On the Job: Laser Scanning Quickly Assesses Soil Erosion in New Orleans

14 Dec, 2005 Cadalyst

USGS uses I-SiTE Studio to map levee damage following Hurricane Katrina

A team of engineers and researchers sponsored by the U.S. National Science Foundation has used I-SiTE Studio software to map structural and geotechnical damage caused by Hurricane Katrina to levees and waterways in New Orleans, Louisiana. The team carried out laser scanning on ten sites where levees failed, recording more than 180 scans in five days, then generated highly detailed 3D models to assist with analysis and reconstruction.

I-SiTE Studio is a software package for laser scan data processing that helps surveyors, engineers and investigators apply laser-scanning technology to a variety of industries, from mining to forensic science.

The laser scanning effort in New Orleans sought to obtain precise measurements of the ground surface to map soil displacements at each levee site, the nonuniformity of levee height freeboard, depth of erosion where scour occurred and distress in structures at incipient failure. Researchers from the United States Geological Survey conducted the laser mapping.

Because laser scanning is so fast, users can survey an entire surface, such as a structure or levee, in minutes, producing a file containing several million points. The points from collected scans are typically transformed into 3D surfaces so users can generate cross-sections and perform volumetric calculations between consecutively scanned surfaces.

At each levee site, researchers imaged the topographical surroundings on 13 or more individual scans. They used I-SiTE Studio software to collect the scan point-cloud data and postprocess multiple scans into georeferenced solid surfaces. Then, a least squares 'best-fit' match is made between scans, augmented by precise survey measurements made with a total station or differential GPS. Unwanted data was then filtered to remove vegetation data points to leave the ground surface.

Modeling laser scan data in I-SiTE Studio provides a clear visualization of damage caused when water broke through damaged levees in New Orleans.

The filtered data serves as the working DTM (digital terrain model) to render a solid surface of the ground. Different surface-modeling methods can be used to fuse and render a surface from multiple scans. The surface model is a highly accurate virtual representation of the scene that can be used for documentation and change detection of volumes, areas and distances. Volumetric calculations are a great advance over traditional geomorphic cross-section analyses.

This I-SiTE Studio model displays a section of broken levee.

Laser mapping allows for highly accurate computation of rotation, length, area and volume, I-SiTE reports. Rotational displacement was common at areas of levee wall distress in New Orleans. Displacement along the canal breaches was measured by identifying the blocks of ground formerly within the intact levee that slid toward the land side of the levee. The image below shows part of the 17th Street dataset, with the bridge crossing the canal upper left. A dense cluster of points is visible at the levee breach in the center, as are the houses in the affected area. Near the levee breach, the remaining wall can be seen in alignment with the crest of the replacement structure. The breach repair of 466 feet can be calculated directly from the scan data.

I-SiTE displays damage to homes caused by the levee failure.

Accurate measurements can be calculated from the I-SiTE Studio model, allowing repair work to begin.

A cross-section through this area, below, shows a section of the intact wall overlaid on the failed section of the levee. The geometry of the repair embankment is clearly visible. The magnitude of displacement of several earth blocks that moved away from the levee break during failure is evident. The scan data shows that blocks translated approximately 46 feet, as measured from the existing alignment of the cyclone fence line to its new position within the displaced blocks. It can also be seen that the canal width has been reduced by about 20 feet. The second image, below, allows a direct comparison of the scour depth to sheet pile embedment.

Cross-sections created in I-SiTE Studio allow engineers and researchers to assess the levee damage and determine which remedial and preventative measures are necessary.

Some information based on Preliminary Report on the Performance of the New Orleans Levee Systems in Hurricane Katrina on August 29, 2005 (PDF) Report No. UCB/CITRIS - 05/01, November 2, 2005.