The risk of flooding in New Orleans from hurricanes had been known for a long time. However, it was only after the catastrophe wreaked by Hurricane Katrina that a reliable flood control system was developed on the basis of a comprehensive study.

A hurricane landfall on the US Gulf Coast is not a rare event. Experts had long been aware that a disaster could happen in New Orleans: even in moderately severe storms the region was susceptible to flooding – with corresponding risks for the people living there. Two years before the disaster, an article entitled "The Creeping Storm" appeared in the magazine Civil Engineering, describing in detail how a hurricane could cripple New Orleans, and how such a disaster could be prevented. An emergency exercise had even been conducted using as its centrepiece a hypothetical hurricane, Pam – a storm roughly equal to Katrina in intensity. With a comparatively short return period of 50 years, New Orleans was one of the regions in which disastrous flooding had to be expected. Neither American society in general nor the local population was particularly concerned that no hurricane protection system had been completed for the city due to lack of funds. In any case, as an analysis conducted by the Interagency Performance Evaluation Task Force (IPET) set up after Katrina showed, it was based on outdated design criteria.

Katrina tracked across the open sea as a Category 5 hurricane, the highest in the Saffir-Simpson scale used by the US National Hurricane Center. Its size and wind field were that of a monster storm. It created a huge wind set-up in the water level long before it reached land. The water level rose by up to 7 metres along the east side of New Orleans and up to 9 metres along the coast of Mississippi. It was the largest storm surge ever recorded in North America. A 200-km stretch of coastline was devastated by strong winds and flooding. In New Orleans, it was the flooding rather than the wind that caused the most damage. The city, which is virtually surrounded by water, was not equipped with sufficient flood protection to cope with the high surge level. The system failed, and dykes and flood walls east of New Orleans and along the numerous canals leading into the city were breached at more than 50 locations. The "bathtub" in which New Orleans is located was filled and remained flooded for over a month.

The storm surge was not the only cause of the catastrophe. Parts of the protection system failed; some were too low, while others were either poorly designed, maintained or constructed. As a result, the water was able to penetrate to the city largely unhindered. But it was the failure of organisational structures that magnified the event into a severe catastrophe. The IPET was established in October 2005 in an effort to understand what had gone wrong during Katrina and why. The aim of the scientists and engineers from various institutions was to analyse and assess the behaviour of the system during Katrina and to implement findings from this for the repair and reconstruction of the storm surge protection system in and around New Orleans.

The IPET report compares the flood risk for New Orleans prior to Katrina with the risk following completion (in 2011) of the new Hurricane and Storm Damage Risk Reduction System (HSDRRS). The IPET team analysed the full range of possible hurricanes. The resultant water levels in various places (storm surge heights plus waves) were applied to the system so that its performance could be studied. This also allowed the reliability of the system to be assessed based on different design assumptions, and the team could additionally estimate what areas would be flooded to what depth at different levels of probability. From the 152 hypothetical hurricanes used for deriving the hazard distribution, 76 were chosen to define surge and wave conditions for the actual risk analysis.

The Hurricane and Storm Damage Risk Reduction System (HSDRRS) that has been in place since 2011 reduces vulnerability to flooding for most of the New Orleans region. Three specific measures:

• Higher and more resistant levees and flood walls were constructed throughout the region.
• Emergency pumps and canal closures were installed at the ends of the outfall canals.
• The pumps were designed to significantly reduce flooding heights in 100- and 500-year events. In turn, this reduces damage and the hazard for the inhabitants.

While some areas could still experience significant flooding and losses, the situation now represents the best structural risk mitigation status New Orleans has ever had. Given similar evacuation conditions to those seen in Katrina, the 2011 system is expected to reduce potential loss of life by as much as 86% without pumping and up to 97% with 50% pumping for a 100-year flood event. It also markedly reduces potential for loss of life from a 500-year event (98%). Given the same property distribution and values that existed prior to Katrina, it would reduce direct property damage by 90% for a 100-year flood event and by 75% for a 500-year event with 50% pumping, compared to the pre-Katrina situation without pumping. It is impossible to avoid risk entirely, but with further improvements to the systems and additional technical and non-technical measures, such as flood-proofing and emergency plans, it can at least be minimised.