Pros And Cons Of Lower Mississippi River Freshwater Diversion Projects
Note: This is the third of three articles on the Louisiana Coastal Protection and Restoration Authority’s plans for freshwater diversion channels in southern Louisiana. Previous articles appeared in the February 7 and April 18 editions.
The highest and most fertile land in coastal Louisiana is along the Mississippi River and former delta channels such as Bayou Lafourche, Bayou Terrebonne and the Atchafalaya River, where annual flooding has left nutrient-rich sand and silt deposits, building land.
The land slowly slopes toward the surrounding marshes, where less silt and sand has been deposited throughout the centuries.
With the creation of levees to protect homes and farmlands from the annual flooding nearly 100 years ago, that deposit of sand and silt mostly ended as the river was cut off from its basins.
The levees remain important to protecting coastal communities but also are contributing to Louisiana’s land loss. Today, coastal land loss is caused largely by land subsidence and lack of sediment delivery, more so than sea level rise. Louisiana is losing more than a football field of land every hour to coastal erosion and subsidence, or about 1,883 square miles of land between 1932 and 2010. That’s 1.2 times the area of Rhode Island.
Sediment Diversion Channels
In an effort to rebuild the vanishing coastline, the state created the Coastal Protection and Restoration Authority (CPRA), which develops and updates a coastal master plan every six years with a 50-year budget totaling $50 billion to plan, prioritize and implement various strategies.
A significant part of the plan is to “reconnect the Mississippi River to the coastal marshes.” CPRA hired an engineering firm to design two sediment diversion channels on the east and west banks to allow sediment-laden river water to flow into the marshes with the intent of re-enacting the natural land-building process.
Locating the diversion canal is critical. It must be on the “land building” bank of the river, not where the current is strong and eroding the bank.
CPRA says connecting the river with the marshes is the most efficient way to build land across vast expanses of what has become open water. The “land” will often be only marshes and other wetland habitats, not grasslands or fields where one can walk, although in some areas that “mostly high and dry” land will be created along the distributary banks.
On the west bank, the $2 billion Mid-Barataria Sediment Diversion project is being designed below the Alliance Refinery, near Mile 61 AHP (Above Head of Passes), to allow as much as 75,000 cubic feet per second (cfs.) of river water, silt and nutrients to flow into Barataria Bay when the Carrolton river gauge in New Orleans shows about 15 feet.
As the river falls, so will the flow of river water, until low river when the diversion canal gates will be closed, allowing for some brackish water to backfill into Barataria Bay. During high river, the increased current allows river water to carry more silt than the slower flow at a lower stage.
On the east bank, the Mid-Breton Sediment Diversion project, proposed just below Wills Point and near Mile 68 AHP, will allow river water and nutrients to flow into Breton Sound. The project was recently downsized to 50,000 cfs. during high river, and, again, will close during low river to allow some brackish water to flow back into the marshes.
The majority of the master plan calls for mechanical marsh creation, using dredged material from sand deposits adjacent to the Mississippi River navigation channel and other borrow sources to pump that dredged material in pipelines, as much as several miles, to build land and rebuild the barrier islands that serve as the first line of defense against hurricanes and the accompanying storm surge. This process can build land above the water line.
Mechanical marsh creation is a quick and effective process, and it continues to be effectively used to create “high and dry” land as evidenced by Port Fourchon, which was built from shallow waters and marsh, by capturing spoil dredged from the navigation channels. The port has developed into the main staging area for companies servicing the lucrative Gulf of Mexico oil industry.
But mechanical marsh creation is limited in the total acreage of land that can be created, as this depends on available sediment sources within reasonable distances from the areas being restored, as well as suitable conditions in the area, said Rudy Simoneaux, CPRA’s chief of engineering.
In all, the coastal master plan envisions spending $17 billion on mechanical marsh creation, by far the largest investment of the restoration project’s budget. In Fiscal Year 2023 alone, 23 of these dredging projects will be underway to rebuild marshes and barrier islands using 86.8 million cubic yards of sediment.
Once complete, the projects will create or nourish more than 16,308 acres of land. Often, the land built by dredged material is “armored” with large rocks to reduce erosion from wave action.
Simoneaux said several dredges are being built by dredging companies in anticipation of continued demand from CPRA for coastal protection projects.
Upriver Navigation Dredging
Much of the dredging to maintain the regulated depth of the navigation channel in the Mississippi River “crossings” is further upriver than where coastal restoration silt is really needed.
Silt from Red Eye Crossing, just below Baton Rouge, or other upriver areas where dredging is required, could be barged to coastal sites, but the cost would be prohibitive, Simoneaux said.
