Floods & The Bonnet Carre Spillway

Written by Donna Bush
Edited by Alison M Jones
Images © Donna Bush

This report on Mississippi River flooding in New Orleans follows last week’s blog on flooding in Natchez MS, to which today’s author Donna Bush also contributed. As a New Orleans freelance writer and photographer, Donna’s passions are focused on the environment, wildlife, nature and travel. Unless otherwise noted, photos in this blog were taken by Donna this year on February 27. [An Addendum on the evolution of flood protection in New Orleans by NWNL Director Alison M Jones is at the end of this blog.]

As Bonnet Carre Spillway floods, a Mississippi River barge heads upstream

IT’S MARDI GRAS IN NEW ORLEANS! People have flocked from all over to experience one of the largest non-stop parties.

But that’s not all that’s happening during our Mardi Gras season. This year, there is more than just beads, king cakes and parades. The Mississippi River is approaching flood stage earlier than usual. Snowmelt in the US Midwest and Mid-Atlantic has begun in earnest this past weekend with Missouri River floods coming down from Nebraska and the Midwest Plains. The contiguous US has experienced the wettest winter on record, with more rain and snow forecasted. Lower elevations that don’t snow get heavy rains.

Water rushing through Bonnet Carre Spillway’s open bays

SWOLLEN TRIBUTARIES north of New Orleans that feed into the Lower Mississippi River from the east include the Eastern Ohio, Tennessee, White, Green, Wabash and Yazoo Rivers and from the west include the Missouri and Arkansas Rivers. They are now all funneling heavy rainfalls to the Gulf of Mexico, via Baton Rouge and New Orleans. As the flooded Mississippi passes Baton Rouge, levees protect that riverside city and its many industrial and petrochemical plants. Further south, New Orleans is protected by levees and additional infrastructure built by the US Army Corps of Engineers [USACE].

THE BONNET CARRE SPILLWAY – a critical piece of the USACE’s flood control system – is upriver of New Orleans in St. Charles Parish. Opening this spillway allows dangerously-high flood waters to spill into Lake Pontchartrain, just north of New Orleans. This prevents flooding downstream and also alleviates pressure on the levee system around New Orleans and upstream. The opening of the Bonnet Carre Spillway keeps the flow below 1.25 million cubic feet per second [cfs]. As of Sunday, March 17, the flow was 1.24 million cfs.

The USACE opened the Bonnet Carre Spillway on February 27. Last year, it was open from March 7 to 30. This was the first time since its construction in 1931 that this flood-control structure has been open in back-to-back years. As noted in the NWNL blog from Natchez last week, the USACE opened 30 spillway bays on Feb 27 and 198 on Mar 10. On Friday, March 15th, they closed 10 bays. They will continue to monitor the river’s flow and height, as high water and snowmelt from up north push towards New Orleans.

If necessary they will reopen bays; halt the opening of bays; or keep bays open longer, possibly into April. It is a waiting game. The Mississippi River is currently at 16.8 feet at the Carrollton Gauge. While Bonnet Carre is the primary resource for flood relief, the Morganza Spillway on the western bank is available as a final backup for control of extreme flooding.

Open bays of the Bonnet Carre Spillway.

This opening of Bonnet Carre allows sediment-loaded water into Lake Pontchartrain at 250,000 cfs. That represents 1/5 of the total volume of the Mississippi River expected at the height of the flood. This unusual addition of water for a full month will have a huge effect on the fishing in the lake, which will be both good and bad.

Per Senior Hydrologist, Jeff Graschel, with the National Weather Service in New Orleans, “The rain and snow melt in the Upper Mississippi and Missouri Valleys will prolong flooding conditions for the Lower Mississippi River. Right now, it is not showing us going any higher, but will keep us at higher levels through March and into April.” It will take approximately two weeks for the current rain and snow melt in the Upper Mississippi and Missouri Valleys to reach New Orleans.

LAKE PONTCHARTRAIN is made up of brackish water. This mixture of marine and fresh water is saltier than fresh water, but not as salty as seawater, and is fine for those species that can tolerate that mixture. On most days, those fishing on the lake will find both fresh water and saltwater species, depending on the location. Speckled trout and a variety of bass, catfish, and crappie delight those fishing year-round. Let’s face it! Louisiana isn’t named “The Sportsman’s Paradise” for nothing!

Bonnet Carre’s open bays & chemical plant across the river in Taft, LA

FLOOD IMPACTS on FISH POPULATIONS: I reached out to Don Dubuc for his insight on impacts of new sediment coming in with the flood waters, since he is a renowned Louisiana fresh and saltwater fisherman and host of the weekly New Orleans TV channel WWL Fish and Game Report. His worry is that muddy water from the Mississippi will displace brackish-water species, especially the speckled trout.

