Cheniers, Natural Levees and Spoil Banks
The Value of High Ground in a Coastal Landscape

While each hurricane season can expose the fragility of the coastal environment, the forces of weather may also reveal the landscape’s natural capacity to buffer the effects of storms. Forested ridges — cheniers and natural levees — are the “bones of the coast,” serving to lessen the destructive power of wind and water while providing habitat for a variety of species.

No more than 10 feet in elevation and running approximately parallel to the shoreline of the Gulf of Mexico, cheniers are remnants of old beach ridges. They take their name from the French word for the oaks found growing on them. Along the banks of streams and bayous, sediment deposited by floodwaters forms ridges described as natural levees. Even the modest heights of cheniers and natural levees are capable of sustaining upland shrubs and trees, providing a habitat distinctly different from the adjoining marshes and swamps and presenting opportunities for certain plant and animal species and human settlers alike to colonize and flourish.

The emergence of cheniers

Concentrated in the Chenier Plain of southwestern Louisiana and in the Moreau-Caminada chenier complex south of New Orleans, Louisiana’s cheniers are a geographic record of weather, water and land interacting over 4,000 years. During past periods of delta-building activity, waterborne sediment drifted from the mouths of distributaries and collected in vast mudflats extending into the Gulf of Mexico. As the mud accumulated new land emerged, eventually supporting vegetation and developing into marsh. When the rivers changed course and cut off the sediment supply, storms, waves and water currents worked the particles of mud, along with sand and shell debris from mud-dwelling creatures, into new arrangements, pushing coarser-grained particles onto shore and into beaches and ridges.

The pattern repeated itself time and again. Gradually the process built a sequence of ridges isolated by stretches of marsh. The most northerly cheniers are the oldest. The youngest, nearest the Gulf of Mexico, were created approximately 1,500 years ago.

Elevations natural and man-made

Spotting a live oak, toothache tree or wild grapevine in Louisiana does not guarantee the discovery of a chenier. Species populating cheniers seek advantageous elevation and may find hospitable conditions on natural levees — even on spoil banks — as well.

Natural levees are ridges formed from sediments delivered over the banks of rivers and bayous during floods. Finer sediments stay suspended longer and spread over the marsh a greater distance from the water body; coarser sediments fall out close to its edge, eventually building into a parallel ridge. These ridges assist in defining a watershed and in maintaining its natural hydrology. Even the modest elevations of natural levees give many species of coastal flora a chance to thrive.

Spoil banks are constructed of material dredged from navigation channels or oil and gas pipeline canals. Like man-made levees, spoil banks often disrupt natural hydrological patterns, damaging or destroying surrounding wetlands. “You can identify spoil banks by shrubs and trees lining only one side of the water,” says Clayton Breland, a geologist with the Louisiana Department of Natural Resources. “But an oak sapling or a palmetto seedling will attempt to grow at two or three feet above marsh level no matter what process deposited the soil there.”

Because the purpose of manmade levees is to provide protection, not ecological functions, federal regulations require clearing them of any flora but mown grass. However, these elevations also are capable of supporting upland vegetation. “The physical differences between a constructed levee and a chenier are principally their dimensions and the arrangement of their materials,” says Darryl Clark, a senior biologist with the U.S. Fish and Wildlife Service. “To build a levee we take specific soil materials and place them in a certain configuration. A chenier consists of sediments reworked through natural processes, with coarser ones remaining to form a mound that generally isn’t as high as a constructed levee, but is much wider.”

High ground in present-day Louisiana

Comparing a chenier to a man-made levee points to a fundamental value of forested ridges in today’s landscape: They stand as an essential line of defense against storms, forming a physical barrier to floodwaters that threaten marshes, waterways and human habitations.

The second fundamental value of forested ridges is the habitat they provide. “Cheniers are islands in a sea of marsh,” says Clark. “They provide a refuge from water for animals that feed in the marsh.” Ridge vegetation — and the insects that feed on it — is vital to the survival of scores of bird species. “For many birds, a chenier or another kind of forested ridge is the first landfall after a long migratory flight over the Gulf of Mexico,” says Clark.

The high ground also provides habitat for the human population. The distribution of roads, houses and buildings in coastal Louisiana follows the lines of natural levees and cheniers.

Fragile bones of a coastal landscape

Louisiana’s coastal environment is a network of interdependent ecosystems. Marshes protect forested ridges from erosion, and forested ridges shield marshes from inundation. But the landscape is naturally in constant change. Decline of one feature may diminish the strength of another.

“Time threatens all of Louisiana’s coast,” says Harry Roberts, former director of the Coastal Studies Institute at Louisiana State University. “Without an influx of sediment to counter subsidence and compaction, marshes deteriorate and break apart. Broken marsh opens into little lakes; little lakes merge and become larger lakes. It’s a natural process — human activity doesn’t cause it, but we do accelerate it by tampering with the hydrology, by managing floodwaters and restraining the flow of sediment into the marshes.”

Shipping channels and pipeline canals also alter hydrology, disrupting drainage patterns and permitting the ingress of salt water. Increased salinity in surface water and aquifers converts marshes to more salt-tolerant habitats, endangers agricultural practices and threatens community sources of drinking water.

While building structures on ridges does not directly reduce their elevations, development can jeopardize their physical structure. Because plants hold soil together, clearing land increases the risk of erosion — denuded sites wash away more quickly than areas where a network of roots resists the forces of wind and rain.

Ridges, particularly cheniers, do face a direct threat in the practice of sand mining — digging up the relatively heavy, coarse chenier soil primarily to use for raising elevations at building sites. Such mining reduces the cheniers’ substantive barrier and weakens their protective capacity. Cheniers close to the edges of beaches risk additional sand loss from the erosive action of waves.

Other practices, while not placing the region’s geography at physical risk, diminish or destroy habitat conditions. Of the live oak ecosystem extant at the beginning of European settlement in the 18th century, estimated to have been between 200,000 and 500,000 acres, only 2,000 to 10,000 acres, or two to 10 percent, survive today. In addition to age-old, natural cycles of renewal and deterioration, ridge habitats coastwide face continued disturbance, fragmentation and destruction resulting from activities such as residential and commercial development, infrastructure maintenance and livestock grazing.

Because forested ridges are not easily restored, CWPPRA favors projects that protect these landscape features. Safeguarding cheniers and natural levees sustains their unique natural habitats and preserves the critical protection against storms, floods and wave action that they provide.