Editor’s Note: This article was originally published in the September 2003 issue of Hydro Review.
Global positioning systems, familiar to many people for surveying and navigation purposes, also are proving to be a valuable tool for precise real-time measurements of deformation in dams and critical landforms at hydroelectric facilities.
By Barry A. Hillman, David R. Rutledge, Steven Z. Meyerholtz, and Cory S. Baldwin
Monitoring and measurement of the deformation of structures and critical landforms at hydropower projects is a major issue for facility operators and regulatory agencies. The timely identification of deformation associated with natural hazards or large structures can save lives, avert large financial liabilities, and avoid severe environmental damage. Dams and related facilities can be especially difficult to monitor, particularly if they are located in remote areas. At many remote, steep, or landslide-prone sites, conventional monitoring techniques are difficult to deploy and maintain, and in many cases provide only periodic information.
Global positioning system (GPS) technology, coupled with specialized software for processing continuous position data, offers a powerful alternative to conventional monitoring systems. A deformation monitoring system utilizing GPS and developed by Condor Earth Technologies, Inc., of Sonora, California, is now being applied at a number of hydroelectric facilities to monitor deformation in dams, penstocks, and landslides. This system, named 3D Tracker, is demonstrating that GPS technology can be cost-effectively deployed to obtain precise, continuous deformation data in a variety of settings.
|A GPS station can be installed on dams or critical landforms wherever sky visibility is adequate. This typical unit, deployed in Montana, is self-contained and operates on solar power.|
What does GPS technology have to offer?
Global positioning systems have moved rapidly from an esoteric military technology to one that is familiar to most people as a navigation tool for vehicles and recreation. A public misconception persists, however, that high-accuracy GPS is only available to the military. In fact, the most accurate GPS systems are available for a wide variety of non-military applications including long-term monitoring. Not only are systems now capable of sub-centimeter accuracy, but they can also provide this information in real time at costs competitive with other monitoring techniques. The instrumentation can be readily installed as a self-contained unit on dams or other structures, as shown in the figure below.
|Figure 1: This schematic shows a typical GPS instrument installation on a dam.|
GPS instrumentation has many attractive qualities when compared with conventional dam safety deformation monitoring techniques like plumb lines, optical survey equipment, and laser alignment systems. For example, GPS equipment is solid state and requires little maintenance and no calibration. GPS measurements are three-dimensional, thereby providing valuable information on vertical deformation as well as horizontal deformation. Equally important, GPS systems are well suited to being automated. Automation is becoming increasingly important as dam operators strive to control labor costs. Many conventional monitoring systems require frequent recalibration to provide accurate data, and some require manual downloading and time-consuming post-processing of data. The result is that critical data from these systems may go for weeks, months, or even years before they are evaluated. An automated GPS system helps eliminate these problems, and is, in most cases, an excellent complement to the existing instrumentation suite on a dam.
Applying GPS to dam deformation monitoring involves processing a continuous stream of position data obtained in real time from GPS receivers located on the dam. Further integration of the data is possible over a selected time period — typically one to 24 hours — to further reduce noise. The GPS solution errors show distributions that match Gaussian distributions very well, and can be dramatically reduced by averaging over time. For example, vertical accuracy of the 3D Tracker system is typically in the range of 1 to 2 millimeters at one standard deviation for a 24-hour solution. GPS systems provide reliable long-term operation, and can be integrated with other instrumentation, such as tilt sensors or plumb lines, to provide redundancy in measurements and to measure vertical deformation that other systems cannot. It is worth noting that GPS provides absolute accuracy because it utilizes a global reference coordinate system. This characteristic of GPS can be very useful because two different, independent measurements of the same point on a dam will yield the same answer (within the GPS error).
Real-time GPS systems can be integrated with the Internet, an intranet, and local area networks (LAN) to provide information to operators and personnel off-site. A favorite method of presenting the real-time GPS data is through a web page being hosted on a LAN. A LAN-hosted web page provides multiple users access to the GPS data in a friendly, secure format and utilizes tools already present on most computer desktops. These real-time GPS systems also can be programmed to reliably send motion-threshold alarms based upon user-defined thresholds. A dramatic reduction in GPS receiver costs in recent years also means that monitoring is cost-effective when compared with other instrumentation.
