levees

above are water lines, exterior and interior. Both are remnants of Katrina. then there are the levees. Do you wonder why they failed?
Natural levees are obviously made by natural means, the river deposits sediments over time through various floodings. These create high river banks called levees, which mean "raised up". They become as high as 15' in some areas, and 8' in others. For instance, when the river runs straight, levees tend to be more narrow and not as high, but in the more meandering sections, levees tend to be faster and have even eroded away entire sections of streets in addition to their own natural levees.

Above you see the man made flood walls, some are this size, some are much taller.

In comparison to other countries...
on the Thames in London
in Japan
Maeslant Storm Surge Barrier in the Netherlands, one of the largest moving structures on Earth
Eastern Scheldt Storm Surge Barrier in Holland
The Hagestein Weir in the Netherlands