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.
Erosion
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.
Oak Trees hanging on to whatever freshwater they can get on Jean Charles Island. You see remnants of bare trunks and limbs of dead trees along many drives down the bayous. - 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.
Subsidence
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.
Besides the containment of the Mississippi River within the levee system, there is a problem with reduced availability of sediment within the river system. The construction of numerous dams upstream in the Mississippi Basin has greatly reduced the amount of sediment that makes its way to the delta. (At the same time, the reservoirs behind those dams are silting in – so they are trapping the sediment that they don’t really want and Louisiana needs!) Blum and Roberts published a nice paper in Nature Geoscience in 2009 estimating past and present sediment loads for the Mississippi River.
The current Mississippi River delta formed under conditions of higher sediment loading and slow sea level rise (< 1 mm/yr). With both reduced sediment and higher relative SLR, it is pretty much inevitable that the delta would be in an erosional stage, unless there is sufficient organic accumulation to offset the mineral sediment deficit (an area that remains poorly studied). So most scientists down here have come to the realization that restoration is going to have to involve a lot of prioritization and difficult decision-making, because even if we could get the sediment and freshwater into the marshes, there is not going to be enough sediment to save everything.
On the SLR issue – one reason you don't hear much about it in coastal LA is because the subsidence component is actually much larger than eustatic SLR. Subsidence rates are as high as 6-8 mm/yr in parts of the delta, for a total relative SLR of 8-10 mm/yr. This is several times higher than the global average eustatic SLR of 2-3 mm/yr. It makes LA an interesting case study, though, because we are already dealing with high SLR that other parts of the country will be experiencing in the future. Maybe Louisiana's struggles now will yield some solutions that will be useful for other coasts as eustatic SLR continues to accelerate.
Hi Melissa, you make some great points in your comment. I have also thought a lot this summer that with the sinking land here that Louisiana is the guinea pig for what is going to happen in many coastal cities with sea level rise in the future. We have also seen in many of the meetings and community events down here that choices and priorities are going to have to be made. I know that there is a Louisiana state master plan for a sustainable coast, but I don’t really have a sense yet about how viable the plan is or if it is clear in the plan what these choices are going to mean for the people and the communities down here.
Another thing we have realized since being down here this summer is that there is not the federal attention and money being spent here on this environment as has been done in places like the Florida Everglades.
The money issue is a big one, but only part of the problem. The other issue is who has the authority to make decisions about how restoration is going to proceed. The state and local stakeholders seem to be largely on board with the basic tenents of the Master Plan, but ultimately, only the US Army Corps of Engineers has legal authority over how the Mississippi River is managed. If they don’t strongly support the plan, then it will go nowhere. So we really need more attention at the federal level.
In any case – I’m thrilled that we have a couple of MEMs down here working on these issues for the summer. Look me up if you ever make it up to Baton Rouge this summer!