The introduction of rich nutrients with floodwaters is a good/bad scenario. The downside is that nutrients can cause an algae bloom; deplete oxygen in the water; and cause a fish kill. Yet, these nutrients and small prey species (such as minnow, shad, crawfish and freshwater shrimp) provide food for the lake’s predator species. The freshwater fish, if disturbed by heavy nutrients loads, will ultimately find their way to fresh rivers and creeks, restocking those areas. Thus, in the long term, the added nutrients will improve the long-term health of the lake. The bottom line from Don Dubuc: “Short term flooding in Lake Pontchartrain is mostly negative. Long term – it’s mostly beneficial [for fish].”

Open and closed bays of the Bonnet Carre Spillway

FLOOD IMPACTS on WETLAND ECOSYSTEMS: The opening of the Bonnet Carre Spillway is a huge negative for Louisiana’s vanishing wetlands. The sediment that floods through Bonnet Carre into the lake is needed to rebuild the Gulf Coast of Louisiana as it used to be. But when the sediment settles and stays in the lake, Louisiana’s coast loses its natural land replenishment. The coast is running out of time. Per RESTORE the Mississippi River Delta, roughly 9 million cubic yards of sediment (that would otherwise go to the Gulf shores) is lost during an opening of the Bonnet Carre. That is soil needed to fill receding wetlands in this region so they can protect people and their properties from major incoming hurricanes.

FLOOD IMPACTS ON FARMLAND, which spreads out for miles and miles beyond the west bank, are minimal. Levees along those western banks can handle floods flowing at flood levels even higher than those at which Bonnet Carre gates are opened.

Tanker Silver Joan headed upriver, seen from Bonnet Carre Spillway

SOLUTIONS: What can be done? Hard infrastructure (i.e., levees, dams and spillway gates) provides one type of solution. Green infrastructure (i.e., restoring wetlands and forests) provides a second type of solution.

Two priority projects for sediment diversion/restoration are being planned to rebuild our freshwater wetlands. One is The Mid-Barataria Sediment Diversion on the west bank of the Mississippi River, near Myrtle Grove. The other is The Mid-Breton Sediment Diversion on the east bank of the Plaquemine Parish. Both of these sediment-deposit projects are currently in the engineering phase.

How will these sediment diversion help? RESTORE The Mississippi Delta states “A sediment diversion is a large-scale coastal restoration project including a structure of gates that will be built into the Mississippi River levee system to allow river water, sediment and nutrients to flow into degrading wetlands to help sustain and rebuild land. Sediment diversions mimic the natural processes that once allowed the river to build the land of coastal Louisiana.” Considered a high priority, these projects will reconnect the river to its wetlands when finished, thus allowing crucial sediment buildup required for the success of our coast.

Water in the Bonnet Carre Spillway and Floodway rushing to the lake

ADDENDUM: A Progression of Flood Protection Options
by NWNL Director, Alison M Jones

In 1927, the Mississippi River let loose the the most destructive flood in US history. The US government reacted to this “mightiest rampage”¹ of the Mississippi River by asking the USACE to control future floods. It built the world’s longest system of levees and floodways. But levees tend to sink, crack and break; so, the USACE added cement and metal infrastructures to its Project Flood mix of flood-control measures. Locks, dams and flood gates were built from Minnesota to Louisiana “to control nature,” as John McPhee described in his 1989 essay. He wrote that flood walls were built to protect New Orleans against a rise up to 20 feet, because, “Something like half of New Orleans [was] below sea level – by as much as fifteen feet.”²

The backup, last-resort option of opening the 4,159-foot-long USACE Morganza Spillway (only ever opened in 1973 and 2011) funnels part of the Mississippi’s flow westward into the Atchafalaya River “distributary” to the Gulf. Scoured and deepened by the 1927 Flood, the Atchafalaya offers a shorter and steeper route than that of the Mississippi River out of the Delta into the Gulf. Careful engineering however is needed to release some floodwaters into the Atchafalaya River, but still keep the majority of the Mississippi’s volume within its longer path to the Gulf via New Orleans. Nevertheless, some predict the Atchafalaya will “soon become the main stream, and the river past New Orleans a deteriorating outlet.”³

Facing the possibly-inevitable victory of the Mississippi over man, today’s engineers who have focused on fighting Nature are now also mimicking and working with Nature. The Sediment Diversions described in today’s blog by Donna Bush are great examples of creating ecosystem resilience and balances.

The Bonnet Carre Spillway when dry (Photo by Alison M Jones, Oct. 2014)

Some say the only option is to go away. Others, who are more pragmatic or economically invested in the City of New Orleans, are betting on restoring the thirsty swamps and forests of roseate spoonbills and prothonotary warblers. Rebuilding Nature’s “flood sinks” and wetlands as an addition to the USACE’s hard-structure repertory of flood protections seems positive and plausibly successful. Perhaps these man-made diversions of the 200 million tons of sediment the Mississippi carries downstream, could eventually turn the Bonnet Carre Spillway into an unnecessary relic.

ADDENDUM SOURCES:

Barry, John M. Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America. New York: Simon and Schuster, 1997. Page 13.
McPhee, John. The Control of Nature. NY: Farrar, Straus and Giroux, 1989. Page 59.
Barry, John M. Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America. New York: Simon and Schuster, 1997. Page 425.