New Orleans & Hurricanes

Prof. Stephen A. Nelson

Why New Orleans is Vulnerable to Hurricanes
Geologic and Historical Factors
Fall 2012

Myths Involving Hurricane Katrina

1. New Orleans survived Hurricane Katrina on August 29, 2005 - then the levees broke the next day to flood the city.
   
   False: All levees breached before 10:00 AM on the morning of August 29, 2005!
   
   
2. Mississippi River levees (or Lakefront levees) breached.
   
   False: No Mississippi River levees in New Orleans breached, some Lakefront levees did overtop, but none breached.
   
   
3. Levees that failed were built by "corrupt" local levee boards.
   
   False: All of the levees that failed in New Orleans were built under the supervision of the U.S. Army Corps of Engineers.
   
   
4. New Orleans is a city below sea level.
   
   Partially False: Roughly half the city and surrounding metropolis is above sea-level (Figure 1)
   

Figure 1

Geologic History of the New Orleans Region

The land on which New Orleans is built has origins that began about 5,000 years ago. As sea level was rising after the last glacial maximum, a series of barrier islands was built outward from the coast of Mississippi across what is now the southeastern edge of Lake Pontchartrain (Figure 2). These islands, called the Pine Islands were composed mainly of sand whose source was the Pearl River along the Mississippi – Louisiana border. At the time the Mississippi River was building its delta out toward the southeast of New Orleans building the Maringouin and Teche lobes of the delta complex (Figure 3). Beginning about 4,300 years ago the Mississippi River began to build the St. Bernard Delta lobe out toward the east. This lobe eventually intersected the Pine Island Barrier Island complex, eventually burying the sands, cutting off the drainage from the north to form Lake Pontchartrain, and building the land on which New Orleans would later be built.

Human History of New Orleans

New Orleans was founded in the year 1718 by Jean Baptiste La Moyne, Sieur de Bienville where the first settlement was at the location of the French Quarter (about 17 feet above sea-level). The city grew along the high ground, which included the natural levee of the Mississippi River and the Metairie and Gentilly ridges (about 3-4 feet above sea-level). Because of its crescent shape along the meander bend of the River, it gained its nickname, The Crescent City. Drainage was a continual problem because of the topography of the region and although Bayou St. John offered a natural drainage from the central part of the crescent to Lake Pontchartrain, it was not enough to drain the often heavy rainfall that occurred. Thus, a drainage system consisting of canals that drained into three outfall canals running from South to North into the Lake was designed and built. Because of the intervening high ground of the Metairie/Gentilly ridges, it was necessary to raise the water at the ridges to get it into the outfall canals.

Figure 8

Sept. 1, 2008 Hurricane Gustav - New Orleans Evacuated.  Did not flood New Orleans, but did fill the Industrial Canal dangerously close to the top of the floodwalls.

August 29, 2012 Hurricane Isaac - A slow moving Category 1 hurricane caused power outages and wind damage in New Orleans.  The new hurricane protection system did will, but the conditions were not severe enough to give it a substantial test.  Areas outside the hurricane protection system suffered severe flooding. In Particular the community of Braithwaite, on the east bank of Plaquemine Parish flooded when storm surge overtopped the non-federal levee and was trapped between that levee and the Mississippi River levee.   LaPlace at the southwestern end of Lake Pontchartrain, and Madisonville, Mandeville, and Slidell on the northern shore of Lake Pontchartrain flooded from storm surge from the Lake.

• Hurricane Betsy in 1965 flooded portions of eastern New Orleans, the Upper Ninth Ward, Lower Ninth Ward, and Gentilly.
  
  
• In response, Congress authorized the Lake Pontchartrain and Vicinity, Louisiana Hurricane Protection Project - authorized by Congress in the Flood Control Act of 1965.
  

• Expected to take 13 years to complete and cost $85 million.

• Designed to protect the New Orleans region from a fast moving Category 3 Storm that would be expected to strike once every 200 to 300 years. Such a storm was designated as the "Standard Project Hurricane".
  
  
• Costs were shared - 70% Federal, 30% Local (taxes collected by Levee Districts)
  
  
• By 1982, the estimated cost had grown to $924 million

• In 2005 (before Katrina) the expected completion date was projected to be 2017.

