Why does the city of New Orleans need protection from flooding?

Answer 1

Most of the city of New Orleans sits on land that is below sea level, because the area was originally shallow swamplands that were drained as the city grew. Once the water was removed, the land compacted somewhat and became even lower. The levees around the various areas of the city keep out the water from the Mississippi River and from the surrounding lakes to the north and east, especially Lake Pontchartrain. Any rainfall must be pumped out, and any breaches in the levees would rapidly flood the city (as has happened in several hurricanes). Only a few areas of the city are slightly higher than mean sea level.

High Ground

When the city was first established by the French in 1718, it occupied the high ground now known as the Vieux Carre (old quarter). Yearly floods had built up sand banks along the river, and New Orleans was the site of a portage from the Mississippi to Lake Pontchartrain, which connects to the Gulf of Mexico. Using the lake to reach New Orleans, ships did not have to come the long way up the Mississippi delta. Conversely, New Orleans was a protected and convenient anchorage when coming downriver, and so became a center of trade.

Answer 2

Much of the city (New Orleans) is below sea level, and the levee system failed catastrophically.