Proponents of the large-scale Mississippi River sediment diversions currently being planned by the Louisiana Coastal Protection and Restoration Authority received a recent boost when scientists from Louisiana State University’s (LSU) College of the Coast & Environment published an analysis of two existing freshwater diversions at the coast of the state, one of which shows a significant amount of land formed in the last 17 years.
The study, conducted by John White, a professor in the LSU Department of Oceanography & Coastal Sciences, or DOCS, John Day, a professor emeritus of DOCS, and Brady Couvillion of the US Geological Survey, examined the Davis Pond freshwater diversion situation on the western shore of the Mississippi, upstream from New Orleans, and the Caernarvon Bypass, which is downstream from New Orleans and on the east bank of the river.
Researchers noted that the Davis Pond Freshwater Diversion, which opened in 2002, is a “dynamic environment that has undergone dramatic changes over the years.” In the time since the opening of the diversion, a new fan-shaped accumulation of sediment, the so-called fissure blast layer, has opened up at the mouth of the inflow canal. Mouthbar deposits and marginal wetlands have begun to fill previously open ponds.
The study estimates that land area has increased between 2 and 4.8 square kilometers (that’s between 1.25 and 3 square miles) in the 17 years since the diversion opened, most of it in the past decade or so. In addition, a ten-year soil sampling study conducted in 2007 and 2018 showed a doubling of the bulk density of wetland soils in over half of the wetland, attributed to the deposition of river sediments.
In contrast, the Caernarvon Diversion shows no significant detectable changes in the amount of land present in the area. This finding is significant in itself, however, as it refutes a previous study that found both diversion areas lost land after the diversion opened.
The need for better wetland data on the Louisiana coast
Diversions at both sites were not created to build land but to combat saltwater intrusion from rising sea levels. While not designed to capture sediment, both sites provide insightful examples of what freshwater diversion results can look like, as well as how best to monitor the success of a particular project.
White said the methods used to collect data are important to the state’s proposed sediment diversion. Wetlands can prove difficult to measure using standard remote-sensing techniques, he said, because the rise and subsequent retreat of floodwaters, depending on changing water levels, make it difficult to determine where land will be built.
“Our study at Davis Pond used appropriate remote sensing tools and combined them with over 140 soil and plant measurements to provide soil truth and certainty for modelling. If a river diversion affects the ground, you need to survey the ground before and after to fully understand how the river diversion affects the country,” he said.
Going forward, he continued, “Louisiana needs a better database of wetland soil spatial data that describes the entire system so that when changes occur, we can understand what is actually causing those changes, and thereby design better systems that do good for the taxpayer Showing value for money The state has a great system, the Coastwide Reference Monitoring System (CRMS), that focuses on the coast, but additional sampling targeted at the sediment diversions will be needed to understand the impact and the fully understand the progress of these recovery projects.”