Two plans were proposed by the Army Corps of Engineers:

The Barrier Plan

• In the 1970s the Corps proposed building barriers and closeable navigation gates across the 2 inlets into Lake Pontchartrain to prevent hurricane storm surge from Lake Borgne and the Gulf of Mexico from entering the Lake.
  
  
• An organization called Save Our Wetlands, filed a lawsuit objecting to the 4 page Environmental Impact Statement (EIS) presented by the Corps, and Judge Charles Schwartz in 1977 ruled that the EIS was inadequate, ruling that the Corps must return with a better EIS (Schwartz, 1977).
  
  
• In 1980, the Corps, rather than issue an acceptable EIS, decided against the construction of the barriers and gates.

High Level Plan

• In the 1980s the Corps proposed building levees and floodwalls to a height necessary to keep storm surge from the Lake Pontchartrain and Lake Borgne from entering the city.
  
  
• For the three drainage canals, the Corps propsed two plans.  One involved putting moveable gates at the Lake ends of the drainage canals (London Avenue, Orleans, and 17th St.).  Alternatively they proposed to raise the height of the levees on the canals.  The latter was called the Parallel Plan.  The Corps stated that it believed either plan would be equally effective.
  
  
  • The Sewerage & Water Board and Orleans Levee Board objected to putting gates at the end of the canals because the Corps refused to consider inclusion of pumping stations at the gates that would pump rainwater out of the city into the Lake when the gates were closed during a hurricane.
    
    
  • For the 17th Street Canal, the Corps had no stated prefence because the cost of the gates was roughly the same as the Parallel Plan.  The Corps approved raising the height of the levees along the canal because local officials preferred this option.
    
    
  • For the London Avenue and Orleans Canals, the Corps preferred the gates plan because it was significantly cheaper than the Parallel Plan (three times less for London Avenue and five times less for Orleans). 
    
    
  • The Sewerage and Water Board and Levee Board successfully lobbied Congress for more money for the Parallel Plan.  The Corps was ordered to raise the levees and floodwalls on the canals and not build the gates, through passage of the Water Resources Development Act of 1990 (passed in 1992).
    
    
• A levee is a mound of dirt piled up to prevent the inflow of water. Raising the height of a levee also requires widening the levee because dirt can’t hold up to steep slopes. In an urban environment like New Orleans, widening a levee would require buyouts of property along the levee. This would have been quite expensive.
  
  
• Instead, concrete floodwalls were proposed and built since a floodwall can be quite steep and still provide a higher level of protection.
  
  
• Two types of floodwalls were used in the New Orleans hurricane protection system (Figure 9)
  
  
  • I-walls were the most commonly used type of floodwall in the New Orleans flood protection system.
    
    
  • T-walls offer more resistance to failure, but are more expensive.

Figure 9

• Failure of I-wall type floodwalls was the most common problem during Hurricane Katrina, and these failures resulted in severe flooding in the main basin of the city.
  
  
• In 1985, the Army Corps of Engineers research branch conducted tests on sheet pile flood walls with the same design that they were using for New Orleans’ hurricane protection system, called the E-99 study. Later analyses of this test found that they would likely fail as a result of deflection of the sheet pile (Figure 10).
  
  

Figure 10

• The results of the E-99 study were misinterpreted and the Corps determined that shorter sheet pile penetration depths could be used, resulting in a cost savings for constructing the canal floodwalls of approximately $100 million. 
  
  
• After Katrina it was determined that sheet piles on the 17th St. and London Avenue canals were too short, a major factor responsible for the floodwall failures on these canals.
  
  
• Poorly constructed levees in St. Bernard Parish resulted in erosion of the overtopped levees.
  
  
• Levee overtopping and levee breaching are two different things.  Levee breaching is a more serious matter (see figure 11)

Figure 11

The commission appointed to investigate what when wrong during Katrina - The Interagency Performance Evaluation Task Force (IPET) concluded the following in their final report:

“The System did not perform as a system: the hurricane protection in New Orleans and Southeast Louisiana was a system in name only ... The system’s performance was compromised by the incompleteness of the system, the inconsistency in levels of protection, and the lack of redundancy. Incomplete sections of the system resulted in sections with lower protective elevations or transitions between types and levels of protection that were weak spots.”

Appendix

Time Line of Important Events Leading up to Hurricane Katrina*

Deposition of St. Bernard Delta Lobe & formation of Metairie/Gentilly/Esplanade Distributary channels

Completion of gates,

Sept. 1, 2008