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The Mardi Gras Diversion at Bohemia

The Mississippi Ridge at Bohemia south of the end of the main line levee

On February 28, New Orleans partied at Mardi Gras, the Mississippi broke through the Mississippi ridge and created a crevasse at the Bohemia Spillway, a couple of miles south of the end of the Main Line Levee, and began delivering precious fresh water and sediment to Breton Sound, which has experienced extreme losses of marshland.

It’s a gift and we can thank the Flood of 2011 for it. All that stood in the way of the crevasse was a slightly elevated gravel road, which the flood washed away last summer. Below are two captions that I cut from The Mississippi: A Visual Biography.

As you will see below, the crevasse represents a potential savings of $6.4 million, if the Corps of Engineers and the Louisiana DNR were to do the work. What the river did by itself may need tidying up, but that work can be done by the Corps and the DNR.

 

Breton Sound at the Bohemia State Wildlife Management Area just off Pointe a la Hache

Mississippi River Ridge: Pointe a la Hache Relief Outlet

Plaquemines Parish, Louisiana

“It is remarkable, that the banks of the river are much more elevated than the circumjacent country. This is occasioned by a more copious deposition along the margins, than at a distance from them. These are thickly covered with grass, and a vast variety of ligneous plants, which serve to filtrate the waters in their progress to the low grounds and swamps, and to retain the greatest proportion of the alluvious substances. Hence the lands along the banks to a certain depth, generally from four hundred to seven hundred yards are excellent for tillage; while the whole surface in the rear of them, extending to the sea, is alternately covered by lakes and impassable swamps.”–Major Amos Stoddard, 1812[i]

The 2004 Louisiana Coastal Area proposal emphasized diverting freshwater and sediments into Breton Sound. So did Louisiana’s post-Katrina 2007 Comprehensive Master Plan for a Sustainable Coast. So did the Corps of Engineers’ post-Katrina proposal. So did numerous unofficial proposals. At the two-year anniversary of the storm, other diversions south of the Caernarvon structure, actual and planned, directed freshwater into Breton sound.

The White’s Ditch siphon, not far from Caernarvon, diverted small amounts of freshwater into northern Breton Sound. The mainline levee ended at Bohemia, where the river was also free to sluice down the Bohemia spillway, but only when it ran high. Built in 1926, the spillway was non-operational by 2005. The Pointe a la Hache relief outlet ran from Bohemia south to the Ostrica Lock. Here, only the natural levee and a gravel road lay between the Mississippi and the marshes in southern Breton Sound. A young natural levee forest had taken root on the ridge. A flooded Mississippi could spill over the levee and wash into the adjacent marshes, or maybe it couldn’t. The gravel road may have impeded overbank flooding. Further south, near Point Pleasant, the Bohemia diversion structure and spillway could have diverted water into Breton Sound through Bayou Lamoque the same rate as Caernarvon, was inoperable, but was included in Louisiana’s 2007 plan for a sustainable coast. At the beginning of the twenty-first century, how much fresh water the overbank flooding introduced into southern Breton Sound had not been measured, nor had any changes in water quality caused by the flooding.

In 2001 the Corps of Engineers and the Louisiana Department of Natural Resources began design work on a diversion at Fort St. Philip, on the east bank opposite Fort Jackson. The agencies planned to divert water from the river at the rate of 2,500 to 5,000 cubic feet per second into 2,252 acres of deteriorating wetlands and open water near the site of the old fort. The existing marshes graded from fresh at the toe of the ridge to saline. The site, short on development and infrastructure, presented a rare opportunity to rebuild marshes in shallow estuarine waters.

Engineers would cut a series of gaps in the bank of the river, armor them, creating channels that would carry river water and sediment to open water adjacent to the natural ridge of the river. Farther downstream they would create a diversion outfall channel, which would connect to Fort Bayou. Holes punched in the outfall channel would allow water and sediment to leak into the shallows between the bank and the bayou. The bayou would carry water and sediment to the east beyond the reach of the channel. The Corps and the DNR expected to create and 624 acres of marsh over the life of the project, recreate the progression of wetlands from natural levee to emergent marsh to mudflats, and reduce the loss of marsh in the rest of the site. The expected cost was $6.4 million. A final note: The Corps and the DNR suspected that the introduction of freshwater into marshes that were largely saline would disrupt existing oyster leases. And, the siltation the engineers hoped for might plugged oil and gas canals and disrupt access to the project area.

