Levees Not War
Infrastructure. Environment. Peace.

New Orleans Is Most Likely Safe from River Flooding

05/12/11

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There’s a certain trepidation in writing that headline, but . . .

Despite over $2 billion in damages, possibly to reach $4 billion from the Mississippi River Flood of 2011, including dramatic flooding upriver around Cairo, Memphis, and Vicksburg—and despite scary images and headlines on screen and paper—the city of New Orleans should be safe from inundation from the historically high waters now coursing down the Mississippi toward the Gulf. The flooding is the result of normal springtime snowmelt compounded by record rainfall from two major storm systems across the U.S. during April. (See Mississippi River watershed map below; click to enlarge.)

The U.S. Army Corps of Engineers has already begun opening the Bonnet Carré spillway upriver from the city (photo below). There will be flooding in the Atchafalaya Basin—possibly around 3 million acres—once water is diverted from the Morganza Spillway, a decision that is expected soon from the Corps of Engineers. And deep-draft shipping may be temporarily suspended by the U.S. Coast Guard if water levels rise to just a little higher than they are now; Americans “upriver” may experience a spike in gas prices as supplies are temporarily interrupted by the halting of oil tanker traffic between Baton Rouge and the Gulf.

Read the Headlines with Some Skepticism

Under the ominous headline “Mississippi River Flooding in New Orleans Area Could Be Massive if Morganza Spillway Stays Closed,” the New Orleans Times-Picayune reported on Wednesday that based upon the best information available from the Corps of Engineers, the staggering volume of water making its way down the Mississippi could cause “levee failures and massive inundation of metro New Orleans, worse than Hurricane Katrina, if the Morganza spillway is not opened to divert river water down the Atchafalaya basin. The bad news for people living along the Atchafalaya: the Corps of Engineers predicts they will flood in either case.” Although the article itself was measured, not alarmist, the headline implied there was a possibility that the Corps might decide not to open the Morganza Spillway. Another headline warned, “Corps Officials Fear Flooding.” Note to the reader: It is the Corps’ job to fear flooding and to try to make sure it doesn’t happen.

Now, reporters don’t write the headlines, and the body of an article is often less alarming than the headline crafted by some news editor. But “Flooding Could Be Massive” sort of got our attention, so we checked around.

An engineer friend in the New Orleans area who knows people at the U.S. Army Corps of Engineers says the Corps is monitoring the situation very closely all along the river, per established protocol, and just waiting for Major General Michael Walsh to give the word about opening the Morganza Spillway. Major General Walsh is president of the Mississippi River Commission and commander of all USACE districts along the river. Times-Picayune environmental reporter Mark Schleifstein writes that General Walsh is expected to announce his decision to open the Morganza spillway between Friday and Tuesday (May 13 and 17).

Our engineer friend says, “Walsh is going to wait until the last possible moment to give the order just in case something changes in the river or they [the Corps] discover a better alternative. And when I say, ‘last possible moment,’ that is taking into account the timeline between when the order is given and everything that has to happen to safely open the structure. This stuff is really well thought out. The hydraulics, structures and levee engineers there have been working this 7 days a week for the past two weeks. . . . New Orleans is safe for now. Never say never, but we have no reason to panic as of right now.”

[ Click here for City of New Orleans Emergency Preparedness / Flood Fight information ]

What Are the Likely Scenarios?

What will most likely happen is that, beginning upriver and working down, the Old River Control Structure will allow some water through, then the Morganza Spillway will be opened gradually to about 50 percent to allow some 300,000 cubic feet per second to divert into the Atchafalaya Basin, and the southernmost relief valve, the Bonnet Carré spillway, will be opened to 100 percent to allow runoff of some 250,000 cubic feet per second into Lake Pontchartrain. There are adverse consequences to the second two openings—flooding in some 3 million acres in the Atchafalaya Basin, loss of homes and crops, and in Lake Pontchartrain, oyster beds spoiled by river water. But these diversions will protect the port and the city of New Orleans.

Along with a press release titled “Corps Analyzes Multiple Mississippi River Scenarios,” the Corps has released a series of Inundation Scenario maps, also known as “decisional documents” to indicate the areas that would be flooded, and how deeply, if certain water control structures—principally the Morganza and Bonnet Carré spillways—are used or not used. (Click here for a big PDF of all 3 maps.)

Map One appears to be the most likely scenario—certainly the one most favorable to the City and Port of New Orleans (that’s why it is most likely). This is what Maj. Gen. Walsh is expected to announce in the next few days. The Corps press release says:

The three inundation maps that are now available to the public outline the following things:

Map One: The same map released on Friday, May 6, which illustrates the effect of the Morganza Floodway at 50% capacity.

Map Two: Outlines conditions that would result from allowing 1.8 million cubic feet per second of water to flow down the Mississippi River without operating the Morganza Floodway.

Map Three: Outlines conditions that would result from allowing 1.5 million cubic feet per second of water to flow down the Mississippi River while diverting the excess water through the Old River Control structure and not operating the Morganza Floodway.

The Bonnet Carré (“bonny carry”) spillway upriver from New Orleans, about 12 miles west of the city, is gradually being opened to divert water into Lake Pontchartrain. That diversion started on Monday, May 9, and gradually the entire spillway will be opened, 100 percent. This is the first line of defense.

