Electronic Design

Rethinking New Orleans' Defenses With Technology

As EEs, you apply the latest technologies and see fast results. Compare this to civil engineering. Started in 1965, the New Orleans levee project was still under construction when Katrina hit this month. Imagine working on the same electronics project for 40 years.

Granted, the science of civil engineering doesn't move at the pace of electronics. But civil engineering hasn't stood still over the course of four decades either. Even the New Orleans geography has morphed, with lots of new construction abutting the levees and silt-deprived land sinking further below sea level. Some coastal areas have dropped as much as two feet since they were surveyed at the start of the project.

By today's standards, the computational models underlying the levee plan gave a coarse, one-dimensional approximation of hurricane forces. The original projections calculated water build-up against a constant wind velocity and then applied wind-speed data from storms that had hit between 1915 and 1947. None of those storms was stronger than a Category 3.

Today, supercomputer-driven programs such as the AdCirc program at University of North Carolina's Institute of Marine Sciences account for such factors as ocean circulation and underwater geography. Then they create 2D storm models that let researchers look at hurricane simulations based on more than a million geographic nodes.

The U.S. Army Corps of Engineers has worked with the AdCirc computer programs over the last decade to model New Orleans storm surges and assess the need to rebuild the levees to withstand Category 4 and 5 storms, according to a 2003 Civil Engineering magazine report. An alternative plan would create an immense storm wall between Lake Pontchartrain and the gulf. According to the article, in a Category 4 or 5 storm, "close to 400,000 people could be stranded in the city. There are an estimated 100,000 people without easy access to automobiles, and those who can drive may not be able to do so."

Louisiana State University models showed that a Category 4 storm hitting New Orleans would cause Pontchartrain's level to rise by 12 feet and top the embankments and fill streets to a depth of 25 feet or more. LSU's Joseph Suhayda suggested back in the 1980s that the city could build a "community haven" by constructing a 30-foot wall and floodgates through the center of town to protect the center of the city, taking advantage of existing Mississippi River levees to form three sides of a waterproof ring.

While upgrading the levee system would take another three decades or longer, the current levee system could be improved via electronic monitoring. If stress monitoring systems had been in place, the Corps could have focused on reinforcing the weakest portions of the levee before they were breached by the storm surges on Lake Pontchartrain.

Kane GeoTech designs and installs instrumentation systems to detect rate and direction of ground movement and slope failures. I spoke with company founder William Kane about time domain reflectometry (TDR), a technique that puts coaxial cable under the soil, then uses electromagnetic pulses to measure cable deformation, corresponding to movement of the surrounding rock or soil (see the figure). "With TDR, sensor cable could be put in when rebuilding damaged levees, and the cost for a monitoring system would be negligible," says Kane. The system also can use IP-addressable modems for system communications via the Internet.

Kane cites the work his company is doing in California. A Kane installation serves the North County Transit District in San Diego County, where railroad tracks run along coastal cliffs. The system runs cables horizontally, along the length of the tracks. The cables are pulsed every four minutes. Movement in the cables can trigger two levels of alarm. The higher level sends signals to a central paging system that alerts railroad personnel to check out the location.

The system can measure very small movements in the lab. In the field, it picks up movement of about 0.25 in. The movement causes a spike in the trace of the cable signature. The spike will continue to grow until the cable breaks after about six or eight inches of movement.

Kane's company works with instruments from Campbell Scientific. Principal TDR installation components include Campbell's CSI datalogger, the TDR100 Reflectometer, SDMx50-series coaxial multiplexers, interconnecting cabling, and TDR probes. The TDR 100 generates the short-rise-time electromagnetic pulse that's applied to the coaxial cable. The onboard processor interprets the elapsed travel time and pulse reflection amplitude to determine movement.

According to Kane, a New Orleans monitor system also would likely include piezometer instrumentation to measure water pressure near the levee, correlating areas of high water pressure with potential failures in the levee. These sorts of electronic tools should be incorporated during the repair process, bringing New Orleans' protective systems into the 21st century. Careful monitoring and maintenance of the current defenses can add strength in the near term, while longterm rebuilding continues.

TAGS: Defense
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