Our top climate scientists say this could not have happened – because there wasn’t enough moisture in the atmosphere back then. Water vapor is a feedback which is determined by the amount of (white) man-made CO2 in the atmosphere.
While New Orleans has experienced numerous floods large and small in its history, the flood of 1849 was of a more disastrous scale than any save the flooding after Hurricane Katrina in 2005; see Effect of Hurricane Katrina on New Orleans. Katrina flooded a larger total urban area. However much of what would become the city of New Orleans and its suburbs in Jefferson Parish was still swampland when it was flooded by the Crevasse of 1849. The water level of Mississippi River which flooded the city in 1849 was higher than that of Lake Pontchartrain which flowed into the city after Hurricane Katrina in 2005. This is particularly evident in areas of Uptown where higher water levels were recorded in 1849 in places which flooded again in 2005. Also, the flooding of 1849 extended into a significant part of Uptown that remained dry during the flooding after Hurricane Katrina.
New Orleans is below sea level in places and prone to severe hurricanes from time to time. That is a bad combination.