The levee were planned and built to withstand the forces related to a Cat 3 hurricane.
Funding for a study that would describe the steps necessary to protect the Lake Pontchartrain Basin from a Cat 5 hurricane was discussed, and cut by Congress in 1998. http://www.freerepublic.com/focus/f-news/1478248/posts?page=82#82 <- details here
There is so much shared blame for this tragedy that it is embarassing...from the local level to the federal, and everyone in between. Although I don't believe I stated so, congress is responsible, not only for not providing the necessary funding for preventing the inevitable (and thus playing Russian Roulette with the lives of the residents of New Orleans), but in confirming (I believe without opposition) Mike Brown to head FEMA when he was clearly unqualified for the position, both in lacking practical experience and qualities of effective and forceful leadership.