As residents of Louisiana prepare for possible disruptions in their drinking water caused by saltwater intrusion, researchers from the U.S. Army Engineer Research and Development Center (ERDC) are assisting the U.S. Army Corps of Engineers (USACE) New Orleans District with assessment and mitigation efforts.
With much of the lower Mississippi River Valley experiencing extreme drought conditions, the lack of rainfall has led to lower levels of fresh water in the Mississippi River, allowing for a denser layer of salt water from the Gulf of Mexico to make its way upstream, threatening the drinking water supplies in several Louisiana communities, including the city of New Orleans.
River conditions are low and have been for some time. The bed of the Mississippi River is much lower than the sea level in the Gulf of Mexico, and if there's not enough fresh water to apply pressure to keep the salt water in the Gulf of Mexico, then it slowly migrates upstream in the shape of a wedge.
“When fluids of different densities encounter each other — which in this case, salt water is denser than fresh water — they tend to stratify,” said Gary Brown, a research hydraulic engineer with the ERDC’s Coastal and Hydraulics Laboratory. “So, the fresh water flows over the salt water, and the salt water flows under the fresh water.”
Salt water has been steadily migrating upstream against the current, and as long as those low river conditions persist, without intervention, that upstream migration will persist.
“Salt is not something that you can conventionally deal with in drinking water filtration,” said Brown. “You can't filter it out of the water, and it corrodes the pipes. It's a significant issue, not only for drinking, but also for agriculture and livestock.”
Though it’s a relatively new term to many, experts at ERDC have been studying these saltwater wedges and intrusion for decades.
“We owe a lot of our knowledge of the salt wedge and salt dynamics to the work that has been done here at ERDC over the years by many different people,” said Brown. “We have much understanding of the basic physics of saltwater wedges, and a lot of that pioneering research was done right here. There was a lot of the early physical and numerical modeling of salt wedges that was developed here, as well.”
To help assess the current conditions, the team is using a basic model they developed with a freshwater layer on a saltwater layer that interact.
“We have a fairly simplified model of the Mississippi River, but it's pretty effective,” said Brown. “It's been successful at predicting where the salt's going to be, and it runs quickly allowing for a lot of ‘what if’ analysis.”
“This is an emergency operation, and we need really quick turnaround of our assessments,” he continued. “We want to be able to do a lot of assessments, not only of what may happen in the future, but also what effects some of our interventions may have. With this tool, we can run very rapid, quick assessments.”
Though ERDC is known for its expertise in research and development, the organization often assists in actionable, emergency operations support — events that are happening right away and impacting a lot of people’s lives immediately.
“Gary and his team have been unwavering in their support to the New Orleans District’s Lower Mississippi River Engineering Branch during this event,” said David Ramirez, chief of the Lower Mississippi River and Tributaries Branch at the New Orleans District. “Their technical assistance by refining an existing hydrodynamic model of the Lower Mississippi River enabled us to respond quickly to this crisis and provide technical information — such as salinity intrusion progress and influence of the natural crevasses along the east bank of the Lower Mississippi River — to USACE leadership and the local governments.”
“The role we play is to try to provide the best information in real-time to support the decision makers,” said Brown. “It's our job to provide the best objective analysis we can — even if that objective analysis is bad news — so that the decision makers can have the opportunity to make the most informed, rational decisions.”
That work is still being used to provide critical data to the Water Management Section and the District Commander, who make real-time operational decisions.
“One of the main products from ERDC is the forecasted salinity locations,” said Ramirez. “The official timeline of when municipal freshwater intakes may be impacted is developed directly from these results. The data is also being used by many local and state government leaders to plan and design saltwater intrusion mitigation strategies.”
“We are working very hard with the district to try to mitigate this issue as much as possible,” said Brown. “We recognize that this is a serious situation for a lot of people in the New Orleans area and the downstream communities. It's a privilege to be able to do something like this and possibly impact people's lives.”
“Gary and his team continue to offer transformative novel, but technically sound, applied solutions to the district at the operational timescale immediately useable for decision making,” said Ramirez. “Cutting-edge technology for decision making today is a rare commodity at the best research intuitions, and we had that luxury through the current emergency saltwater intrusion operations thanks to Gary and the team at ERDC.”