Northwest New Orleans after Hurricane Katrina.

From RAWorkbook

The city of New Orleans is situated on the Mississippi River in the southeastern United States. The city has developed around the river, and for over two centuries has been a major commercial seaport- utilizing the river as a transportation corridor that links the Midwestern US with the rest of the world. The economic and social development of the city has been based on controlling the river.

Because of flood control and water management in the Mississippi River basin, the sediment supply that once replenished the soils of the delta is moved out to the Gulf of Mexico. The lack of sediment input, combined with soil subsidence, has led to many parts of the city now being below sea level.

Floods can occur from high rainfall over the drainage basin or from storm surges associated with tropical cyclones. At such a precariously low elevation, the city is protected from flooding by a system of levees and canals. This system of defense was built in a piecemeal fashion over time as successive governments invested in infrastructure to control floods. Structures and operations were added in response to flood events that revealed the inadequacy of the system to control natural variability. According to the federal agency that builds and manages the flood-control system, the systems resilience was overwhelmed in 2005 by Hurricane Katrina.

Hurricane Katrina moved inland from the Gulf of Mexico in August 2005 and passed over the city. The accompanying storm surge raised water levels in the surrounding open waters. A number of levees failed because the hydraulic pressure from the high water caused part of the substrate in the levees to slip, resulting in levee failure. Nearly 80% of the city was inundated, with some areas lying 4 meters under water for weeks following the storm.

Fifty levee breaches were recorded, and much of the levee system needs to be rebuilt. Losses are estimated at greater than 20 billion US$. More than 1,200 lives were lost. The population of the city decreased by one-third, as many moved away and didn’t return. Economic impacts persist, as oil and gas production facilities were shut down or damaged. The federal government, which takes a lead role in disaster relief, was seen as slow to react and incompetent. The myth of flood protection by the federal agency was shattered. Wide spread looting and anarchy occurred after the storm, as law enforcement was non-existent and informal networks were unable to maintain order.

Hurricanes and floods are two types of ecological disturbances. Both are disturbances that originate from processes that occur at larger scales. Hurricanes are self-organized systems that disperse heat from the tropics to the temperate regions of the globe, and as such can be described at spatial scales of a hemisphere. Yet, at the scale of cities, these cyclonic storms cause massive disruption through intense winds, surges, and rainfall. Hurricanes such as Katrina have hit coastal Louisiana before, and will do so in the future. As such, accumulated observations can be used to develop statistical estimates of how often a type of storm will hit a given area. Four major hurricanes have affected New Orleans during the 20th century.