New Orleans Levee Improvement after Hurricane Katrina
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The levees and floodwalls protecting New Orleans from hurricane’s and floods were designed to withstand a category 3 hurricane. When making landfall on August 29, 2005 Hurricane Katrina was designated a category 4 hurricane; later it was downgraded to a severe category 3. Hurricane Katrina, the costliest natural disaster in US history, was also a warning shot. Located in one of the lowest spots in the US, the Big Easy is already as much as 17 feet below sea level in places, and it continues to sink, by up to an inch a year. Upstream dams and levees built to tame Mississippi River floods and ease shipping have starved the delta downstream of sediments and nutrients, causing wetlands that once buffered the city against storm-driven seas to sink beneath the waves. Louisiana has lost 1,900 square miles of coastal lands since the 1930s; Katrina and Hurricane Rita together took out 217 square miles, putting the city that much closer to the open Gulf.
Most ominous of all, global warming is raising the Gulf faster than at any time since the last ice age thawed. Sea level could rise several feet over the next century. Even before then, hurricanes may draw ever more energy from warming seas and grow stronger and more frequent. The levees and floodwalls developed by the USACE (U.S. Army Corps of Engineers) in the 1970s and 1980s reduced the risk of flood damage and provided economic development opportunities. At the time the USACE designed the system, its analysts believed that it protected New Orleans against 100-year flood (that is, a flood of such magnitude that would occur, on average, only once in 100 years).
However, due to many optimistic assumptions, the analysts overestimated the level of protection and underestimated the consequences of such a major flood. Furthermore, the levels of protection decreased overtime due to natural and man-made changes. Natural changes included continuing subsidence, lack of sedimentation and declining vegetative growth. Land use changes such as road building and increased residential densities induced hydrologic changes that reduced the level of protection provided by levees and floodwalls. And, while these levees and floodwalls required regular and extensive maintenance, their record of maintenance quality was spotty. The long odds led Robert Giegengack, a geologist at the University of Pennsylvania, to tell policy makers a few months after the storm that the wealthiest, most technologically advanced nation on the globe was helpless to prevent another Katrina. He recommended selling the French Quarter to Disney, moving the port 150 miles upstream and abandoning one of the most historic and culturally significant cities in the nation. Others have suggested rebuilding it as a smaller, safer enclave on higher ground. But history, politics, and love of home are powerful forces in the old river town.
Instead of rebuilding smarter or surrendering, New Orleans is doing what it has always done after such disasters: bumping up the levees just a little higher, rebuilding the same flood-prone houses back in the same low spots, and praying that hurricanes hit elsewhere. More than a third of the city’s pre-Katrina population has yet to return. In modeling future floods and their expected consequences in New Orleans, many input quantities can only be estimated, and, as such, they have an inherent degree of uncertainty. A model that explicitly specifies the range of uncertainty in its inputs can provide more realistic and informative estimates than deterministic assessments. Influence diagrams are a useful tool in mapping out the decisions, events, and variables that influence the potential consequences of decisions and events. Below are two fault trees, a fault tree analysis (FTA) is used to follow the trail of a problem or risk providing a visual tool that enhances team brainstorming to resolve the problem or risk.
Figure 1.1 depicts the flow of a levee failure in the where the gate 2 follows the water overtopping the floodwalls and excessive rainfall. The gate 3 follows the failure to premature failure of the levee due to damage of older hurricanes or soil movement. Gate 1: Two different events that can happen in the failure of a levee are, the water level exceeding the height of the levee and a premature failure due to earlier damage. Gate 2: The water level can exceed the height of the levee because the design did not take in account the rising sea level because of global warming, as well as the changes of weather and rain fall. Gate 3: Premature failure are natural and man-made changes that include continuing subsidence, lack of sedimentation and declining vegetative growth have weakened the levees. Land use changes such as road building and increased residential densities induced hydrologic changes that reduced the level of protection provided by levees and floodwalls