Optimizing the sediment, freshwater and nutrients in the Mississippi River is one of the primary tasks for scientists, engineers and policy makers charged with restoring Louisiana’s coastal wetlands.
The goal of the often-debated Mid-Barataria and Mid-Breton sediment diversions south of New Orleans is to capture as much sediment suspended in the river’s currents as possible, primarily in the late winter and early spring, and use that sediment to rebuild coastal marshes lost over the last century. The diversions are designed to mimic the natural land-building ability of the river prior to levees and jetties blocking water and sediment from disbursing into adjacent wetlands.
Recently, researchers at Tulane University and the Louisiana Coastal Protection and Restoration Authority have been examining how the restorative power of the Mississippi can be used upstream of New Orleans. The hope is to rejuvenate and sustain swamps that have been cut off from the river by the same levees that isolated coastal marshes.
In the process of restoring those swamps, researchers have also found a way to reduce the use of the Bonnet Carré Spillway in lessening river flooding that threatens New Orleans.
“The sediment, water and nutrients that come out of the river when we open the Bonnet Carré Spillway aren’t being utilized as much as possible when the spillway dumps straight into Lake Pontchartrain,” said Dr. Ehab Meselhe, Tulane professor of River Coastal Science and Engineering. “But, if we route some of that water coming through Bonnet Carré into swamps on the east and west side of the river, we can use the sediment and nutrients to keep those swamps alive.”
Potential benefits
Meselhe is leading the team of researchers analyzing the potential benefits of increasing flows into the Maurepas Swamp near the town of Union east of the Mississippi River about 40 miles north of the Bonnet Carré. They are also looking at routing the river into the upper Barataria Basin west of the river near the town of Ama in St. Charles Parish.
The swamps in both basins are suffering. A combination of sinking land due to subsidence and a lack of sediment input plus saltwater intrusion from hurricanes and sea level rise along with stagnant water and invasive vegetation is gradually killing the cypress and tupelo gum trees. As the swamps die, their wildlife and fisheries production decreases along with the natural protection provided to nearby communities.
Eventually, the swamps will convert to floating marshes, called flotant, that can easily become open water over a few decades or when powerful hurricanes make landfall. Hurricane Ida tore apart approximately 106 square miles of marsh in all, most of it flotant in the northern Barataria Basin.
“A big part of the Coastal Master Plan is reconnecting the Mississippi River to its historic floodplain,” said CPRA Executive Director Bren Haase. “Connecting the river back to the Maurepas Swamp can protect and rebuild the swamp for fish and wildlife and for the surrounding communities.”
The Bonnet Carré Spillway has been opened seven times in the last 14 years after being used just eight times from 1937-1997. Each time the spillway opens, between 90,000 and 250,000 cubic feet per second rushes through the gates into a narrow swamp that separates the river from Lake Pontchartrain.
Mountains of sediment pile up in the swamp during each opening but have limited direct benefit in building land as they spread out into the lake.
Relatively small freshwater flows from the Mississippi River, like those from the historic natural crevasse at Bonnet Carré prior to the spillway’s construction, help encourage submerged vegetation and marsh and swamp growth along the rim of Lake Pontchartrain and spur populations of forage fish like pogies, mullet and white shrimp.
However, too much freshwater can cause algae blooms and disrupt the basin’s fresh-saltwater balance. During the 2019 opening, Bonnet Carré flowed for 122 days, affecting the Pontchartrain Basin and areas as far away as coastal Mississippi.
Reducing salinity impact
Routing the floodwaters through coastal swamps east and west of the river would slow the water, reducing the impacts to salinity. It would also allow trees to absorb the nutrients, leading to fewer algae blooms while sediments help elevate soils, offsetting subsidence and allowing the swamps to return to a natural drying cycle in the late summer and fall, according to Haase.
The team has modeled flows in the two proposed diversions at rates of 25,000cfs to 100,000cfs to determine how much water can be directed into the swamps and the resulting reduction in the volume and the duration of a Bonnet Carré opening. According to Messelhe’s team, operating the Union and Ama diversions could reduce Bonnet Carré’s volume by as much as 60 percent as well as reduce the length of the openings.
Modeling will continue to be refined to determine how much water can be safely routed into the Maurepas Swamp and the northern Barataria swamps and the impacts on existing infrastructure like roads and levees.
Refining the engineering and moving into the design and permitting is a process that could take a decade for the two proposed diversions. CPRA is still considering the best areas to locate the diversions as well, with the potential for Ama Diversion to move farther west near Hahnville.
While those projects continue to evolve, CPRA is planning to begin construction on a smaller, 2000cfs diversion into the Maurepas Swamp near Garyville in the next year that will help rejuvenate the swamp near the Blind River. Haase said CPRA will also have details of a smaller diversion in the Upper Barataria Basin in the upcoming 2023 Coastal Master Plan, designed to revive the swamps and improve water quality in and around Lac Des Allemands.
Messelhe said additional modeling showed the operation of the Morganza Spillway, a sparingly-used flood control structure in Point Coupee Parish that routes floodwaters into the Atchafalaya Basin, could further reduce Bonnet Carré’s flows and diminish the impact of the Ama Diversion on the lower Barataria Basin.
“That would take a rethinking of how we manage the river,” he said. “I think it’s time.”