Risk Management Solutions works for the insurance industry and others and published a comprehensive review of the city's risk in 2006.  Their 29 page assessment is worth review (click to read it for yourself). An excerpt from the Executive Summary follows:

Hurricane Katrina marks a turning point for the city of New Orleans. While the city had been flooded by hurricane storm surges three times in the past century, the flooding was most extensive in 2005 when more than 80% of the city was left underwater after the passage of Hurricane Katrina.

After each of the earlier storm surge floods in the city in 1915, 1947, and 1965, modest programs of investment in improved flood defenses were followed by decades of relative neglect. Throughout the postflood periods, tens of thousands of new buildings were developed in the partially protected flood plains, effectively increasing the number of people and properties exposed to flooding from hurricane storm surges. As the years passed it must have appeared that flood risk had been vanquished, yet no comprehensive flood risk analysis was ever performed to discover the true situation – the risk was continuing to rise.

The threat to New Orleans from flooding is increasing due to a combination of three physical temporal factors. First, as a result of its location on thick recent delta sediments along the edge of an oceanic basin, the city is sinking at geologically rapid rates. Second, over the last decade, global sea level rise has increased as a result of climate change and is predicted to accelerate into the future. And third, the level of Atlantic basin hurricane activity has also risen, with the biggest increases for the strongest storms (with the largest surges), particularly in and around the Gulf of Mexico. These factors all serve to increase the storm surge flood hazard faced by New Orleans, and will significantly raise the risk of flooding in the city through the 21st century.

First RMS looks at the history of flooding and flood control efforts. Then they look at the emerging risks, mitigation and policy implications. RMS modeling techniques are familiar:

1. Historical hurricanes evaluated to predict future storm magnitudes and period of return (i.e. the number of years until a similar strength storm hits again).
2. Local topography is evaluated in light of predicted storms to determine how great a surge can be expected.
3. Defenses are evaluated and failure modes explored. Levees, flood walls and gates, pumps, etc all participate in this review.

RMS looks hard at floods coming from the east (Lake Borgne) and the north (Lake Pontchartrain) but curiously seems less concerned about storm surge from  the south and the west which would primarily affect the west bank of Jefferson Parish and Algiers.

Selecting sites from around the city, the baseline shows that the lowest elevations currently face a 55 year return period for flooding and the highest 275 years. However the equation is changing as sea levels rise and subsidence continues. They predict that unchecked, these factors reduce the return period to 40 years and 125 years respectively by 2050. Their assumptions include accelerating sea level increases due to global climate change especially after 2030. They also look at predictions of more intense hurricanes and independently generate similar conclusions for reduced return periods because the storms are stronger.

Finally RMS looks at mitigation based on higher levees. They achieve the expected result that higher levees reduce the risk of flooding. However even with improvements, the underlying theme is that flood risk is increasing over time and will require additional mitigation efforts over time to keep the risk within tolerable bounds. Reading between the lines it is obvious that the authors are wondering if the authorities can show the continuing resolve necessary to face the risks.

In summary RMS poses the alternatives. "It [the city of New Orleans] can either embrace transparency around flood risk and flood risk management, or, as has happened on three previous occasions over the past century, it can simply resume business as usual and pray that the floods stay away." Which will it be?