Section 1.3: Coastal Erosion in Southeastern Louisiana
In a sense, we can think of erosion as being the opposite of sedimentation. Erosion can be defined as the transport of rock or sediment from its original context by the various forces of weathering, and especially the flow of water.
Once again, we could imagine being on a mountainside somewhere in the Rocky Mountains. Over thousands of
years, the forces of wind, rain, snow, and the freezing and thawing of water break up rocks. The resulting sediment is then carried downhill by a system of streams and rivers. Eventually, fine-grained sediment is carried into the Mississippi by its system of tributaries, and it is transported downstream. After a long sequence of deposition and erosion events along the way, this sediment is finally deposited in the Mississippi Delta. In this way, sediment erodes from one place on the landscape and is eventually deposited in some other place on the landscape.
The sediments that have formed the coastlines of deltas are either being deposited or they are being eroded away by the action of sea water. When sediment is being deposited, the land surface area of that part of the coast increases, and we say that it is prograding. When erosion is removing sediment from a coastline and the land surface area is decreasing, we say that it is retrograding. In this way, stability in the location of the coastline is rare, happening only when the rate of coastal erosion is balanced by the rate of sedimentation.
Coastlines are a part of the landscape that is extremely susceptible to erosion. This is because the rate of erosion is determined by the energy, or the force, of the weathering and sediment-transporting agent involved. Slow-moving water in a river, for example, can be considered a relatively low-energy erosional force. In moving slowly, the water has little effect on the rocks and/or sediments that it contacts, and it is only capable of transporting the smallest kinds of sediments, such as silts and clays. In contrast, fast-moving water is a very powerful force in terms of erosion and sediment transportation. High-velocity water—let’s say in a steep mountain stream—is capable of carving deep valleys and canyons, and it can move sediments as large as boulders.
Coastlines are generally high-energy erosional contexts because they involve water moving with great force. Here, we could imagine standing on a beach and watching the waves crash against the shore. Those crashing waves carry huge amounts of energy and they are capable of removing vast amounts of sediment from that beach. Furthermore, we could imagine being on that same beach during the landfall of a tropical storm, where wave forces are significantly amplified. Unless sediment is somehow being introduced from some other source, such as the mouth of a river, the great energy associated with actions of waves along coastlines will lead to erosion.
Coastal erosion is a prevalent and destructive problem for much of Southeastern Louisiana. Given the balancing act between sedimentation and erosion, parts of the Louisiana coast should be expected to experience some coastal erosion. The Mississippi River is the main source of sediment along the Mississippi River Delta Gulf Coast, and portions of the coast that are distant from this source of sediment naturally experience erosion. In contrast, certain portions of the Mississippi River Delta Gulf Coast should be accumulating Mississippi River sediment and therefore their land surface area should be prograding.
Unfortunately, human activities in the channeling of the Mississippi River have had enormous consequences for coastal sedimentation and erosion. As I will explain in greater detail in Part 2, human populations have interfered with the natural actions of the Mississippi River by building a network of levees and jetties. Levees protect low-lying areas from river flooding, prevent the Mississippi channel from moving or shifting courses, and keep the channel open to commercial boat traffic. However, they also have the effect of preventing sediment from being deposited along the coast through the distributaries of Mississippi in its lower course by cutting off the flow of water to adjacent wetlands. In addition, these levees and jetties accelerate the water in the Mississippi River channel, which ejects virtually all of the Mississippi’s sediment through the Birdfoot Delta far out into the Gulf of Mexico. For these reasons, very little sedimentation is currently taking place along the coast of the Mississippi River Delta, especially in comparison with what had been occurring prior to the human channeling of the river.
The amount of erosional energy associated with the ocean along a particular part of the coast can go up or down a lot based on many natural factors, such as the weather and/or the tides. Again imagining standing on our beach, there will be calm days in which the waves are quite small and their force in eroding coastal sediments will also be small. In contrast, there will be stormy days in which the waves are very large and capable of doing a lot of erosional damage.
Furthermore, extremely destructive weather events, such as tropical storms and hurricanes, generate great destructive erosional force through the actions of waves. On the one hand, storm surges— in other words, the short-term sea level rises associated with the arrival of tropical storms—increases the amount of moving water present at the coastline, causing waves to penetrate further inland. On the other hand, the great energy of the winds
from tropical storms causes size and the velocity of waves to increase. This massively increases the erosional force of oceans during these extreme weather events.
