Restoration of wetlands and barrier islands for storm protection in the Northern Gulf of Mexico
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Spray dredging applying a thin layer slurry of dredged material to the marsh
Mark A Ford, National Park Service, US
The northern Gulf of Mexico coast frequently experiences tropical weather systems and rising sea level impacts. Communities are at risk. Restoration of two US National Parks play a role in the protection of the coastal communities. Disaster risk reduction is enhanced through restoration of their wetlands and barrier islands. These ecosystems reduce wind and storm surge intensities, and protect risk reduction levees.
Last update: 06 Feb 2023
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Contexto
Défis à relever
Rising sea level and increased major storm frequency and intensity mean that the need for restoration of coastal ecosystems is greater than in the past. Increased inundation, salt water intrusion and encroachment by human development are resulting in lost coastal habitats in non-protected places. Additionally, lack of necessary funding for large-scaled restoration projects is limiting our ability to conducted planned projects.
Ubicación
Northern Gulf of Mexico USA
North America
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Summary of the process
These restoration techniques and their outcomes work together to provide a set of multiple lines of defense for coastal communities against the impacts of climate change, sea level rise and surges from storms. Alone each is useful, but combined they create a stronger and more resilient coastal risk reduction. There is no particular order to which these blocks should be completed. As an analogy, they are like members of a team, each of which has a role to play in the overall goal of coastal protection and the reduction of disaster risk.
Building Blocks
Filling old canals
Spoil banks from canal dredging are being pushed back into the canals, as funds for projects come available, in "backfilling" events. This helps restore the normal hydrology and keep the marsh healthy and less stressed. Canal backfilling eliminates the hydrology barrier present from the dredged material spoil banks, as well as partially filling in canals long unused for oil exploration or production extraction. Using heavy equipment, such as excavators on amphibious marsh buggies, old spoil banks are pushed into the canals. Trees are knocked down and the final elevation is similar to the surrounding marsh. This allows natural flows of water and allows for the mixing and equilibration of fresh and brackish waters. Canals have been backfilled at a cost of US$12K/0.5 ha.
Hydrologic barriers are removed, water flow is no longer restricted, an exchange of fresher and saltier waters can take place and sediments within the water can move into the marsh, nourishing the marsh and adding to the vertical soil accretion. Submerged aquatic plants thrive as the elevation of the canal bottom is raised.
Enabling factors
Old unused canals must be present on the landscape. Funding to push the old dredged material spoil banks must also be available. Once all required permitting is completed, projects can proceed.
Lesson learned
This technique is relatively easy from a logistical point of view. It is essential to have all required permits completed before work begins. Also, be sure that all stake holders of the lands have been included in planning. In the USA, there are multiple owners/users in some places, such as National Parks, where fossil fuel companies own the subsurface rights to extraction. Old canals are often relics of oil exploration and some companies do not want these filled, though they have often gone unused for decades.
In the case of canals filled at Jean Lafitte NHPP, the canals have expanded and broadened in width since the time they were dug. Pushing the spoil banks and vegetation back into the canals does not completely restore them to land, but sufficiently raises the water bottoms to allow submerged aquatic plants to thrive and for hydrology to quickly return to baseline levels. This impact improves the health of the wetlands overall.
Thin Layered Marsh Nourishment
Dredged materials from periodic dredging events are sometimes broadcast onto the marsh, including the floating marsh, in a marsh nourishment effort. When possible, spray dredges are used. These dredges collect the material, mix it with ambient water and spray the slurry on to the marsh surface. The thickness of the applied slurry can vary from very light, which would primarily nourish the marsh with fresh sediments and light nutrient loads, or applied to the marsh surface at depths up to and around 30 cm. Thicker applications would be desired if there is a strong vertical elevation deficit due to high soil subsidence rates.
Enabling factors
Dredging of nearby waterways would need to be conducted. Given the relative short distances the slurry can be sprayed, areas of application would need to be within fifty (50) meters of the dredging operation. Funding and permits would also need to be secured.
Lesson learned
Thin-layered application of dredged materials can be very beneficial to the marsh. Applications of greater than 30 cm can bury an existing plant community too deep, thereby killing the vegetation and perhaps causing more harm than good. It is important to have close oversight as dredging contractors often will apply the slurry randomly to suit their desires, versus applying where the restorationist would prefer.
