Using Nature’s Blueprint for Coastal Reconstruction

TO REBUILD LOUISIANA’S COASTAL LANDSCAPE, scientists and engineers are turning to a natural concept that appears quite simple: deliver sediment directly to areas that need replenishment, just as the Mississippi’s floodwaters did in years past. But each step in a sediment transfer project demonstrates the complexity of replicating this natural process.

Finding Sources

Every project begins by selecting sediment that has characteristics suited to the project’s goals. Sand is best for rebuilding barrier islands, for example, while nutrient-rich silt boosts the growth of fragmented marshes.

Both the Mississippi River and the Gulf of Mexico are rich sources of available sediment. Because sediment from neither of these sources enters the wetlands naturally, it’s referred to as “new” sediment. New river sediment is also renewable. “Draining the interior of the continent, the Mississippi continuously replenishes its sediment load,” says Shea Penland, director of the Pontchartrain Institute for Environmental Sciences at the University of New Orleans. “It carries all the kinds of sediment needed for wetland restoration. We can choose to dredge a point bar for silt and clay, for example, or a mouth bar for coarser sand.”

Although the most immediate source may be the huge deposits at the mouth of the Mississippi River, sediment can also come from bays, waterways and the Gulf of Mexico. Maintenance dredging of navigation channels can provide material for rebuilding wetlands, as can offshore shoals. “There are large, ancient bodies of sand 10 to 30 feet underwater that we can excavate without digging big holes in the sea floor or altering the wave fields,” says Penland. Access to these sources may be restricted by oil and gas infrastructure and buffer zones, but, Penland says, “There is plenty of sand within reach to restore our coast.”

Whatever the source, sediment delivered via pipeline is tested for grain size and contaminants. “Because contaminants don’t stick easily to the relatively coarse sediment we use,” says Nancy Powell, hydrologic engineer with the U.S. Army Corps of Engineers, “the risk of contamination is actually quite low. A greater concern is the degree of salinity in the sediment’s slurry. Too much salt will kill a freshwater marsh.”

Moving the Material

Sediment is usually delivered by pipeline over distances ranging from a few yards to many miles to places where it is most needed, or to locations difficult to rebuild by other restoration methods. “In theory, we could move sediment for hundreds of miles, but typically in Louisiana, pipelines run less than 10 miles,” says Rachel Sweeney, ecologist and project manager with the National Marine Fisheries Service.

Whenever possible, pipeline routes are laid along existing channels and canals to minimize their impact on the environment. Although pipelines don’t take up much space, Sweeney says installing them raises a lot of questions: “Who resolves property rights along transport routes? What footprint does setting up pipes leave in the fragile marsh? Is it better for the pipes to float or sink? How do you protect oyster beds and fisheries, and who maintains the pipelines? These are some of the transport issues we have to consider in designing a successful project.”

Shaping the Landscape

After a pipeline route is established, deciding how much material to transport depends on the site’s projected elevation. “Elevation dictates how wet a wetland is,” says Powell. “Engineers set a target height and then calculate how much and how quickly the transported sediment will compact.”

Once the pipeline starts to discharge its load, the sediment is shaped to fulfill the project’s objectives. The sediment may be piled up with earth-moving equipment, or sprayed in a thin layer directly onto existing marsh. Or a pipeline might discharge sediment into water where drifts and currents carry it to its intended destination. Sediment enhances the skeleton of a landscape, pumping up components such as ridges and bay rims. Underwater, sediment can form features like berms and reefs that reduce erosion by breaking up the energy of waves and tidal flow.

Keeping It in Place

Sediment consisting of large particles drains quickly and will stay put on its own. Earth-moving equipment packs down layer after layer of coarse sediment to build barriers to waves and storm surge. Fine clays and silts, however, may need structures to hold them in place while they compact and become stable. Low dikes of rock or earth, or barriers of synthetic fiber confine the material during settling until vegetation takes root, either through hand planting or through natural colonization. “We’ve used tubes and bags and fabric baskets that pop open and fill with water,” says Powell. “We position them to ensure drainage, then leave them until elevation builds up.”

Long-term Health

An area newly constructed from transported sediment can be quite barren. Given time, natural colonization is likely to take place, although hand plantings can jump-start natural vegetative growth and encourage fauna habitation. But the health and longevity of created marshes, like natural ones, depend on regular doses of rejuvenating sediment and nutrients. One method of feeding the marsh is to leave transport pipes in place permanently. Another is to periodically flood project sites with sediment-laden river water. Near the river’s mouth, where there are no levees, this can be done by diverting water through gaps cut into the river banks.

While building a functional wetland with transported sediment is not simple, its possibilities excite wetlands scientists. “Pipeline transport creates new land quickly,” says Penland. “We need to use it on a scale that has never been tried before. In building the river levees, we demonstrated the power we have to control huge floods. We can use that same power to restore the environment, too.”