Flood sediment helps some coastal areas, but misses others
6th June 2011 · 0 Comments
By Susan Buchanan
The Louisiana coast needs every bucket of river sediment it can get to rebuild marshland that evaporated in the last 70 years or so. Precious silt coming down the Mississippi River in this year’s flood is helping some spots, like areas east of the Bohemia Spillway in Plaquemines Parish and deltas at the mouth of the Atchafalaya River. But many sections are missing out on it because of levees and other constraints, experts say.
P.J. Hahn, coastal director for Plaquemines Parish, said “Mississippi River water that’s being diverted into Lake Pontchartrain now via the Bonnet Carré Spillway probably won’t do much to rebuild marshes and coastline. However, the Bohemia Spillway—which adds sediment to the east bank of our parish in high-river events—is helping to nourish marshes.” When the Mississippi swells as much as it has this spring, water flows naturally over the Bohemia Spillway—located 45 miles below New Orleans—and enters adjacent marshlands.
Built in 1926, the Bohemia extends below the end of the Mississippi River levees, is over 11 miles long and needs no intervention by officials to ease high-river pressures as the Morganza and Bonnet Carré Spillways do. The Bohemia system was formed by knocking down river levees and back levees. A remaining berm acts as a relief valve for river water in excess of five to six feet in elevation so anything exceeding that level flows into Breton Sound.
Hahn said “the Bohemia Spillway will deposit more sediment into surrounding wetlands this year than when it was overtopped in the past because of the exceptionally high river now. We’ll be reviewing satellite imagery over time to gain a better understanding of its impacts this year.”
Hahn also said “for several years, Plaquemines President Bill Nungesser has advocated pumping river sediment into marshes to build ridges that will provide storm protection.” The Mississippi River runs down through the center of the parish, and sediment builds in bends in “borrow pits” that can be mined.
Meanwhile, “much of the sediment entering Lake Pontchartrain from the Bonnet Carré Spillway now is settling to the lake bottom and not doing the marshes much good,” said John Lopez, acting director of the Lake Pontchartrain Basin Foundation. “That sediment would be stirred up in a major storm, however.”
And, said Lopez, “the Bohemia Spillway, which has been under the public radar recently, has added sediment this spring to marshes along the east side of the river extending to Breton Sound. The Bohemia’s diversion structure is dilapidated, and though there’s talk of opening it, that’s not necessary now because water flows through the spillway when the river’s high.”
The volume of water entering the Bohemia is relatively small, however, especially given what’s passing through the Bonnet Carre, Lopez said. The Lake Pontchartrain Basin Foundation has been studying the flow through the Bohemia for several months. Lopez noted that the U.S. Geological Survey is looking at turbidity in Louisiana waterways and Louisiana State University is conducting several studies on the impact of river sediment on marshes.
Anyone living near the Mississippi knows that the river has been alarmingly high for a month. Paul Kemp, Baton Rouge-based director of the Louisiana Coastal Initiative Program and vice president of the National Audubon Society, said “a flood like this occurs every 50 years and brings loads of material down with it. Whether the flow is delivering beneficial sediment now depends on where you look.” He is a coastal geologist and oceanographer.
Satellite images from the National Aeronautics and Space Administration, or NASA, and the National Oceanic and Atmospheric Administration, or NOAA, show a plume of muddy water flowing out of the Bonnet Carre Spillway and into Lake Pontchartrain, Kemp noted. To the north and west, “photos show Mississippi River water entering the Atchafalaya River and going into a shallow coastal system, spread over a hundred miles. Manmade structures and spoil banks from oil-and-gas canals crisscross the Atchafalaya Basin and usually affect the flow, but flood water is going over almost everything in that basin now.”
Kemp said the Mississippi River running past New Orleans with levees contrasts with the “more natural” Atchafalaya River, and among the two systems, the Atchafalaya — especially at its mouth — is reaping the most sediment now. “In the Morgan City area, we expect to see a major, new land-building episode in the Atchafalaya deltas—something we haven’t seen since the 1970s,“ he said.
Kemp said “we’ve tried to cut the rivers off from the wetlands, but some sediment has succeeded in getting in. We do not see land loss where it’s gotten in around the Atchafalaya.”
Much of the Lower Mississippi won’t benefit from this historic flood, he said. “Sediment is getting into some Lower Mississippi marshes, but it’s going through Lake Pontchartrain, out into sea and then coming into the marsh on tides from the coast. That’s not the way a river normally nourishes its delta.”
