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News Source: nola.com
Where The Metro New Orleans Levees Don’t Reach The Forecasted Crest Height
overtopped ward algiers

These New Orleans Levees Could Be Overtopped By Mississippi River Surge Ahead Of Storm

Storm surge accompanying potential Category 1 Hurricane Barry may cause overtopping Saturday of much of the Mississippi river levee in the Lower 9th Ward, Algiers and St. Bernard Parish, according to Army Corps of Engineers levee data. 

The National Weather Service's Lower Mississippi River Forecast Center in Slidell expects the surge will add between 3 and 5 feet to the unusually high river in New Orleans and locations to the south, reaching as high as 20 feet at the Carrollton Gauge in New Orleans. 

South Louisiana should brace for the effects of a storm expected to be named Hurricane Barry by the time it makes landfall with maximum winds …

The river was at 16 feet at the Carrollton Gauge on Wednesday morning. 

The official flood stage in New Orleans is 17 feet, but a combination of recently elevated earthen levees and floodwalls on both sides of the river in the New Orleans area protect most locations to water heights of between 20 and 22 feet.

But a map of levee heights in the New Orleans area that's part of the Corps' National Levee Database shows that the top of large segments of river levees along the Lower 9th Ward in New Orleans and St. Bernard Parish on the east bank, and some locations in Algiers on the West Bank, were between 18 and 19.99 feet.

Thus, a 20-foot river height could cause overtopping at those locations, something that has never happened in New Orleans' modern history, and only rarely in St. Bernard.

Officials with the Corps, which oversees construction and repair of the river levees, say there is little risk of levee failures, even if they are overtopped.

“We’re confident with the integrity -- the levees are extremely robust and designed to handle a lot of pressure,” spokesman Ricky Boyett said.

A corps spokesman said Wednesday it would consult with local levee districts to determine whether any areas need Hesco baskets -- huge plastic buckets filled with sand -- to increase the height of earthen levees or floodwalls.

If the river does rise to 20 feet in New Orleans, Boyett said, corps officials think the surge would also overtop river levees south of Oakville and Caernarvon in Plaquemines Parish, allowing some water to flow into protected areas. There also could be intermittent overtopping of levees farther upriver from wave action in the river, though that would not create the constant flow expected at low areas downriver, he said.

Water overtopping the levees along the river would likely flow into internal drainage systems, and in New Orleans on the east bank, would move through canals to Lake Pontchartrain. 

"On our end, we are monitoring it and we’ll pump out any related inundation to the best of our system’s ability," said Richard Rainey, spokesman for the Sewerage & Water Board.

One low spot farther upriver happens to be the campus of the Corps' New Orleans District office, which is built along a 3,000-foot stretch of the river along Leake Avenue in the Riverbend neighborhood. According to the levee database, the campus levees protect to water heights of between 19 and 19.7 feet, making the corps' office one of the lowest points along the east bank river system in New Orleans.

In 2014, Congress authorized construction of new floodwalls that would increase protection around the campus to more than 24 feet, but only appropriated money for the project this year. The corps has been unable to begin construction, however, because of this year's unusually long high-river period, which actually dates back to November of last year. 

Corps officials had put together a stop-gap plan to reduce potential overtopping at the campus by erecting Hesco baskets. On Wednesday, Boyett said the agency expected to build the Hesco wall around the campus over the next few days. The river is expected to crest at 20 feet on Saturday.