Levees and floodwalls surrounding New Orleans - no matter how large or sturdy - cannot provide absolute protection against overtopping or failure in extreme events, according to a new report by the National Academy of Engineering and the National Research Council. The 68-page report, "The New Orleans Hurricane Protection System: Assessing Pre-Katrina Vulnerability and Improving Mitigation and Preparedness," recommends voluntary relocation of people and neighborhoods from areas that are vulnerable to flooding.