There are many ways to explain land loss in Louisiana. We’ve attended several forums and workshops in which several presentations on the topic have been provided. One core challenge I have noticed with these presentations is finding the most simplistic way to explain a very dynamic issue.
From my perspective, land loss in Louisiana is the result of two different processes: erosion and subsidence. To not oversimplify these two processes, there are multiple causes behind the erosion and subsidence of Louisiana’s coast.
This one appears to be very obvious at the surface. But how exactly is the erosion happening and why did it start? I think it is important to recognize that this is the result of multiple dynamic events.
- Canals: Canals have been dredged throughout Louisiana’s marshland over the past century for the purposes of navigation and mostly for the use of the oil and gas industry. These canals typically run perpendicular to the Gulf of Mexico and allow saltwater to move further into freshwater marsh areas. Due to this saltwater intrusion you see a conversion of what once was freshwater marsh with luscious trees to grass dominated salt marsh.
- Nutria: Nutria are invasive, herbivorous rodents that eat the roots and tubers of marsh plants. This poses a problem because land-building sediment is more easily eroded without plants present. Nutria were originally brought to Louisiana for the fur industry and then with time I assume these little critters roamed free and multiplied. Louisiana now has a state program run by the Department of Wildlife and Fisheries that involves an incentive payment for harvesting nutria (I believe you have to provide them with the tail). There are also some vendors that make clothing and jewelry from nutria fur and their distinctive orange teeth!
- Storms: A statistic we often hear is that Louisiana loses land at an annual rate approximately equivalent to one football field every 30 to 38 minutes. (WOW! Right?) I imagine a good portion of that average rate is contributed during hurricane season when erosion from high winds and storm surges is increased. This is true especially now as the multiple lines of defense are becoming weaker each year as more land is lost. A “Catch 22” so to speak.
- Natural Erosive Processes: One interesting comparison I have heard was the analogy of eroding marsh to a melting ice cube. An ice cube which is broken into smaller pieces will melt much faster than a whole ice cube. It’s all about surface area and exposure.
- Freshwater and Sediment Deficiency: At the root of the erosion problem… As the Mississippi Delta became more populated and vital to the U.S. (and North American continent), the Army Corps of Engineers has made efforts to protect this area from flooding by the construction of levees and to maintain ship channels by dredging for navigation needs. Basically, the Mississippi River has been contained and natural processes that would have delivered freshwater and sediment to areas throughout the coast have been muzzled. There is very little new sediment being introduced into Louisiana’s eroding coast.
For the non-geologists out there, this term is used to define the sinking of land. It can be the result of both natural and man-made events. In Louisiana, there are two central causes of subsidence:
- Soil Compaction: Marsh soil is typically highly porous and organic in composition. Over time, the porous soil will become compacted and thus the surface elevation will lower.
- Natural Resource Extraction: We often hear this in reference to groundwater extraction, but it is also true for areas where oil and natural gas are being removed. Basically, these natural resources provide a subsurface bubble of support and once the matter is removed the land sinks.
Additional Side Note: As Sarah has pointed out, there is a third contributor to future land loss in coastal Louisiana–sea level rise. We have noticed this is a subject most people here are not prepared to discuss. I do have to say with the central focus of my Master’s Project being sea level rise it has been hard for me not to think about it while we have been waist deep in the marsh doing restoration planting.