Let’s Get Muddy: How A Mixture of Mud and Sand Can Help Revive Louisiana’s Wetlands
With the recent unanimous passage of the 2017 Coastal Master Plan, the State of Louisiana is working hard to advance priority restoration projects, including numerous sediment diversions. These project types have been called a “cornerstone” of our efforts to restore and protect Louisiana’s coastal wetlands. As such, it’s important to understand the valuable resource they help capture and deliver to our wetlands: sediment. This is a fancy word for the sands, silts, clays and muds that flow through the Mississippi River each year and out into the Gulf of Mexico. This wasted sediment is a huge missed opportunity, and sediment diversions aim to capture it. (Take a quiz here to see if you can guess how much wasted sediment we’re talking about!)
In recent years, there has been a growing understanding of Sediment Retention Efficiency (SRE), a measure of the ability to capture sediments to productively build land in large-scale projects such as sediment diversions 1. The sediment-laden water of the lower Mississippi River carries large amounts of sand and mud, and there are a variety of factors that influence how well these materials can help build land.
A new study titled Efficient Retention of Mud Drives Land Building on the Mississippi Delta Plain shows that 80 percent or more of the incoming sediment load in most rivers is dominated by mud, which can be a powerful substance to build land effectively. The study analyzes the Attakapas Crevasse Splay, an area of land that has been built by the river breaking its natural levees and depositing sediment across its floodplain. The Attakapas Crevasse Splay, located on the Lafourche subdelta of the Mississippi, is comprised of 95 percent mud and has been stable enough for centuries to support farming along the Bayou Lafourche channel. This site is inland and vegetated, protected from waves and tidal energy, resulting in higher SRE than other parts of the Mississippi Delta 1.
Crevasse Splay. Credit: R.H. Meade, USGS
The broadest conclusion from this study, in lead author Chris Esposito’s words, is that “site selection is very important” 2. This study concludes that sediment deposited in vegetative environments, sheltered from waves, tides and currents retains more than 75 percent of incoming sediment, while delta lobes subject to tides retain a range of 5 to 30 percent of incoming sediment.
Although this study demonstrates that mud-dominated crevasse splays have a high SRE in protected inland areas, it is important to account for a variety of sediment grain-sizes as tools in building land along coastal areas of the delta. Coarser-grained sediment such as sand is essential in the land-building process because it settles close to the river and provides a stable surface for vegetative growth, which aids in the longer-term trapping and deposition of mud 4. In tidal portions of the lower Mississippi, sand makes up a large portion of the sediment load that is transported both suspended in the water column as well as along the river bottom through bedload transport. Utilizing a sediment diversion structure to capture this sediment load would allow for a greater potential for building and maintaining wetlands, rather than wasted sediment flowing into the open waters of the Gulf of Mexico.
These are important considerations in advocating for projects in Louisiana’s Coastal Master Plan designed to build and maintain land, particularly sediment diversion projects. Decades of studies and modeling efforts have gone into the design and location of the large-scale Mid-Barataria Sediment Diversion. This information helped refine the design and location of the diversion to maximize the capture of both sand and mud during its operation, helping to establish the (literal) groundwork for the growth of stable and sustainable wetlands in coastal Louisiana.
As we look ahead to address the coastal challenges our region faces, no detail is too small to understand, even down to the sediment grain size. Thankfully, studies like the one referenced here show the opportunities that exist by putting the mud and sand given to us by the Mississippi River back in our wetlands where they are desperately needed.
1 Esposito et al., 2017 https://www.earth-surf-dynam.net/5/387/2017/esurf-5-387-2017-discussion.html
2 Houma Today http://www.houmatoday.com/news/20170717/researchers-use-mud-instead-of-sand-to-build-land
3 Allison et al., 2013 https://link.springer.com/content/pdf/10.1007/s11269-014-0731-y.pdf
4 Nittrouer et al., 2012 http://www.nature.com/ngeo/journal/v5/n8/full/ngeo1525.html