Using Controlled River Diversions for Restoration
River diversions have been built along the Mississippi River in Louisiana to help replace some of the lost function of periodic pulses of fresh water, historically delivered during periods of high water, into the coastal wetlands. River diversions are water control structures that allow controlled flows from the Mississippi River into various basins of the delta. The Davis Pond diversion, with a 300 cubic meter per second discharge, upstream from New Orleans, releases fresh river water into the Barataria Basin which strongly influences salinities and sediment loads into the Jean Lafitte NHPP Barataria Preserve. Abundant fresh water and nutrient input into the preserve results in a robust growth response from submerged aquatic vegetation (SAV).
Enabling factors
Local and state governments provide funding for these expensive structures. Public agreement is essential as changes to the hydrology and the ecosystem will impact users, though most see this as an improvement.
Lesson learned
Not everyone agrees that freshwater inputs are good. Commercial fishermen worry that their fishing grounds may change and they will have to travel farther, and use more fuel, to find the same species they currently catch. Excessive water and sediment inputs can also change the floating and submerged plant communities in ways not desired. Periodic monitoring is essential to be sure the diversions are managed and operated in an optimal manner.
Barrier Island Restoration for Disaster Risk Reduction
Barrier Islands have long been reported to provide disaster risk reduction from storm surge. Modelling shows that barrier islands contribute to storm surge attenuation, but do not remove it completely. Benefits include annual hurricane and storm damage risk reduction to the mainland, annual recreation benefits, and annual fishery losses avoided. By replacing sand back into the littoral zone, there is a long-term contribution to the sediment budget of the islands and an opportunity for island chains to replace lost sands from one place to provide growth in others. Sands are either barged in from other locations, or dredged from nearby sources, then pumped into degraded area in stages, which allows for some natural settling to occur.
Enabling factors
Major funding, typically by federal government sources, is needed to conduct these restoration projects. Permitting and environmental impact studies must be completed to ensure no non-intentional adverse impacts occur.
Lesson learned
Near shore and splash zone fauna will be temporarily displaced. Activities of nesting sea turtles may also possibly be displaced. Oversight of contractors performing sand placement is essential to ensure sand is placed properly in the correct place and to correct elevations.
Impacts
Wetlands and barrier islands provide disaster risk reduction in the form of storm surge and sea level rise attenuation and the protection of the hurricane protection levees that border the metropolitan New Orleans in Louisiana. Restoration of the Gulf of Mexico islands at Gulf Islands National Seashore also contributes to disaster risk reduction and the attenuation of storm surge. These ecosystems serve to protect people, homes, businesses and critical infrastructure such as roads, bridges and railroads. Disaster risk reduction levees have been documented to have been protected by swamps in recent hurricanes. Coastal marsh has lowered impacts from storm surge and sea level rise by being able to absorb rising water. Barrier islands decrease storm surge. In mainland areas closest to barrier islands, damage from storms such as Hurricane Katrina was less than areas which were more exposed. Restoration through the replacement of lost sands have protected structures such as Fort Massachusetts on West Ship Island. Wetlands and barrier islands also are a critical component of the local tourist economy.
Beneficiaries
The people and businesses of coastal communities are the beneficiaries of restoration projects.
Story
The city and metropolitan area of New Orleans in Louisiana, USA is divided by the Mississippi River. The central business district, the historic French Quarter and some of the suburban communities lie to the east side, known as the East Bank. Remaining communities of the area are on the West Bank. In 2005, Hurricane Katrina made landfall just east of the city of New Orleans. Loss of life, mostly from failure of the storm protection system, was nearly 1,600 people in metro New Orleans alone. This loss of life occurred primarily on the East Bank side. The West Bank side of New Orleans did not flood. The biggest difference is the storm protection system for the West Bank communities are providing additional protection from healthy swamp and marsh ecosystems. Research indicated that levees with marsh and swamp protection did not fail during Hurricane Katrina. Later, in the year 2012, Hurricane Issac, a category 2 storm on the Saffir-Simpson scale, also hit the metropolitan New Orleans area. Now, 7 years post Katrina with restoration projects in place throughout the New Orleans area, similar to what has been done in Jean Lafitte NHPP, there were no levee failures. Healthier restored marshes, filled canals, restored swamps all provided sufficient protection to the storm protection system to spare all of theses communities from floods.
