Revised corps list ranks metro new orleans levees as ‘not screened’ what that means in each parish nola.com data recovery boston

The east bank levee system includes levees in St. Charles Parish overseen by the Pontchartrain Levee District, and levees in New Orleans and east Jefferson and St. Bernard parishes that are overseen by the Southeast Louisiana Flood Protection Authority-East. For that system, the Corps new web site points out that the risk characterization summary for both hurricane and river levees remains in progress.

The web site also provides basic statistics about the number of people and structures at risk, and their value, indicating those levees surround more than 677,000 people, as well as more than 238,000 structures with a value totaling $129 billion.

"The overall levee system is composed of twelve (12) separate segments composed of Hurricane and Riverine levees. Although these 12 segments make up one overall system with a single protected area, the individual segments have different required design heights (either from hurricane storm surge or a riverine event), have experienced different maximum historical flood events, and have different performance issues to be evaluated."

It also indicates that evacuation procedures for the hurricane levees and the Mississippi River levees are different, because an approaching hurricane offers days to evacuate, "while a potential inundation scenario for a riverine event may only provide hours."

"The HSDRRS is one of, if not the best, hurricane and storm surge defense systems in the world and continues to become more robust with the armoring program currently underway," said Ricky Boyett, spokesman for the corps’ New Orleans District office, using the initials for the Hurricane and Storm Damage Risk Reduction System — the region’s upgraded levees and floodwalls built after Hurricane Katrina.

The "armoring" is a fabric mat material that will be applied in long sheets to the interior of earthen hurricane levees on the east and west banks, anchored at the crown and base of the levee and with grass growing through it to provide an additional anchoring mechanism. The combination of fabric mat and properly grown grass will reduce the chance of the earthen levees eroding when overtopped by storm surge, something that occurred in numerous locations during Hurricane Katrina in 2005.

The levee system has been designed to an elevation aimed at blocking surges caused by a hurricane that has a 1 percent chance of occurring in any year, the so-called 100-year storm. However, the LSAC risk ranking also takes into account the potential for loss of life and economic damage caused by larger storms, those with 200-year or 500-year or greater surge levels.

The corps agreed to delay adding the fabric mat to many levee segments at the request of state and regional levee authorities because the levees already have been in place long enough to result in subsidence. The state and local agencies wanted to avoid the cost of tearing out the armoring and having to replace it, at a cost of millions of dollars.

"Our confidence in the system’s ability to perform as designed in defending against a 1-percent storm surge event is taken into account when establishing the Levee Safety Action Classification," Boyett said. "However, we must also consider the inherent risks for this region and the potential consequences associated with these risks.

"We know that eventually we will experience a storm greater than the system is designed to defend against," he said. "Additionally, we must take a conservative view of the potential consequences associated with a storm given the dense population and infrastructure that is served by this system."

Boyett said he could not specifically say if improvements since 2011 will change the classification. He said a key lesson learned from Katrina was the importance for both residents and public officials to understand that they are still facing "residual risk," meaning the risk that a catastrophic event could exceed the level of protection provided by the hurricane and river levee systems.

"However, HSDRRS was one of the first systems to undergo a risk assessment. Because of its importance, we revisited our assessment to ensure the classification was done in an approach consistent with the other national levee risk assessments," Boyett said.

"We anticipate that whatever the corps‘ official classification is for New Orleans, it will take into account that we have the best flood defense system in the United States (as per the corps) and also account for our geographic location on the Gulf plus the hundreds of thousands of people and billions of dollars of property we protect," said Derek Boese, chief administrative officer for the east bank authority. He said the authority "is confident that the system we consistently operate and maintain to a very high standard on a daily basis will perform as designed during a storm."