Recently, we have seen the huge erosional energy and destructive power of tropical storms along the Gulf Coast of Louisiana. For example, during Hurricane Katrina, the Chandeleur Islands (a chain of sandy barrier islands at the mouth of the Chandeleur Sound) nearly vanished, losing as much as 84% of their land surface area (Fearnley et al., 2009). The storm surge from Hurricane Katrina inundated the islands and the forces of the waves removed much of the sandy sediment from which the islands were formed. In the last decade, Hurricanes Gustav and Isaac also caused major damage to Louisiana’s coastal wetlands through their storm surges and erosional forces.
Finally, ecological changes along the Gulf Coast have accelerated processes of coastal erosion over the course of the last century or more. The presence of wetland vegetation tends to inhibit coastal erosion because the root systems of plants act to hold sediment in place. In addition, vegetation can buffer against the energy of waves from the open ocean and shelter coastal sediments. In this way, coastal wetlands, and the vegetation communities that make them up, have protected the coast from the full erosional force of the Gulf of Mexico.
For this reason, ecological changes involving Louisiana’s coastal wetlands have been highly problematic in terms of erosion. Rising sea level has introduced more salt water and increased the salinity of coastal wetlands. This phenomenon, called saltwater intrusion, has resulted in the death of many major freshwater wetland plant species—especially those with deep root systems capable of holding sediment in place in the face of erosional forces. For example, many of Louisiana’s coastal wetlands are dotted with the remains of dead bald cypress trees (Taxodium distichum). With their deep root systems, bald cypress trees were important stabilizing forces in the coastal wetland landscapes of Louisiana. Aside from human forestry activities, the destruction of cypress swamps has largely been caused by the increasing salinity of the wetlands in which they have lived. This phenomenon has been a major factor in the destruction of coastal freshwater wetland plant communities. It has also been accelerated by rising sea levels, destructive storm surge events, and various human activities that have exposed freshwater wetlands to larger amounts of salt water (this will be discussed further in Part 2).
As native freshwater wetland plant communities have degraded, they have often become clogged with invasive plant species, such as hydrilla (Hydrilla verticillata), salvinia (Salvinia molesta), water hyacinth (Eichhornia crassipes), and Chinese tallow (Triadica sebifera). These invasive plant species damage wetland ecosystems by competing with native species. Furthermore, they hold fewer benefits in buffering landscapes against coastal erosion. Likewise, nutria (Myocastor coypus) are an invasive species of rodent introduced from South America in the 1930s. Nutria damage the plant communities of coastal wetlands through overgrazing. Thus, invasive plant and animal species have contributed significantly to coastal erosion by eliminating native plant communities that historically stabilized sediment in the face of high-energy erosional events.
As with sea level rise, coastal erosion is likely to become an even greater problem for Southeastern Louisiana in the future. First, without taking some fairly radical actions to remediate the problem (discussed further in Part 3), the lack of sediment from the Mississippi River accumulating in the coastal wetlands of the delta will continue to be a major factor causing coastal erosion. As stated above, if the coastline is not prograding—or at least held in equilibrium by processes of sedimentation—it will be eaten away by erosion and it will retrograde. Without more sediment from the Mississippi, coastal erosion and retrogradation are inevitable.
Second, many current weather models suggest that increasing global temperatures and other related climatic effects will increase the frequency, size, and intensity of tropical storms in the future (Knutson et al., 2010). More and more powerful tropical storms would have terrible consequences for erosion along the Gulf Coast. Furthermore, warmer global temperatures and a warmer Gulf of Mexico would increase the amount of energy present in the seas adjacent to the coastline. Through a variety of mechanisms, this is likely to contribute to increased coastal erosion in the future.
Third, if the ongoing degradation of coastal wetland vegetation communities continues, coastal erosion will accelerate. Without the presence of coastal wetland vegetation communities to buffer the effects of high-energy storm events and marine energy, more and more land will be lost to erosion. Radical action is required to protect our remaining wetland areas from human-made sources of degradation and destruction, as well as the impacts of invasive plant species.
In short, the balance of sedimentation from the Mississippi River and erosion from the enormous energy associated with the Gulf of Mexico are key factors in determining the relative stability of Louisiana’s coastline. Tropical storms, storm surge events, and other extreme forms of weather can have dramatic effects in accelerating coastal erosion. Similarly, the loss of the vegetation communities associated with coastal wetlands contributes to further erosion, as the root systems of vegetation that once anchored sediments disappear. If the current dynamics of coastal erosion remain unchanged, Louisiana is likely to continue to face a difficult future in terms of land loss, as well as a host of ecological, economic, and cultural consequences.