Tropical Storm Barry could breach New Orleans' river levees. Here's how the levee system works and how much it can withstand.

The Mississippi River approaches a levee at left in New Orleans, La., Thursday, July 11, 2019, ahead of Tropical Storm Barry. Never in the modern history of New Orleans has water from the Mississippi River overtopped the city's levees. (AP Photo/Matthew Hinton)The Mississippi River approaches a levee at left in New Orleans, La., Thursday, July 11, 2019, ahead of Tropical Storm Barry. Never in the modern history of New Orleans has water from the Mississippi River overtopped the city's levees. (AP Photo/Matthew Hinton)Associated Press

Tropical Storm Barry is getting stronger as it heads for New Orleans, and is likely to become a Category 1 hurricane by the time it makes landfall in Louisiana tomorrow morning, according to the National Hurricane Center.

A hurricane warning is in effect for a swath of the Louisiana coast, and forecasts suggest the Mississippi River could crest as high as 19 or 20 feet- the highest level the river has reached in New Orleans since 1950. (The river has already swelled to 16 feet.) 

That could create the biggest test ever of the city's river levees, which were built in 1927. 

 

Louisiana Gov. John Bell Edwards has declared a state of emergency and warned that there could be "a considerable amount of overtopping" of levees in Plaquemines Parish, a suburban district southeast of New Orleans.

Here's everything you need to know about what levees are, how they work, and what the system looks like in New Orleans.

Levee systems rely on embankments, flood-walls, and pumps 

Most levees are trapezoid-shaped, elevated embankments that separate bodies of water from inhabited flood plains. They're meant to protect those areas in the event a lake or river level rises. 

Roads and railways sometimes cross a levee, so flood-walls - which are usually made of concrete or steel - and other structures are used to close those gaps. Flood-walls are also often built to supplement levee systems in high-density urban areas where there isn't enough space for a large levee.

The US Army Corps of Engineers classifies levees by the environment they protect (urban or rural) and the body of water they protect it from (river, coastal, or estuary).

New Orleans 9th ward Mississippi river leveeA view from the top of the levee that protects the Ninth Ward from the Industrial Canal is pictured as Tropical Storm Barry approaches land in New Orleans, Louisiana.Jonathan Bachman/Reuters

Different types of river levees can run parallel the main river channel, encircle a protected area, or provide backup or protection to an existing levee.

New Orleans has two levee systems along the Mississippi River

Two levee systems hold back the Mississippi in New Orleans: the East Bank System and the West Bank System. Together, these systems boast 192 miles of levees and 99 miles of flood-walls. 

But it's unclear just how much water the river levees can withstand. The official levee database run by USACE shows levee heights as low as 18 feet above sea level in some parts of the city. That puts it below forecast for peak river heights due to Tropical Storm Barry. 

However, the Army Corps has disputed the information in its own database. Ricky Boyett, a Corps spokesperson, told The Times-Picayune and New Orleans Advocate that the group's model still "does not show overtopping of the levees in the 9th Ward."

 

The Corps even has an internal analysis of the river levees in New Orleans that ranks both the East Bank and West Bank System as at moderate to high risk of a breach.

Levees can fail, and this will be the biggest test yet

If the river water rises higher than the levee it will spill over, but a levee breach is not just a question of height. Levees can also crumble under pressure due to poor maintenance, inadequate foundations, or erosion.

Too much water can seep through a levee and cause it to slough away, or simply seep through the ground below and spout up on the other side in what are known as "sand boils." Animals also burrow below levees sometimes, creating unintentional paths for pent-up water.

Of course, a levee that breaks is far more dangerous than a levee that's too short. In 2005, Hurricane Katrina caused a New Orleans coastal levee to break, releasing a wall of water. The flooding killed over 1,000 people. Since then, Congress has invested $15 billion in repairing the levee system around New Orleans. 

The river levees at risk of a breach due to Tropical Storm Barry, however, held during Katrina.

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