Saltwater Intrusion Complicates Lower Miss Operations
For the better part of a decade, the Mississippi River seemed to be stuck on flood, with 2011 one of the worst floods on record and 2019 the longest.
But it’s a different story this year, with drought conditions persisting across the United States and flow rates and stages extremely low throughout the Mississippi River and its tributaries.
Near the mouth of the Mississippi, low water conditions mean that saltwater from the Gulf of Mexico, called a “saltwater wedge,” is extending dangerously far upriver, putting the freshwater intakes of several communities at risk. The New Orleans Engineer District and Kirk Lepine, president of Plaquemines Parish, held a joint press conference September 28 to announce measures underway to continue delivering fresh water to Plaquemines Parish’s east bank communities and to Boothville-Venice on the west bank and to ensure the water supplies of larger communities like Belle Chasse, St. Bernard Parish and New Orleans are protected.
As of September 28, the toe of the wedge was located around Mile 43.5 and was forecast to reach Mile 65 within about 10 days, which is the trigger for dredging a sill across the river bottom to hold back saltwater. According to Heath Jones, emergency manager for the New Orleans District, the Corps has already signed the contract to build a sill at Mile 63.7. The estimated cost for a cutterhead dredge to construct the sill is about $10 million, and construction will likely take between two and four weeks.
“The sill will be built to the necessary height that slows the salinity levels down,” Jones said. “We’re out there monitoring it, and we can tell if the sill is working.”
Jones said the sill will be built in 5-foot increments and will likely not need to go higher than negative 45 feet. Jones admitted that negative 45 feet is, in fact, 5 feet less than the newly certified 50-foot channel in the Mississippi River.
“We would have, likely, if we go to negative 45, some draft restrictions on ships,” Jones said. “We’re also going to have some restrictions during construction of the sill.”
Jones said the issue with the saltwater creeping up the river is that, simply, there is not enough flow in the river to push the salt water back out. Currently, the river flow through New Orleans and 20 miles down in Belle Chasse is about 180,000 cubic feet per second (cfs). A flow of around 300,000 is sufficient for naturally pushing the saltwater wedge back toward the sea.
Lepine noted that in 2012, when the wedge compromised the water supply in Port Sulphur, Buras, Boothville-Venice and the east bank of the parish, the parish had to barge and truck in fresh water until the wedge retreated after a tropical storm. The parish has taken steps this year to avoid that costly endeavor.
“We have secured two reverse osmosis machines that will run at the Boothville plant and at the east Pointe à la Hache plant,” Lepine said.
Each water plant supplies about 1 million gallons of potable water per day to those communities, and the reverse osmosis machines will allow that to continue. Also new since 2012, the Belle Chasse water plant pipes water down the road to Port Sulphur. That system supplies roughly 6 million gallons of water per day, Lepine said.
Jones said the Corps has actually moved up its triggers for installing the sill, and for that, Lepine said he’s thankful.
“We want to thank the Corps for jumping on this right away,” he said. “We’re at the mercy of the river, and every 10 years we have to go through this.”
All the way up to St. Louis, low water on the Mississippi River is creating problems for operators, with some barge companies having to light-load barges to avoid groundings. The issue couldn’t come at a worse time, with farmers in the Midwest and Mississippi Valley nearing harvest season.
According to Mike Steenhoek, executive director of the Soy Transportation Coalition, a typical hopper barge can be loaded with about 1,500 short tons of cargo, or 50,000 bushels of soybeans.
“A 15-barge tow can, therefore, easily accommodate 750,000 bushels of soybeans,” Steenhoek said. “Each reduced foot of water depth will result in 150 to 200 fewer short tons—5,000 to 6,700 fewer bushels of soybeans—being loaded per barge.”
Multiplied across the entire soybean harvest that would normally use the Mississippi River to get to market, the effect is staggering.
“Let’s assume a soybean-growing region will need to transport 100 million bushels of soybeans via barge,” he said. “Under normal conditions, 100 million bushels divided by 750,000 bushels per 15 barge tow equals 133 barge tows required. If the water level for loading is decreased by 1 foot and if you utilize the conservative 150 fewer short tons (5,000 fewer bushels) per load, the 15-barge tow will be loaded with 75,000 fewer bushels. This is the equivalent of removing the entire production of three soybean farms from a single barge tow.”
Instead of 133 barge tows, that scenario would require 148 barge tows, Steenhoek said.
Channel width restrictions due to low water are also leading barge companies to limit the size of tows to 25 barges, Steenhoek said.
“When attempting to transport an expected robust harvest via a less efficient inland waterway system, it becomes analogous to attaching a garden hose to a fire hydrant,” he said. “Having to load barges lighter and restricting the number of barges results in needing more roundtrips to accommodate a given amount of volume. The expected result of this is higher barge shipping rates.”
According to Steenhoek, the estimate from the U.S. Department of Agriculture for transporting 1 ton of soybeans just between St. Paul, Minn., and St. Louis for the week ending in September 20 was $51.02 per ton, which represents a 33 percent increase over the same week a year ago.