a hurricane ...
http://www.weather.com/newscenter/specialreports/hurricanes/vulnerablecities/neworleans.htmlLake Pontchartrain forms New Orleans' northern boundary. The lake spans 630 square miles, but it's only 25 feet deep. Many experts say the lake is the city's greatest threat during a hurricane because of its relatively shallow depth.
As recent as 1998, when Hurricane Georges skimmed the city, gales pushed the water of Lake Pontchartrain over the man-made seawall and onto roads and yards that face the Lake. But, the city has not seen the worst devastation possible.
A hurricane approaching the city from the east, virtually at the mouth of the Mississippi River, "would drive the lake water southward into the city. So under the right circumstances, the flooding may be more severe coming from the lake than that coming from the Gulf (of Mexico)," said Jay Grimes, Louisiana State Climatologist.
Many see the threat of a surge from the Gulf of Mexico as minimal because there is a complex series of levees between New Orleans and the Gulf. Many of the levees have been built and improved since 1966, when construction on the Lake Pontchartrain and Vicinity Hurricane Protection Project began. The levees that protect New Orleans from the lapping waves of Lake Pontchartrain have holes in them formed by three large canals that are used to pump water out of the city and into the lake on a daily basis.