The coastal scientists and engineers who published the Comprehensive Recommendations Supporting the Use of the Multiple Lines of Defense Strategy to Sustain Coastal Louisiana in August 2007 recommended encouraging the Mississippi to wash over its banks, a rates totaling up to 100,000 cubic feet per second, over the twenty miles south of Bohemia. Doing so would rebuild its natural levee. It was a habitat that, in the twenty-first century at least, was unique to Louisiana, where all other ridges were under intense development. The process would offer scientists and engineers an opportunity to observe and understand how a river develops its natural levee.[ii] Photograph 2007

 

Bohemia Siphon along the Mississippi

 

Mississippi River Ridge: Small Siphon at Bohemia

Plaquemines Parish, Louisiana

James Buchanan Eads solved the problem of sandbars blocking the passes at the mouth of the Mississippi with jetties, which opened Southwest Pass to shipping, but as early as 1832 Major Benjamin Buisson, Chief Engineer of the State of Louisiana, proposed and the State Legislature approved dredging a canal to connect the River at Fort St. Philip to Breton Sound. The State sent the proposal to Congress, which ordered a survey in 1837. Major W.H Chase did the survey and presented the War Department with a plan for the canal at an estimated cost of ten million dollars, a price tag that was way beyond Congress’s reach for public improvements in 1837.

Mr. R. Montaigu, a civil engineer, revived the idea twenty years later, but it was not until 1871 that Congress ordered the Corps of Engineers to make surveys and plans for the canal. Major C.W. Howell drew up the plans and a board of army officers, that included Major Howell, made a report that favored the plan over Eads’ proposed jetties in 1873. The cost would be thirteen million dollars. General A.A. Humphreys, Chief of Engineers and Eads’ rival, approved the plan, but General J.G. Bernard dissented, saying the cost estimates for the canal were unreliable, the canal would not solve the problem, would not be finished until 1884, and at least one of the passes in the river should be improved. And, should South Pass be jettied, it would function like a canal.

In February 1874 Eads proposed delivering a channel twenty-eight feet deep and 350 feet wide at a cost of ten million dollars, and he offered to work for free until he had achieved a channel twenty feet deep when he would receive a million dollars and a million for every two feet until the channel reached twenty-eight feet. The rest, five million dollars, would go for maintenance.[iii]

Eads got the job and got the job done. His jetties and those that followed delivered sediment to very deep water, where it was useless for land building. Combine the jetties with the levees and the wetlands in the Barataria Basin to the west and Breton Sound to the east were starved for freshwater and sediment.

In the wake of Katrina the canal was back and Fort Philip, located south of the end of the mainline levee and already the site of a planned diversion, was the proposed site of a major diversion of freshwater and sediment to Breton and Chandeleur Sounds.

First, the diversion: In 1994 Ivor van Heerden proposed diverting Mississippi water and sediment to Breton and Chandeleur sounds through the Bohemia Wildlife Management Area near Fort Philip and creating a new delta. He reiterated the proposal in a 2003 paper and again in his 2006 book, The Storm. The diversion would be huge, 200,000 cubic feet per second, would create more than five thousand acres or eight square miles of wetlands every year, leading to 140 square miles of new land within twenty years. The Bird-foot Delta would be abandoned, though enough water would be left the river to maintain the shipping channel through Southwest Pass. Or, a new navigation channel would be dredged just north of Empire through Adams and Bastian Bays in the Barataria Basin to the Gulf of Mexico. A new lock and other control structures would have to be built to make it possible. The delta would evolve into a series of barrier islands, which would coalesce with the Breton and Chandeleur islands to the east and the Barataria Bay islands to the west, and create a continuous arc of islands from Grand Isle to the northeastern tip of the Chandeleur Islands, a speed bump to hurricane storm surges. The new wetlands would further reduce storm surges. New Orleans’ levees would be protected. Eventually, the new delta would extend across MRGO, leading to its closing, but after Katrina that was on Louisiana’s agenda anyway. The proposal followed van Heerden’s dictum: “Barrier islands protect the wetlands, the wetlands protect the levees, the levees protect the home.” Photograph, 2007

 


[i]      Stoddard, 159.

[ii]             Multiple Lines of Defense Assessment Team, “Comprehensive Recommendations Supporting the Use of the Multiple Lines of Defense Strategy to Sustain Coastal Louisiana,” Lake Pontchartrain Basin Foundation, August 17, 2007, 71-72, http://www.saveourlake.org/pdfs/JL/LPBF%20-%20CRCL%20Final%20Draft%20MLODS%20report%208-17-07%20for%20release%20part1.pdf; Lane, Robert R., John W. Day, Jr., Burnell Thibodeaus, “Water Quality Analysis of a Freshwater Diversion at Caernarvon, Louisiana,” Estuaries, Vol. 22, No. 2A, June 1999, 329, http://estuariesandcoasts.org/cdrom/ESTU1999_22_2A_327_336.pdf; Louisiana Coastal Wetlands Conservation and Restoration Task Force, Delta Building Diversion North of Fort St. Philip (BS-10), October 2003, http://data.lacoast.gov/reports/gpfs/BS-10.pdf.

[iii]             Corthell, Elmer Lawrence, A History of the Jetties at the Mouth of the Mississippi River, New York: John Wiley & Sons, 1881, 17-23; Barry, 68-71;

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