Lessons learned from early investigations
Shortly after the GPS satellite constellation became fully operational in the late 1980s, the system’s potential for monitoring of large structures became apparent to many dam safety engineers. An early attempt to use the system for deformation monitoring was reported from the Naret Dam in Switzerland in 1992.1 This study report asserted that permanent, fixed GPS monitoring on baselines less than 5 kilometers could providesub-millimeter horizontal accuracy and 1- to 2-millimeter vertical accuracy. The report’s authors also predicted that many potential errors could be overcome through long occupation time. Although the relatively short occupation times used by the authors may have failed to reveal longer-term scatter in the data, the study nonetheless demonstrated the potential value of using GPS for high-accuracystructure monitoring.
GPS monitoring was introduced at Los Angeles County’s Pacoima Dam in southern California in the mid-1990s.2,3 The 113-meter-high Pacoima Dam was the tallest dam in the world when it was constructed for flood control in 1929. It had been subject to severe shaking and significant damage during the 1971 San Fernando and 1994 Northridge earthquakes.4 Two GPS stations were deployed on the dam. One was installed on the roof of a concrete operations building on the left thrust block of the dam, and the second was installed near the center of the concrete arch about 100 meters away. A reference station was installed on a ridge almost 2.5 kilometers from the structure and about 160 meters above it.
After processing nearly three years of data, the investigators concluded that the center of the dam arch experiences an annual cycle ofupstream-downstream motion having an amplitude of approximately 15 to 18 millimeters. The data also allowed the researchers to distinguish between the annual motion cycle and shorter-period ambient temperature variations.
The Pacoima Dam study also highlighted the importance of carefully processing GPS data. Good processing is necessary to filter out errors and minimize measurement noise. In the processing methodology typically utilized for land-survey applications, integers are fixed to quickly compute a several centimeter solution. This fast and productive land-survey methodology, however, is not necessarily the best choice for long-term monitoring. It can produce false position jumps, which in a real-time dam safety application could trigger a motion threshold alarm. This problem can be overcome by using a non-integer fixing technique, such as a triple-difference technique in which data are compared across successive measurements in time. The triple-difference technique, when combined with the well-established technique of Kalman filtering, provides a very robust processing technique that is virtually immune to artificial position jumps.5
Building a reliable GPS monitoring system
Condor and its partner company, The XYZs of GPS, of Dickerson, Maryland, adopted the triple-difference approach for the processing algorithms used in the 3D Tracker system. These algorithms are embedded within the 3D Tracker software, and receive and process raw GPS data sent from remote GPS receivers located on a dam or other target structure. This technology provides real-time, millimeter accuracy for surveillance of deformation in dams and other structures.6
This approach differs from the typical survey or navigation application of GPS where real-time processing occurs inside the GPS unit rather than on a centrally located personal computer running a software program. Transmitting the GPS measurements for processing at a centrally located computer offers several benefits. First, a personal computer can easily handle the sophisticated algorithms necessary to provide high-accuracy information for multiple sites in real-time. The cost of the GPS component of the system also is reduced significantly because the system integrator is able to deploy less expensive GPS receivers in the field. Finally, data held in a central computer can easily be distributed to other users and archived at a secure location.
Monitoring the motion of Libby Dam
The U.S. Army Corps of Engineers and Condor completed the installation of a 3D Tracker real-time GPS monitoring system at the 525-MW Libby Dam hydroelectric project in northwestern Montana in February 2002. Libby Dam, a straight-axis concrete gravity dam on the Kootenai River, is part of the network of dams operated by the Corps in the Columbia River Basin.
The monitoring system is comprised of six remote GPS stations deployed across the crest of the dam at strategic monoliths and two GPS reference stations. One GPS reference station is located on a ridge above the left abutment and the second is on a promontory upstream from the right abutment and at approximately the same elevation as the crest of the dam. The two GPS reference stations enable facility operators to run two sets of independent baselines to each of the remote GPS receivers on the dam crest, while also monitoring for reference station integrity by running baselines between reference stations. These baseline distances range from 100 meters to almost a kilometer. Real-time data are processed at a computer located in an instrument room at Libby Dam and conveyed to the Corps’ offices in Seattle through a LAN connection. The good sky visibility at the site and the use of two GPS reference stations result in horizontal and vertical measurement accuracies of ap- proximately 2 to 4 millimeters in real time and 1 to 2 millimeters at one standard deviation over a 24-hour period. Corps engineers compare these data to information from other monitoring technology at the dam, including plumb lines and joint meters. The first year of monitoring data demonstrates very good agreement between the GPS and the existing plumb lines.
