By Brittany Smith, Environmental Scientist at Princeton Hydro
As a graduate student in the geology program at the University of Texas at Austin, I worked on a study that used remote sensing to explore links between coastal geomorphology and ecology at the Wax Lake Delta in Louisiana. In this blog, I provide a snapshot of my research, which was recently published in the journal Remote Sensing.
What is the Wax Lake Delta?
The Wax Lake Delta is a small, young river delta in Louisiana that began growing in the 1940s after the construction of the Wax Lake Outlet. In 1941 the U.S. Army Corps of Engineers dug the Wax Lake Outlet from the Atchafalaya River – it extended out to the coastline and was designed to reduce the severity of floods in nearby Morgan City.
This outlet provided a constant flow of water to be diverted from the river before reaching the banks of Morgan City, which had experienced several devastating floods. Approximately 40 percent of the Atchafalaya’s discharge gets channeled through the Wax Lake Outlet, which has the capacity to carry a maximum of 440,000 cubic feet per second.
Following the creation of the Wax Lake Outlet, the turbulent flow of water began to carry sediment down the outlet, which deposited at the mouth of the outlet and, over time, caused an underwater delta to grow. In just over 40 years, the Wax Lake Delta grew from nothing to an area twice the size of Manhattan. Research shows that it receives 34 million tons of sediment per year. Today, it spans roughly 7 miles out into the Gulf of Mexico and provides valuable habitat for a variety of animals.
The Why Behind the Research
Many coastal areas have been retreating or drowning as a result of subsidence and decreased sediment availability due to upstream dams and levees. According to the Population Reference Bureau, “Today, approximately 3 billion people — about half of the world’s population — live within 200 kilometers of a coastline. By 2025, that figure is likely to double.”
This population is increasingly vulnerable to flooding and erosion due to sea level rise and storms, especially in coastal Louisiana, where land loss is prevalent due to subsidence and decreased sediment supply.
The Wax Lake Delta is one of the few places in coastal Louisiana that is building rather than losing land, so is seen as an example of processes that could be applied elsewhere on the Gulf Coast to mitigate subsidence and restore coastal wetlands. Additionally, it is an ideal study site because it is relatively small, young enough that it has a good historic record, and has been largely unaltered by human activities.
The Research in a Nutshell
Elevation is a very important variable in coastal ecosystems, as it controls how frequently a site is flooded. This in turn controls how frequently sediment can be delivered or removed from the site, and also what type of vegetation will grow there.
To understand how the Wax Lake Delta is growing, it would be ideal to have an understanding of how the topography has changed over time. Unfortunately, very little elevation data was available for the Wax Lake Delta, so I had to develop an indirect way of getting this information.
What I did have available was a significant amount of Landsat satellite imagery. This was useful to me for two reasons: the delta is extremely flat and low-lying (less than 3 feet above sea level), and the tidal cycle typically fluctuates between 0-3 feet above mean sea level. This means that since each satellite image is taken at a different water level, different parts of the delta are exposed above the water in each picture. Taken together as a group over time, we can start to get a sense of which areas are higher and more likely to be exposed, and which areas are more likely to be flooded, and therefore at a lower elevation.
To do this in a quantitative way, I took all the images taken over a three year period and converted them to binary images, where land was classified as 1, and water classified as 0. I then added the images together, to create a composite image where pixels with higher values corresponded to areas that were exposed more frequently, and pixels with lower values were more frequently flooded.
Using water level data from a USGS gauge station that was installed at the delta in late 2008, I was able to develop a probability distribution of water levels. Taking these together – a probability map of flooding frequency and a probability distribution of water level elevations – I was able to create topographic maps based on the Landsat satellite imagery.
By performing this method over a number of years, patterns emerge about how the delta is evolving over time. The island changes from a relatively amorphous, unorganized shape to a defined outer levee and inner island platform.
A deeper knowledge of the delta topography, allows us to look at connections to the delta ecology. We know that elevation controls hydrology and therefore plant growth, but we have also seen situations where plants can in-turn affect elevation by contributing organic matter to the soil, preventing erosion due to the root mat, or trapping sediment with their stems when sediment-laden water flows through.
In the case of salt marshes, previous studies have shown that if there is a feedback between the two, it occurs because a) plants tend to be most productive at a specific elevation and b) plants are in some way contributing to sediment accumulation relative to their productivity.
For example, if a plant grows best at an elevation of two feet, it grows really densely at two feet, contributes more organic matter to the soil, bigger roots grow that help increase cohesion and reduce erosion, and the stems are denser and trap more sediment when the area is flooded. These all help increase the elevation of the marsh over time. However, if the elevation starts to get too high, the plant grows less densely, contributes less to the marsh surface, and the elevation will drop back down until the plant is happy again. Over time, the surface of the marsh will start to organize around these ideal elevations, creating a terraced effect with platforms corresponding to different plant types that do particularly well at that elevation.
When we look at how a transect down the center of the delta island has changed over time, we see that it goes from a relatively smooth, straight line, to a stepped system comparable to the models from other studies. When we compare the elevations of these platforms to the vegetation communities at the delta, we find that they correspond positively to high-marsh and low-marsh plants. This suggests that there is feedback occurring between plants and sediment accumulation at the delta.
This research a) developed a new approach for investigating changes in coastal topography using satellite imagery, and b) confirmed that there is likely to be feedbacks between sediment deposition, marsh elevation, and vegetation growth. These feedbacks should be considered in any coastal wetland restoration and land building efforts.
Brittany Smith is an Environmental Scientist with an extensive background in hydrology and ecology. At Princeton Hydro, she has been involved in a wide variety of projects including stream assessments, wetland water budget modeling, soil field assessments, GIS analysis, permitting, and aquatic ecology. She holds a Master of Science in Geology with an emphasis in hydrology and geomorphology from the University of Texas at Austin, and a Bachelor of Science in Plant Biology from the University of California at Davis. Brittany has strong skills in data analysis and management, as well as experience in a broad range of field and laboratory techniques.