Coastal Restoration is Critical to the Future of New Orleans

The single greatest threat to the future of New Orleans is the continued degradation of the Louisiana coast. The building and rebuilding of levees and floodwalls is not truly sustainable without having land surrounding the levee system. The demand for Category 5 hurricane protection must include a fully funded large-scale restoration of the Louisiana coast.

For those of you that are unfamiliar with the geographic wonders of New Orleans, it is a truly unique environment for a major city in North America. The Continental Shelf actually ends at Baton Rouge, and the Mississippi Deltaic Plain south of Baton Rouge has been built by the rapid deposition of Mississippi River sediment over thousands of years. One of the largest delta complexes in the world has formed in Louisiana due to the combination of a major river with a large discharge and a low energy receiving basin (Gulf of Mexico) that does not rapidly rework the deposited sediment. Many major river systems of the world, even those with a greater discharge, do not form large deltas because they empty into higher energy receiving basins.

The rapid deposition of sediments leads to subsidence as those sediments settle and dewater under their own weight. Prior to the leveeing of the river for flood control and shipping needs (the Corps maintains a 45-foot wide deepwater channel south of Baton Rouge), the Mississippi River flooded the adjacent marshlands and provided sediments that allowed the marshes to combat subsidence and maintain their elevation relative to sea level. Also, the Mississippi River naturally switched course every couple of thousand years to a shorter, more efficient course. Bayou Lafourche and Bayou Teche are actually abandoned Mississippi River channels. The Mississippi River is kept in its current course by control structures north of Baton Rouge that allow only 30 percent of the Mississippi River flow to go down the Atchafalaya River, which is the alternate, more efficient course that the Mississippi River would prefer.

It is the natural flood and sediment deposition cycle that has created marshes across the deltaic plain, extending from the Mississippi state line, westward to the Chenier Plain. This cycle has also created all of the higher elevation upland areas along rivers and bayous where most coastal Louisiana residents live well above sea level. For New Orleans, like the rest of coastal Louisiana, areas near the river and along historic bayous (Metairie and Gentilly Ridges) are higher elevation levee ridges. The areas between the levee ridges (called interdistributary basins) were swamps and marshes, and in New Orleans were drained to create much of the city. Because dewatering of sediments in the deltaic plain naturally leads to subsidence, the physical draining of marshes and swamps leads to even more rapid subsidence.

The loss of natural flooding of the coastal marshes from the Mississippi River has caused massive degradation of the coastal zone and has led to numerous studies and projects to address the catastrophe. Currently most of the Louisiana coast between the Mississippi state line and Morgan City is fragmented broken marsh similar to the first photo of these degraded marshes in Terrebonne Parish. In the photo below you can see what is left of a natural levee ridge (where the camps were built) and spoil banks along canals. The rest of the marsh has subsided and deteriorated to the point where it is nearly impossible to differentiate between the bayous and the marshes.

Photo 1: Degraded Marshes in Terrebonne Parish.

The solution is incredibly simple and incredibly complicated at the same time. Restoration of the coastal zone can occur naturally through river flooding. The only location in coastal Louisiana where marsh building is occurring is in the Atchafalaya Delta south of Morgan City. This area is capturing 30 percent of the Mississippi River flow and is not constrained by levees south of Morgan City allowing the sediment rich water to flood across the adjacent marshes. The second photo is of the marshes located just west of the Atchafalya River Delta, and shows how marshes in coastal Louisiana should look. As can be seen in the below photo, the bayous are distinct and the marshes are fully intact with little to no open water areas.

Photo 2: Healthy Marshes in St. Mary Parish.

The complication to coastal restoration is wrestling with existing infrastructure. Levees and channel flow in the Mississippi River are critical for maintaining shipping lanes, minimizing dredging requirements, providing flood control and protecting private property. The simple solution to coastal restoration of letting the Mississippi River flood the adjacent marshlands would destroy the lives and homes of many people in the coastal zone.

However, the State of Louisiana and the U.S. Army Corps of Engineers are embarking on a large scale ecosystem restoration program for the coast and it is critical that it is fully funded and implemented. The Coast 2050 Plan will attempt to make major changes to the river in support of restoration and the protection of infrastructure. New Orleans cannot afford to wait any longer. Without a complete restoration of the coast, New Orleans will be more and more susceptible to hurricane damage in the future as it becomes an island. The west bank is becoming closer and closer to open water environments each year. Levees alone as a protection method are not sustainable because they too subside, often at a rate faster than the surrounding land due to their greater weight. If we really want protection from major storms in the near term, and from sea level rise in the long term, coastal restoration for Louisiana must receive priority funding, even before raising the levees to a level that provides Category 5 protection. The cost tag will be many billions of dollars. Although it sounds like a lot of money, the Federal government has no problem providing this kind of money for Everglades restoration or the restoration of the Sacramento/San Joaquin Delta in California. Coastal restoration planning and implementation is every bit as important as the urban planning efforts to the rebuilding and sustainability of New Orleans, and must be taken into consideration in any discussion of the future of New Orleans.



TAGS: Katrina, New Orleans, NOLA, Coastal Restoration, Louisiana, Corps of Engineers