrier island beach, dune, and marsh. Coastal Engineering had already prepared its design plans and construction specifications for this project prior to Katrina striking.
While little has changed in the actual design of the project in the wake of the hurricane, the degree of restoration will indeed be greater because of the storm. To meet this need, the sponsoring state and federal agencies are requesting a $1 million addition to the initial budget of $19 million from state and federal funding agencies, legislators, and Congress. If funding is granted in the spring of 2006, work will begin immediately, with a projected completion date of Dec. 31, 2007. The project will have a 20-year life expectancy.
Background and challenges - As with the demise of the rest of Louisiana's barrier islands, Pass Chaland has been affected by a deadly combination of natural forces and human impacts, including extraction of oil and gas from the Louisiana Continental Shelf - which has caused subsidence - and interruption of sediment flow to the delta because of Mississippi River flood control measures.
To make matters worse, miles of pipeline canals have been cut over the years through the beaches and marshes to deliver raw products from drilling platforms to shipping ports, thus severely altering the hydraulics of the area. This allows more damage to occur when a storm hits because the natural drainage flow-ways are interrupted, and storms now have a pathway to interior marshes and land masses - areas that previously were protected from coastal flooding.
As more barrier islands are affected, erosion advances exponentially until it occurs at a rate that is nearly impossible to reverse, and the Gulf and bays shielding the mainland become one. At Pass Chaland, the one-two punch of Mother Nature and man has been ongoing since the mid-1800s, which accounts for the current astounding rate of erosion.
Over the centuries, the Mississippi River overflowed its banks significantly every five to seven years. This natural and productive process deposited sediment and replenished the marshlands, keeping them strong and healthy and sustaining habitat for abundant plant and animal life critical to the region's ecosystem. With extensive construction of levies and redirection of the river's flow in many areas, some of these sediments are now trapped upstream or flushed out into the Gulf of Mexico. They are not reaching the areas dependent on them to produce soil and minerals needed to sustain not only the shoreline and the barrier islands, but also the Gulf itself. Clearly, the situation has reached crisis proportion.
Strategy - Coastal Engineering Consultants proposes to build the beach and dune landward of the existing beach and dune - rather than building seaward - to increase fill stability (see Figure 1). If the marsh is going to survive future storms, it must be built higher than the surrounding marshland. This requires replacing marsh material over a very consolidated sediment (basically, Mississippi mud).
When placed, this material will settle and compact over the existing bay during a three-year period to an elevation within the normal tidal range. It is projected to endure for the next 17 years.
Specifications indicate a narrow tolerance because only 1-1/2 feet of differential elevation exists in the area between high and low tide. There is an art and a science to determining a constructed marsh elevation that will maintain water exchange through the marsh, thus keeping it healthy.
The initial strategy was to block off an existing flow-way to the Gulf of Mexico, but when Coastal Engineering conducted its study of the area's tides and currents, engineers realized that maintaining the tidal exchange with the Gulf was imperative to maintaining the health of the new and existing marshland.
A new flow-way will be excavated through the existing marsh to create a channel to sustain the tidal flow to the back barrier marsh; about 10 acres of marsh will be excavated to save 350 acres.
Because of large volumes of very fine sediments pouring out of the Mississippi River, good sand and compatible sediment needed to save the marsh are limited and locating them can be challenging. Preliminary work by the U.S.
Geological Survey and other federal agencies laid the foundation for Coastal Engineering's geophysical and geotechnical surveys.
Fortunately, the firm found a buried distributary channel that is a relic riverbed loaded with the types of sands, silts, and clays needed to build a marsh, beach, and dune system. With the dredged material from this relic channel, Coastal Engineering's design will be able to save the marsh, and restore the beach, the dunes, and the island.
The objective of the second project is to restore the 13 miles of Caminada Headland and related dune, beach, and marsh.
Background and challenges - Bayou Lafourche Peninsula, an abandoned delta peninsula that juts out into the gulf from Caminada Headland, has been eroding at a rate of 45 feet per year for the past 100 years through the same process of subsidence, storm, and human impact that has caused erosion in other areas of Louisiana.
Central to this area is Port Fourchon, a major gateway for 25 percent of the nation's total domestic oil and gas consumption. Clearly, if the erosion continues unabated, it will be only a decade before the direct effect is felt nationwide, and the indirect effects - having to purchase this gas and oil elsewhere - will ripple throughout the world.
Moveover, and of greater concern to many Louisianans, is the preservation of numerous, rare ecosystems (Chenier ridge habitats) and many shorebirds and wildlife unique to coastal Louisiana that reside in them.
Because of its promontory position, Caminada Headland's sediment transport is divergent, some going east and some west, which accelerates the natural erosion process. Regardless of which strategy among the many under consideration is undertaken, reconstruction of the headland will require millions of yards of sand initially and millions more during the coming years to ensure its stability and success. To date, Ship Shoal is the only source identified that contains enough sand. Unfortunately, it is located 40 miles into the Gulf. Given the size of the dredge plant and equipment needed to transport the significant quantities of sand between Ship Shoal and the headland, not to mention the variable weather conditions throughout the year, the cost and time to accomplish the project will rank it among the largest in U.S. history.
Strategy - Coastal Engineering Consultants is still in the process of analyzing several strategies to restore the headland.
In addition to sand fill where practical, the firm may recommend construction of complimentary structures, such as offshore breakwaters, to reduce wave energy, as well as other solutions.
Regardless what is finally incorporated, all parties agree that this will be a huge project spanning 50 years - requiring as many as three years to finish just the design and permitting, and an additional three years to complete the initial construction, followed by four maintenance renourishments occurring once every 10 years thereafter.
At this time (late November), federal and state authorities and agencies are still reviewing the breadth of the project, especially in light of future hurricanes, and hope to turn their findings over to Coastal Engineering for its final feasibility report due in early 2006.
Michael F. Stephen, Ph.D., P.G., is president, and Michael Poff, P.E., is vice president of engineering, at Naples, Fla.-based Coastal Engineering Consultants Inc. They can be contacted at firstname.lastname@example.org and email@example.com, respectively.