From an email I recieved today:
Several other factors warrant careful consideration, though based on preliminary analysis these did not contribute as significantly to the breach at the 17th Street Canal. These factors include selection of soil strengths for design in a location with a high degree of variability: selection of factor of safety (1.3 at 17th Street), which should be reexamined in all situations where populations are at risk; and selection of the length of the sheetpile used. Significant additional analysis will be needed to determine the role of these factors, if any, in the performance of the levee breaches.
It is highly unusual for preliminary results from a technical assessment to be released at this stage, when there remain as many questions as answers. ASCE commends the U.S. Army Corps of Engineers for recognizing the importance of these findings as a tool to guide assessment and reconstruction of the New Orleans hurricane protection system. Engineers now have a reliable method to assess the safety of floodwalls in New Orleans.
It is important to remember that this is a technical assessment that allows us to understand the facts as they relate to the design and actual performance of the hurricane protection system and its components. This assessment does not answer questions as to the understanding of these factors at the time of the design, the procedures in place for establishing design criteria or conducting review and oversight. Those questions can and must be addressed, both for the benefit of those who lost their loved ones and their livelihoods as the tragic consequence of the flood, and to ensure that the potential for such events in the future can be prevented.
I urge each of you to familiarize yourself with the IPET report and its findings and to be prepared to engage in discussions with your colleagues, your clients, your family and your neighbors about the engineering process, our evolving understanding of the performance of the hurricane protection system and the implications for our society.
As engineers, we must carry the memory of the catastrophic flooding of New Orleans with us each day as a sobering reminder of the responsibility that rests in our hands.
Phone: 703.295.6300 Fax: 703.295.6415
March 10, 2006
A Message from ASCE President Dennis Martenson:
Today, the team conducting the Federal performance assessment of the New Orleans hurricane protection system released the second of its four scheduled reports. This report was issued during a meeting in Vicksburg, Mississippi, between the Interagency Performance Assessment Team, led by the U.S. Army Corps of Engineers, and the ASCE External Review Panel.
Based on the assessments conducted to date, we know that there was no single cause for the catastrophic flooding of New Orleans and the tragic loss of life and property. Some parts of the system performed as designed, while others were simply overwhelmed by the force of the storm. Some were compromised by uncoordinated interaction by multiple jurisdictions, including levee boards, and long-time under-funding of maintenance and improvements. Several breaches, however, appear to have resulted from soil foundation failures and failed before their design capacity.
The more than 700-page report released today includes a preliminary finding on the cause of the breach at the 17th Street Canal levee. This levee is among those that breached before reaching its design capacity. It appears that a combination of factors are responsible for the failure, in particular:
The more than 700-page report released today includes a preliminary finding on the cause of the breach at the 17th Street Canal levee. This levee is among those that breached before reaching its design capacity. It appears that a combination of factors are responsible for the failure, in particular:
- The design appears to have not accounted for soil strengths that were significantly weaker at the toe of the levee (near the adjoining backyards) than under the centerline of the levee embankment.
- The design did not anticipate that the storm surge could cause the floodwall to deflect, opening a gap in the foundation at the base of the floodwall. This gap allowed significantly greater force to push against the floodwall, extending the full length of the sheetpile foundation, and shortened the sliding surface.
Several other factors warrant careful consideration, though based on preliminary analysis these did not contribute as significantly to the breach at the 17th Street Canal. These factors include selection of soil strengths for design in a location with a high degree of variability: selection of factor of safety (1.3 at 17th Street), which should be reexamined in all situations where populations are at risk; and selection of the length of the sheetpile used. Significant additional analysis will be needed to determine the role of these factors, if any, in the performance of the levee breaches.
It is highly unusual for preliminary results from a technical assessment to be released at this stage, when there remain as many questions as answers. ASCE commends the U.S. Army Corps of Engineers for recognizing the importance of these findings as a tool to guide assessment and reconstruction of the New Orleans hurricane protection system. Engineers now have a reliable method to assess the safety of floodwalls in New Orleans.
It is important to remember that this is a technical assessment that allows us to understand the facts as they relate to the design and actual performance of the hurricane protection system and its components. This assessment does not answer questions as to the understanding of these factors at the time of the design, the procedures in place for establishing design criteria or conducting review and oversight. Those questions can and must be addressed, both for the benefit of those who lost their loved ones and their livelihoods as the tragic consequence of the flood, and to ensure that the potential for such events in the future can be prevented.
I urge each of you to familiarize yourself with the IPET report and its findings and to be prepared to engage in discussions with your colleagues, your clients, your family and your neighbors about the engineering process, our evolving understanding of the performance of the hurricane protection system and the implications for our society.
As engineers, we must carry the memory of the catastrophic flooding of New Orleans with us each day as a sobering reminder of the responsibility that rests in our hands.
American Society of Civil Engineers
1801 Alexander Bell Drive Reston, VA 20191Phone: 703.295.6300 Fax: 703.295.6415
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