He was lying. He was specifically warned in no uncertain terms of this possibility, in advance. And the videotaped proof has now been made public:
In dramatic and sometimes agonizing terms, federal disaster officials warned President Bush and his homeland security chief before Hurricane Katrina struck that the storm could breach levees, risk lives in New Orleans' Superdome and overwhelm rescuers, according to confidential video footage of the briefings.Via Talking Points Memo.
Bush didn't ask a single question during the final government-wide briefing the day before Katrina struck on Aug. 29 but assured soon-to-be-battered state officials: "We are fully prepared."
... the video and transcripts show federal and local officials discussed threats clearly, reviewed long-made plans and understood Katrina would wreak devastation of historic proportions. "I'm sure it will be the top 10 or 15 when all is said and done," National Hurricane Center's Max Mayfield warned the day Katrina lashed the Gulf Coast.
UPDATE (March 5, 2006):
Associated Press has issued this "clarification":
WASHINGTON - In a March 1 story, The Associated Press reported that federal disaster officials warned President George W. Bush and his homeland security chief before Hurricane Katrina struck that the storm could breach levees in New Orleans, citing confidential video footage of an Aug. 28 briefing among U.S. officials.
The Army Corps of Engineers considers a breach a hole developing in a levee rather than an overrun. The story should have made clear that Bush was warned about flood waters overrunning the levees, rather than the levees breaking.
The day before the storm hit, Bush was told there were grave concerns that the levees could be overrun. It wasn't until the next morning, as the storm was hitting, that Michael Brown, then head of the Federal Emergency Management Agency, said Bush had inquired about reports of breaches. Bush did not participate in that briefing.