Overwhelmed and underwater: An aerial view of New Orleans in September 2005, shortly after Hurricane Katrina and subsequent floods ravaged the city (Source: NOAA)

Situated at the juncture of Lake Pontchartrain, the Mississippi River, and the Gulf of Mexico, New Orleans has served as an entrepôt of cultural and commercial exchange for nearly three centuries. However, the city’s unique geography has also rendered it vulnerable to dangerous hurricanes and storm-induced floods. In 2005, this became startlingly clear when a Category 3 storm and the levee failures it triggered turned much of New Orleans into a disaster zone.

In the five years after Hurricane Katrina, the U.S. Army Corps of Engineers spent $15 billion building levees with specially engineered floodgates, closures, and surge barriers around Louisiana’s largest city. In the event of another major hurricane, the enhanced defenses are designed to stand up better than their predecessors and hold back turbid floodwaters from the lakes, rivers, and channels coursing through Greater New Orleans.

(Click to enlarge) Crescent City Encircled: A map of new levees and floodgates in the New Orleans metropolitan area since Hurricane Katrina (Source: The New York Times)

But will they? While the massive new floodwalls are impressive, we believe that these reinforced earthworks (designed to withstand 100-year floods) must be paired with natural defenses to effectively protect New Orleans from storm surges. In addition, non-structural floodproofing measures like home elevation must be incorporated into the urban fabric of this low-lying city and others like it across coastal Louisiana. This is especially important in an era of global warming, when bigger successors to Katrina could wallop Louisiana’s levees with stronger surges of wind-whipped water. Without action now, lofty expectations of improved flood resilience could come crashing down in the fierce winds of future storms.

Hurricanes from a Hotter Gulf

(Click to enlarge) Line of fire: Tracks of Category 3 through Category 5 hurricanes that hit Louisiana between 1965 and 2007. Each line indicates the path of the respective storm’s eye. The location of New Orleans is marked by the red bull’s eye. (Source: NOAA Coastal Services Center)

New Orleans lies along a stretch of the Gulf Coast that isfrequently hit by hurricanes. Because of the region’s importance to national trade, the federal government has taken an active role in building flood defenses around New Orleans for decades. Indeed, the levees that failed in 2005 were part of an unfinished system of floodwalls authorized by Congress after Hurricane Betsy hit Louisiana in 1965.

Hurricanes like Katrina and Betsy draw their strength fromheat in the surface waters of the ocean. As a result, Louisiana’s most devastating storms often occur during the late summer and early autumn, when average sea surface temperatures in the Gulf of Mexico are close to their peak (see chart below at left). Bodies of water like the Gulf are likely to get even warmer over the next century due to anthropogenic climate change. Until recently, scientists were unsure what effect this warming would have on hurricane strength and frequency. Fierce debate raged between competing camps over how the storm vulnerability of coastal cities like New Orleans would increase or decrease due to climate change.

Monthly mean sea surface temperatures in the Gulf of Mexico from 1985 through 2009 (1 = January, 2 = February, ... , 12 = December) (Source: Atlantic Oceanographic and Meteorological Laboratory, NOAA)

To resolve the debate, the World Meteorological Organization convened a panel of ten experts to examine the evidence on climate change and hurricane strength. The researchers co-authored a paper earlier this year in which they agreed that global warming will likely lead to fewer but stronger hurricanes in the coming decades.

This suggests that anthropogenic warming could increase the power of low-frequency, high-risk storms in the Atlantic Basin, meaning that a 100-year storm several decades from now would pack a much bigger punch and trigger much bigger floods than a 100-year storm of the present. Thus, it isn’t enough for Louisiana’s largest city to take the easy way out and build for today’s challenges. Instead, it must address those of tomorrow.

Past as Prologue

Ironically, lessons from the past could provide guidance for flood defense in the future. Cognizant of their settlement’s vulnerable position, the earliest residents of New Orleans relied on a combination of manmade defenses and natural barriers to protect their community from flood damage. In the same way, the modern-day city could be buffered from hurricanes if swamps and marshes on its outskirts like the Central Wetlands Unit were restored. As an added layer of defense, homes and community centers could be floodproofed and elevated to render them more resilient to water damage.

Rising to the challenge: A newly-constructed home, elevated ten feet above ground level, by the Make It Right Foundation in the Lower Ninth Ward (Source: Center for Hazards Assessment, Response and Technology (CHART), University of New Orleans)

Many structures already comply with base flood elevation mandates from FEMA, but consultations must be made with relevant climatologists, hydrologists, and construction specialists to anticipate potential requirements for flood protection against what are now considered 200-year or 300-year storms. In this way, New Orleans and its neighbors could proactively prep themselves against powerful hurricanes of the future.

Engineers like Robert Bea and scientists like Ivor van Heerden agree that a metal-and-mortar solution to flood defense, even one with a $15 billion price tag, won’t be enough to guard the city against future storms. It would be even more tragic if an expensive but inadequate system contributed to a false sense of security in New Orleans, setting up the city for a disastrous sequel to Katrina’s devastation. That is why efforts to increase the resilience of the city’s neighborhoods and initiatives to restore “horizontal levees” like the area’s wetlands should serve as necessary complements to the peripheral floodwalls nearing completion around New Orleans. This multi-faceted approach will guarantee that the Big Easy remains buffered from flood damage well into its next century.