Section 1.4: Geological Subsidence and Land Loss in Southeastern Louisiana
Geological subsidence is a complex problem that affects coastal Louisiana, contributing to the loss of land and causing various other environmental problems. In a nutshell, subsidence occurs when the weight of alluvial sediment (in this case, deposited by the Mississippi River) presses down on edge of a continental plate and causes it to sink.
At first, this may sound a bit absurd: how can the weight of the dirt deposited by a river cause an entire continent to sink? The answer is simple: like everything else, the earth’s crust is slightly flexible and it will bend and fault when billions and billions of tons of sediment are placed on top of it.
The Mississippi River has been dumping sediment on the edge of the North American plate for 100 million years or more. Over the last 7,000 years, the Mississippi focused its sedimentation particularly along the coast of Southeast Louisiana, which has caused this region to subside. Today, despite the fact that the Mississippi is currently dumping its sediment offshore below the Birdfoot Delta, much of the Lower Mississippi Valley is experiencing subsidence.
There are some other secondary factors that also serve to bring about subsidence in coastal Louisiana. One of the most important of these is the drying of the clays deposited in the Mississippi Delta—a phenomenon referred to as clay dewatering. When dry clays become wet, they expand significantly. Some very fine-grained clays may actually double in volume when they become wet and many Mississippi Delta clays may expand by 50% or more. The problem that this poses is that the human draining of wetlands can often lead to the drying and shrinking of clay deposits—a problem discussed in greater detail in Part 2. When these clay deposits shrink, the land surfaces that they make up sink. In Southeast Louisiana, where many wetland areas have been reclaimed via the construction of levees and the use of water pumping systems, subsidence from clay dewatering poses a great danger.
The effects of geological subsidence can be disastrous. It amounts to a relative rise in local sea level, since the sinking of the land is practically the same thing as rising sea level. Over much of Southeast Louisiana, subsidence rates exceed 10 millimeters per year and in certain places, such as the Birdfoot Delta of Plaquemines Parish, subsidence rates exceed 25 millimeters per year. This means that over the course of the next century, subsidence might account for a relative rise of sea level of 2.5 meters or more. This is a much higher rate than the absolute rise in global sea level and it clearly constitutes one of the main threats to Louisiana’s coastline.
In summary, geological subsidence is already causing alarming problems for the Mississippi River Delta Gulf Coast and adjacent regions of the Gulf Coast. Subsidence is a significant factor in the loss of land in Southeast Louisiana with few simple solutions in sight. While certain aspects of the problem of clay dewatering may potentially be resolved through engineering solutions, our capabilities of intervention are limited.
Go to: Section 1.5: Exercises