Kemp continued, saying “these big flood events come once in a generation, and if we don’t take advantage of one, it’s a serious setback given the state of collapse of the wetlands. It would have been nice if the two, relatively small river diversions, built in the past 20 years in the Lower Mississippi to save the marshes—Davis Pond in St. Charles Parish and Caernarvon in Plaquemines Parish—could have been opened this spring to get sediment into the wetlands from further up.”
But, Kemp noted, those diversions or openings were closed last fall and weren’t reopened for the spring flood, partly because the local oyster industry is recovering from open diversions that kept oil at bay, but injected more freshwater into beds, during the BP spill. “Oysters spawn in the spring and fall, if salt levels are right,” he said.
Alex Kolker, wetlands scientist at the Louisiana Universities Marine Consortium or LUMCON in Cocodrie, said large areas of Southeast Louisiana won’t receive much sediment from this year’s flood because the region is engineered with levees and other structures. “A few spots at the mouth of the Mississippi are getting more sediment now, but places like Terrebonne Bay and Barataria Sound are not,” he said.
Kolker said that river diversions can be used to let more sediment into marshes, but authorities have kept some Lower Mississippi River structures closed this spring to prevent flooding. “A large flood is potentially dangerous if not managed correctly,” he noted. “Safety issues come into play.”
And, he said, “most of the diversions on the Lower Mississippi aren’t large enough anyway to take full advantage of the current sediment flow, though a medium-to-large diversion exists at West Bay at the Mississippi River’s mouth.”
Kolker continued, saying “a lot of the apparent solutions to the lack of sediment are value judgments. You have tradeoffs, including public safety and the interests of fishermen who don’t want too much fresh water mixing with salt water.” He asked “do you choose safety, or fishermen or marshes?”
He said that scientists are watching how the river delivers sediment now, and added “I’m not advocating one approach or another” for capturing sediment.
“The Atchafalaya River is engineered as a large diversion so that in a normal year 30 percent of Mississippi River water runs down it,” Kolker said. “At the mouth of the Atchafalaya, south and southwest of Morgan City, two deltas have being forming from river sediment deposited over the years.”
Morgan City, he said, is an example of tradeoffs in water management and public safety. “The river is six to seven feet above flood stage there now, following the mid-May opening of the Morganza Spillway. But river water passing through the Morgan City area will continue to develop wetlands and a delta nearby.”
At the U.S. Army Corps of Engineers, Barb Kleiss, director of the Louisiana Coastal Area Science and Technology Program, said her office started a project late last year to produce a suspended-sediment assessment or “budget” for the Lower Mississippi River. Federal, state and university researchers involved in that project have recorded sediment levels at stations along the Lower Mississippi and Atchafalaya Rivers to gauge the silt flow from the Old River Control structure in Concordia Parish, La. southward.
However, “progress on the sediment-budget effort has been temporarily delayed because of work to collect data and study the sediment dynamics of the river during this historic flood,” Kleiss said last week. Field crews from the Army Corps’ Engineering Research and Development Center or ERDC, the U.S. Geological Survey and the University of Texas, assisted by the Louisiana Office of Coastal Protection and Restoration, have been sampling sediment and measuring discharge near the Old River Control structure, the Morganza and Bonnet Carre Spillways, Myrtle Grove in Plaquemines Parish and other sites along local waterways. Over the next several months, data will be analyzed to examine sediment movement in a flood.
Kemp said in this flood event, many of the marshes around the Lower Mississippi River remain thirsty for river sediment. “It’s like being in the desert, where you hear water gurgling through but can’t take a sip,” he said. Because of decades of manmade changes to the river and wetlands, Louisiana’s coast is rapidly shrinking, he said, and added “it will take lots of water and sediment to rebuild, and we have to work out compatible approaches quickly to keep from losing it all.”
P.J. Hahn said that less than a century ago the Mississippi River replenished existing marshes and created new ones with each spring flood. Since the 1950s, however, Plaquemines Parish has lost over 250 square miles to natural and induced subsidence or sinking of land; oil-and-gas exploration canals and navigation channels that allowed saltwater in; Mississippi River levee construction; major hurricanes; sea level rise; and invasive species, like nutria—a rodent that gnaws at Cypress trees. Louisiana as a whole has lost an area equal to the state of Delaware since the 1930s.
This article was originally published in the June 10, 2011 print edition of The Louisiana Weekly newspaper