The storm surges produced by Hurricane Katrina on August 29, 2005, breached the levees protecting New Orleans in numerous places, flooding approximately 75 percent of the metropolitan area. Most of the levee failures were caused by overtopping, as the storm surge rose over the top of a levee and scoured out the base of the landward embankment or floodwall. Three major and costly breaches appear to have been caused by failure of the soils underlying the levees or failure of the earthen levee embankments themselves; in several places, levee foundations failed when water levels were below the tops of the levees. Transitions between levees of differing heights or materials proved to be weak points in the flood-protection system; a significant number of levee washouts occurred, for example, where the weaker of two adjacent materials was at a lower elevation.
In the aftermath of the flooding of New Orleans, the U.S. Army Corps of Engineers requested an external review of the levees' performance by teams of engineers and scientists sponsored by the National Science Foundation (NSF) and the American Society of Civil Engineers (ASCE). Various teams worked in New Orleans from September 28 through October 15, 2005a time chosen by balancing the need to gather ephemeral data against the need to avoid interfering with emergency operations. On numerous occasions, team units arrived and investigated sites only hours before vital information was buried by ongoing emergency repairs.
Two U.S. Geological Survey (USGS) scientists, research civil engineer Robert Kayen and Mendenhall Postdoctoral Fellow Brian Collins, both of the USGS Western Coastal and Marine Geology Team (WCMG), were asked to join as investigators on the NSF-sponsored team. They were in New Orleans from October 9 to 14, using a ground-based terrestrial laser-mapping system to conduct detailed surveys of the levee breaches. Gathering information about the magnitude and geometry of structural and soil deformation is paramount for analyzing how and why the levees failed.
Kayen and Collins arrived on the only United Airlines flight into New Orleans on October 9 and stayed in what appeared to be the only available hotel near the Mississippi River. The roof of their rental vehicle, a Nissan Pathfinder, served as a platform for the terrestrial laser-mapping instrument. They used a global-positioning-system (GPS) unit to navigate around the city, which was overwhelmed with flooded sections, downed trees and powerlines, and windblown and waterborne debris. Each day they drove on whatever surface allowed them get to a levee breakboth sides of the road, sidewalks, lawns. In some areas, especially in the Lower Ninth Ward, the debris was so thick that they were unable to drive to their destination and had to carry in gear on foot.
"Both Brian and I are familiar with post-event [earthquake] damage, but the devastation of Hurricane Katrina was so unusually severe and affected so much of urban New Orleans that we were taken aback by the magnitude of this natural and manmade catastrophe, and the absence of all services and people," said Kayen. "By analogy, imagine the impact of 50 to 80 percent of the San Francisco Bay area rendered uninhabitable by some event. That was the state of New Orleans as we worked."
The two researchers used laser-mapping techniques they developed in their WCMG studies of coastal-seacliff erosion and earthquake ground deformation to capture the surface evidence of levee deformation and distress at 10 sites in the greater New Orleans area. To do this, they brought to the field area a new terrestrial tripod-mounted laser-mapping tool to perform lidar (light detection and ranging) data collection. The terrestrial lidar method consists of sending and collecting laser pulses from the surface of objects to build a data set of three-dimensional coordinates. The USGS laser-scanning system can measure the location of as many as 8,000 surface points in 1 second. Thus, within a few minutes, an entire surface can be imaged efficiently, producing a data set that contains several million position points. The data sets from collected scans are typically transformed into virtual three-dimensional surfaces so that cross sections can be generated and volumetric calculations can be performed.
The objective of the laser-scanning effort in New Orleans was to obtain precise measurements of the ground surface in order to map
Overtopping was most severe on the east side of the flood-protection system, as the waters of Lake Borgne were driven west toward New Orleans, and also farther to the south, along the lower reaches of the Mississippi River. Significant overtopping and erosion caused numerous breaches in these areas. The magnitude of overtopping was less severe along the Inner Harbor Navigation Canal (IHNC, also called the "Industrial Canal") and along the western part of the Mississippi River Gulf Outlet (MRGO) channel, but overtopping in these areas nevertheless caused erosion and levee failures. Although field observations suggest that little or no overtopping occurred along most of the levees fronting Lake Pontchartrain, evidence of minor overtopping or wave splashover was observed in several places. A breach in the levee system occurred at the northwest corner of the New Orleans East protected area, near Lakefront Airport, at a complex transition between levee segments of varying heights and materials. It appears that many of the levees breached by overtopping might have performed better if conceptually simple details, such as scour protection on the land side, had been added during or after original design and construction.
Farther west, in the New Orleans East Bank Canal District, three levee failures occurred along the banks of the 17th Street and London Avenue Canals. Kayen and Collins observed evidence indicating that the failures occurred when water levels were below the tops of the concrete floodwalls lining the canals. These three levee failures were likely caused by failure of the foundation soils underlying the levees. Signs of an incipient failure were observed at a fourth distressed levee/floodwall segment on the London Avenue Canal, where lateral displacements, sinkholes, and sand boils all indicate that water was flowing through a weakening embankment.
The levee-investigation teams pooled their findings to produce a joint NSF-ASCE report that was presented to Congress in early November, along with Congressional testimony from the NSF and ASCE team leaders. The 17 MB PDF file of the report can be downloaded from URL http://hsgac.senate.gov/_files/Katrina/Preliminary_Report.pdf.
Kayen and Collins were among the experts featured in a 10-minute television news feature about the NSF-ASCE investigation of the New Orleans levee failures after Hurricane Katrina (see Sound Waves article, USGS Scientists Featured in National News Segment on Investigating Broken Levees in New Orleans). The segment aired on October 20, 2005, on the "News Hour with Jim Lehrer," a national Public Broadcasting Service (PBS) news program. A transcript, photographs, and links to video and audio files of the television feature, called "Investigating Broken Levees," are available online at http://www.pbs.org/newshour/bb/science/july-dec05/levees_10-20.html.
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