The second line of defense, used only rarely, is the Morganza Spillway upriver in Pointe Coupee Parish, near New Roads. This spillway’s opening to release water westward into the Atchafalaya Basin is what is expected to be announced soon by Maj. Gen. Walsh. This, too, will be opened gradually to allow a slow, gentle flooding. The Atchafalaya will overspread its banks and water may rise some five feet or more, but the water will not rush in in a raging torrent. The gradual opening of the Morganza’s gates will see to that. Louisiana Governor Bobby Jindal has warned that some 3 million acres in the Atchafalaya Basin may flooded to some extent. “That includes about 18,000 acres of cropland just within the Atchafalaya basin.”

When to Open the Morganza?

Corps spokesman Bob Anderson said in an e-mail to the Times-Picayune’s Mark Schleifstein that the opening of the Morganza has a “trigger point” of 1.5 million cubic feet per second of water at the Red River Landing near New Roads, where the Corps maintains a water monitoring gauge. “We should see that exact scenario sometime on Saturday, May 14.” NBC Nightly News reported on Tuesday that at Vicksburg, where the Yazoo flows into the Mississippi, the big river was carrying 1.8 million cubic feet per second—enough volume to fill the Louisiana Superdome in 30 seconds—and rising about a foot a day. Already at Vicksburg the river is one foot higher than in the 1927 flood. (The 1927 flood, of course, was before the super-levees now girding the river were built; it was that flood plus the terrible flood of 1937 that prompted the building of the massive earthen levees now keeping the river in place—most of the time. For an awesome account of the 1927 flood, see John M. Barry’s Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America.) The river levees, by the way, held during Hurricane Katrina; what failed were the walls of industrial canals, outflow canals from the city to Lake Pontchartrain, and the sandpiles along the banks of MR-GO and the Intracoastal Waterway in St. Bernard Parish and New Orleans East—in short, just about everything but the river levees.

Deep-Draft Shipping May Be Halted

Late Wednesday in the Times-Picayune Mark Schleifstein reported that the Coast Guard is considering closing deep-draft shipping from Baton Rouge to Boothville—essentially all the way to the nails of the so-called Bird’s Foot where the Mississippi meets the Gulf of Mexico. The concern is that the ships’ wakes could push waves against the levees and add unnecessary pressure on the earthen walls. Port of New Orleans Captain Stanton says that down-bound deep-draft ships are moving at 18 miles per hour.

Stanton cited the case of the Bright Field in December 1996 when a Chinese-owned ship lost engine power and crashed against the Riverwalk, causing $15 million in damage. The river then was not in flood stage or moving at high velocity. What if a ship lost control when the water is 17 or more feet higher than normal and rushing at higher than normal speeds? What if that ship were an oil tanker?

Captain of the Port Edwin Stanton said the closure will happen “when the surging river reaches 18 feet at the Carrollton Gauge in New Orleans.” The National Weather Service Lower Mississippi River Forecast Center in Slidell anticipates that the river’s level will reach 17.9 feet at the Carrollton Gauge by Monday if the Morganza spillway is not opened. The river is projected to be at 19.5 feet on Monday, May 23.

Can the Old River Control Structure Help?

We asked our engineer friend about the Old River Control Structure upriver from the Morganza spillway, and whether it is primarily a containing wall or levee keeping the Mississippi from swerving westward to the Atchafalaya Basin—this is what Old Man River has been wanting to do for decades—or whether it also has outlet valves that can relieve the river’s pressure on the levees keeping it in its channel. In other words, in addition to the Morganza and Bonnet Carré spillways, can Old River help too?

He explained, “The Old River Control is actually several structures that are operated as valves all the time. The Corps does have the option of opening it all up to allow more water, but with Morganza open that will not be necessary. They actually did look at opening Old River in lieu of Morganza, but the results are almost the same regarding flooding downstream and it pushes the design capacity of the structures, so that scenario was rejected. Part of what is taking the time for the Corps to make the decision is working out all the possible scenarios and then evaluating advantages and consequences for each.”

For those who have read the gripping account by John McPhee in his New Yorker essay “Atchafalaya” about the 1973 collapse of a wing wall due to scouring erosion under the structure, that has since been repaired and reinforced with an auxiliary structure about 1,000 feet to the side so that water can flow a slower velocity, with reduced likelihood of scouring action by swift river currents.

Will the Mississippi Shift Westward to the Atchafalaya?

It’s going to happen eventually: the path of the Atchafalaya River is a more direct course to the Gulf of Mexico and a steeper slope, too. Gravity, as it were, is pulling the Mississippi toward the Atchafalaya. The Old River Control Structure is there to prevent this westward shift—it diverts about 30 percent of the Mississippi to the Atchafalaya—but the Mississippi is stronger than anything man can build. When it does swing to the west, probably at the point where the Old River Control Structure is situated, Baton Rouge and New Orleans will be left, if not high and dry, as backwaters, and so will the petrochemical industries along the Mississippi.

The map below shows the Mississippi’s present course in blue, and the Atchafalaya Basin outlined in yellow, with the present course of the Atchafalaya River in red.

 

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