Hurricane Preparedness For New Orleans - The Impact of Louisiana's Sinking Coast

The Impact of Louisiana's Sinking Coast

When the Corps of Engineers started systematically leveeing the river in the 19th century, it cut off the region's main source of silt, the raw material of delta-building in the Mississippi Delta and the Wetlands of Louisiana. The weight of large buildings and infrastructure and the leaching of water, oil and gas from beneath the surface across the region have also contributed to the problem. Following the great floods of 1927, the Mississippi River was surrounded by a series of levees meant to protect the city from such floods. In 1965, New Orleans was hit by Hurricane Betsy, which caused tremendous flooding in the New Orleans area. The federal government began a levee-building program to protect New Orleans from a Category 3 hurricane (the same strength as Betsy). These series of levees were completed in recent years before Hurricane Katrina.

However, an unintended consequence of the levees was that natural silt deposits from the Mississippi River were unable to replenish the delta, causing the coastal wetlands of Louisiana to wash away and the city of New Orleans to sink even deeper. The Mississippi River delta is subsiding faster than any other place in the nation. While the land is sinking, sea level has been rising. In the past 100 years, land subsidence and sea-level rise have recently added three feet to all storm surges. That extra height puts affected areas under deeper water; it also means flooding from weaker storms and from the outer edges of powerful storms spreads over wider areas. The marshes that ring New Orleans, as well as the land depression of the city itself, originally above sea level, have sunk the quickest.

The problem with the wetlands was further worsened by salt water intrusion caused by the canals dug by the oil companies and private individuals in this marshland. This erosion of the wetlands not only caused Louisiana to lose 24 square miles (62 km2) per year of land annually and 1,900 square miles (4,900 km2) of land since the 1930s, but it also destroyed Louisiana’s first line of defense against hurricanes.

Hurricanes draw their strength from the sea, so they quickly weaken and begin to dissipate when they make landfall. Hurricanes moving over fragmenting marshes toward the New Orleans area can retain more strength, and their winds and large waves pack more speed and destructive power. Scientists working for the Louisiana Department of Natural Resources measured some of these effects during Hurricane Andrew in 1992. Andrew's surge height dropped from 9.3 feet (2.8 m) at Cocodrie to 3.3 feet (1.0 m) at the Houma Navigation Canal 23 miles (37 km) to the north. For every mile of the marsh-and-water landscape it traversed, it lost 3.1 inches of height, sparing some homes farther north from more flooding. Currently Louisiana has 30% of the total coastal marsh and accounts for 90% of the coastal marsh loss in the lower 48 states. The engineering of the river has basically brought the Gulf of Mexico much closer New Orleans, making it more vulnerable to hurricanes.

The combination of sinking land and rising seas has place the Mississippi River delta as much as 3 feet (0.91 m) lower relative to sea level than it was a century ago, and the process continues. That means hurricane floods driven inland from the Gulf have risen by corresponding amounts. Storms that once would not have had much impact can now be devastating events, and flooding now penetrates into places where it has rarely occurred before. The problem also is slowly eroding levee protection, cutting off evacuation routes sooner and putting dozens of communities and valuable infrastructure at risk of being wiped out by the flooding.

State and federal officials have recently pushed a $14 billion plan to rebuild wetlands over the next 30 years, to be funded by oil and gas royalties, called Coast 2050. Louisiana will receive US$540 million under the energy bill enacted in August 2005. More money for this program is likely to come with aid from Hurricane Katrina. The original actual cost needed for the project, however, was $14 billion.

Wetlands have the capacity to absorb storm surges at the rate of 1-foot (0.30 m) per 2.7 miles (4.3 km). However, due to the systematic, long-term nature of wetland loss, and because wetland remediation can take decades, it is not possible to pinpoint blame for wetland loss on any specific Congress, legislature, President, or governor.