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Three scientists from the U.S. Geological Survey (USGS) visited New Orleans in March to collect highly accurate global-positioning-system (GPS) data from sites where levees failed after Hurricane Katrina made landfall on August 29, 2005. The data are being used to georeferenceassign real-world geographic coordinates tohigh-resolution laser scans of the levee breaks collected by USGS scientists last fall. The georeferenced laser scans will have many uses, including the accurate determination of floodwall heights in this area where subsiding land has lowered many survey benchmarks and rendered them unreliable as elevation markers.
Brian Collins, Tom Reiss, and Diane Minasian of the USGS Western Coastal and Marine Geology team were in New Orleans from March 13 to 17, working with a group of researchers supported by the National Science Foundation. They revisited levee breaks at the 17th Street Canal, the London Avenue Canal, the Lakefront Airport, and the Inner Harbor Navigation Canal (IHNC, also known as the Industrial Canal). They also visited an area along the Orleans Avenue Canal at one of New Orleans' many pump stations, where floodwaters entered neighborhoods via a spillway.
These breaks were responsible for massive flooding in the aftermath of Hurricane Katrina, which left hundreds of thousands of people homeless. Collins and USGS research civil engineer Rob Kayen had traveled to the city in October 2005 to conduct high-resolution, ground-based laser scanning of numerous levee breaks (see "USGS Scientists Investigate New Orleans Levees Broken by Hurricane Katrina" in Sound Waves, December 2005/January 2006). Their work was part of a multiagency review of the levees' performance that was reported to the U.S. Senate (download the 17 MB PDF report from URL http://hsgac.senate.gov/_files
During their visit last fall, Kayen and Collins were struck by the desolation of vast areas of New Orleans, and when he returned in March, Collins' first impression was that "not a lot had changed. For the most part, the neighborhoods were still abandoned and the stores still closed. While the levee breaches that we visited were under reconstruction, they were also a tragic reminder of what had happened just 6 months earlier." After spending a few days performing surveys, however, he "realized that New Orleans was trying to rebuild and that signs of this were widespread. For one, Bourbon Street was alive and well, and restaurants in other parts of the city, saved from floodwater (but not from looting), were reopening as well."
The March fieldwork focused on high-precision, high-accuracy surveying of "registration points" to be used for georeferencing the laser-scanning data sets gathered last October. The rapid pace of data collection after the drying out of the city in October precluded collecting survey registration points at that time. In preparation for the March surveying work, the researchers used the laser scans collected in October to virtually "revisit" each levee-breach site, pulling potential registration points from three-dimensional objects visible in the laser scans. Corners of structures, concrete foundation pads, fence posts, and levee floodwalls all made attractive targets. In the field, the team discovered that many points were no longer accessible because of reconstruction and repair efforts in the vicinity of the levee breaches; nevertheless, they succeeded in gathering a sufficient number of points to register and georeference the data sets to standard horizontal and vertical survey datums. The team was able to determine the geographic position of each registration point with centimeter-level accuracy.
After completion of the registration process currently in progress, the team's georeferenced data sets will be made available to other researchers and to engineers and construction personnel involved in rebuilding efforts through the U.S. Army Corps of Engineers. The data can be used as a layer depicting postfailure conditions in any geographic-information-system (GIS) study of the levees. A common use of the USGS data sets will be in combination with geotechnical drilling data to see how subsurface geology influenced levee failure.
In view of recent concerns regarding actual floodwall elevations in relation to supposedly "fixed" survey-control benchmarks, the USGS data sets will also be useful for measuring floodwall elevations. Throughout southeastern Louisiana, ground subsidence has caused survey benchmarks, typically used as permanent and reliable elevation indicators, to sink by different amounts, some by more than a foot after their installation. Before beginning the March survey, Collins contacted researchers at Louisiana State University (LSU)'s Center for GeoInformaticsheaded by Roy Dokka and J. Anthony Cavellwhich has been working with the National Geodetic Survey to establish a network of high-precision GPS reference stations throughout Louisiana (see URL http://www.c4g.lsu.edu/modules.php?name=LSRC). The LSU researchers helped Collins ensure that the USGS survey was tied into accurate benchmarks that had been resurveyed after Hurricane Katrina, and they assisted the USGS team with logistical support in New Orleans.
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