He confirmed that spoil from navigation dredging by the Corps of Engineers is required to be “disposed of in the most cost-effective and timely manner.” Often the dredges simply pump the spoil back into the river for the current to carry it downstream.
Most likely, the same silt drops out at the next crossing, to be dredged again. And at the next.
Engineers are looking at that possibility and how to prevent the recurring dredging of the same silt as it moves from one crossing to the next. If the same silt is dredged again and again, is simply pumping it into the channel really the least expensive means of disposing of dredged material?
Pushback From Fishermen
The massive amount of fresh water the diversion canals will dump into the brackish coastal marshes to build land has created serious concerns expressed by oyster and shrimp fishermen, specifically that the required salinity the aquatic species need will be reduced below what is essential for survival.
Mitch Jurisich, a third-generation oyster fisherman and chairman of the Louisiana Oyster Task Force, said 10 years ago a breach in the East Bank levee of the Mississippi River, near Mile 44 AHP, allowed fresh water to flow into the brackish marshes, reducing the salinity and destroying what he called the world’s most productive oyster grounds in Breton Sound.
Now called Mardi Gras Pass, the breach occurred on Mardi Gras Day in 2012 and allows as much as 25,000 cfs. of fresh water to flow into the Breton Sound marsh during high river. The breach scoured a channel 300 feet wide and 100 feet deep. In recent years, CPRA said the pass has typically flowed around 15,000 cfs., even during high river.
Brian Lezina, CPRA chief of planning, agrees that the sediment diversion canals will lower the salinity in the upper reaches of Breton Sound and Barataria Bay.
But Lezina suggested there may be a renewal of older oyster cultivation grounds in the lower reaches that are no longer productive because of the loss of barrier islands and salt water intrusion into the basins from the Gulf of Mexico, resulting in higher salinity than what an oyster can survive in.
Those older, dormant oyster grounds could be brought back into production as the influx of river water drops salinity in the lower reaches of Breton Sound and Barataria Bay, he said.
Louisiana’s brown shrimp season from May to August could also be a causality of the fresh water diversions lowering salinity in brackish marshes where juvenile brown shrimp matriculate, said Acy Cooper, president of the Louisiana Shrimp Association.
Lt. Gov. Billy Nungesser opposes the sediment diversion projects, identifying with the fishermen’s concerns about lower salinity and mortality of oysters and brown shrimp, and because he says he is concerned about the influx of fresh water on the prolific shallow water bottle-nosed dolphin community along the Louisiana coastline.
Vessels transiting Belle Pass at Port Fourchon are “escorted” by numerous dolphins frolicking off the bow wakes and feeding on redfish and speckled trout in the harbor channels.
Nungesser said the two openings of the Bonnet Carré Spilllway in 2019 were needed to prevent over-topping or damage to levees that protected New Orleans further downriver, but he said three times as many dolphins died as a result of the spillway openings that year than were killed by the crude oil pollution from the massive BP oil spill in 2010.
Nungesser said that as a result of the influx of fresh water during the spillway openings, many dolphins developed skin lesions that led to the fatalities of more than 300 of the aquatic mammals.
The Louisiana congressional delegation secured a waiver from the Marine Mammals Act allowing monitoring dolphins, but only after the diversion canals were completed and operating.
Lezina said the comparison with the spillway openings was not accurate because the spillway flow approached 300,000 cfs., while the Mid-Barataria opening will have a maximum flow of 75,000 cfs.
Bayou Lafourche Increased Flow
Another project to introduce fresh water into the coastal marshes proposes increasing the flow of water in Bayou Lafourche. Once a natural outflow channel of the Mississippi River, Bayou Lafourche stretches 106 miles from Donaldsonville, La., about Mile 175 AHP, to Belle Pass at Port Fourchon on the Gulf of Mexico.
In 1903, a dam was built to block the flow of water at the confluence of Bayou Lafourche and the Mississippi River, with the intent of preventing the natural downstream flooding during high river.
Currently, pumps flow 500 cfs. of river water into Bayou Lafourche, making it a sleepy, low-flow waterway with little current. During times of low water, salt water intrusion would back up into Bayou Lafourche and threaten the municipal water purification plant many miles upstream at Lockport, La.
CPRA is proposing increasing the water flow to 1,500 cfs. by installing additional pumping capacity. The water flow will freshen bayou water, but will not create a salinity issue at the mouth because of locks at Larose and Golden Meadow, which serve both navigation interests and flood control.
The Leon Theriot Lock in Larose is at the intersection of Bayou Lafourche and the Gulf Intracoastal Waterway (GIWW), and fresh water will divert both east and west into the GIWW when blocked by the closed lock gates.
The additional flow will protect the water purification plant from future salt water intrusion and add river nutrients to marshes connected to the GIWW.