The two GPS reference stations enable two independent solutions to be computed for each remote GPS station on the dam. Since GPS accuracy is absolute, these two independent measurements can be instantly compared to gauge system accuracy and provide integrity monitoring. Longer-term comparisons also can be made of relative deformation. An extremely close agreement between the independent processes for each remote station on the dam reinforces confidence in the accuracy and precision of the computed positions. Disagreement between the independent solutions would alert an operator to investigate the cause for the deviating solutions. At Libby Dam, the solutions from each GPS reference station agree very well with each other (within the GPS error).
To ensure long-term precision, a GPS monitoring system must have stable reference monumentation and a method for continuously monitoring the integrity of the coordinates for each GPS reference station. If motion of the target structure is suspected, an accurate measurement of any reference station displacement must be determined to eliminate the possibility that the motion is propagated error. Analysis of GPS error, data repeatability, and accuracy of the computed solutions provide three comprehensive indicators of system perform- ance. These three indicators have been defined at Libby Dam, and enhance the ability of the Corps of Engineers system operators to monitor any motion of the dam at the several millimeter level.
|GPS receivers are installed along the crest of Libby Dam, and GPS stations are situated on natural ground above both ends of the dam. Libby Dam, owned and operated by the U.S. Army Corps of Engineers, is on the Kootenai River in Montana.|
Putting the technology to work at other sites
Kalman filter-based, triple-difference, real-time GPS systems have been deployed for several other monitoring applications within the hydropower industry and elsewhere by Condor. For example, Pacific Gas and Electric Company has installed a real-time GPS system to monitor movement of a penstock in northern California. The Northern California Power Agency uses the technology to monitor a landslide upstream from one of its dams.
The U.S. Geological Survey uses real-time GPS systems to monitor numerous volcanoes, and has deployed an extensive GPS system at the Long Valley caldera at Mammoth Lakes in east-central California. This and other systems at Rabaul, Papua New Guinea, and Three Sisters in the Cascades of central Oregon have proven the reliability and cost-effective application of the system in very rigorous environments. The system is also in use to monitor oil field subsidence. The city of Long Beach, Calif., for example, has deployed a real-time GPS system to monitor deformation occurring within the greater Long Beach area related to oil production and steam injection.
The combination of GPS instrumentation with sophisticated processing algorithms and communication devices is proving to be a reliable technology for long-term monitoring of structures and critical geologic features. Like any instrumentation, GPS has limitations. The biggest limitation is the requirement for each GPS antenna to have sky visibility. Operators of GPS systems also must have good data management skills due to the fact that the system runs continuously and produces large amounts of data. A familiarity with computers and LAN connections is also necessary. For sites where good sky visibility is available, and operators have good data management skills, GPS can be a very attractive choice for deformation monitoring. The integration of real-time GPS with other conventional monitoring techniques, including tilt sensors, inclinometers, robotic total stations, and plumb lines, provides the redundancy sought by facility operators and regulatory agencies. It is likely that this technology will be rapidly adopted for monitoring North America’s infrastructure as the demand for real-time position information increases.
Dr. Hillman and Messrs. Rutledge and Baldwin may be contacted at Condor Earth Technologies, 21663 Brian Lane, Sonora, CA 95370; (1) 209-532-0361; E-mails: [email protected], [email protected], cbaldwin @condorearth.com. Mr. Meyerholtz may be contacted at the U.S. Army Corps of Engineers, P.O. Box 3755, Seattle, WA 98124; (1) 206-764-3449; E-mail: [email protected] army.mil.
1Frei, E., A. Ryf, and R. Scherrer, “Use of the Global Positioning System in Dam Deformation and Engineering Surveys,” reprinted in WILD GPS-System 200 Service Manual, Leica AG, Heerbrugg, Switzerland, 1992.
2Behr, J., K. Hudnut, and N. King, “Monitoring structural deformation at Pacoima Dam, California, using continuous GPS,” Proceedings of the 11th International Technical Meeting of the Satellite Division of the Institute of Navigation, ION GPS-98, Nashville, Tenn., 1998, pages 59-68.
3Hudnut, K., and J. Behr, “Continuous GPS monitoring of structural deformation at Pacoima Dam, California,” Seismological Research Letters, Volume 69, No. 4, 1998, pages 299-308.
4Swanson, A. and R. Sherma, “Effects of the 1971 San Fernando Earthquake on Pacoima Arch Dam,” International Commission on Large Dams, Proceedings, New Delhi, India, 1979, pages 797-824.
5Remondi, B. and G. Brown, “Triple Differencing with Kalman Filtering: Making it Work,” GPS Solutions, Volume 3, Issue 3, 2000.
6Rutledge, D., and B. Hillman, “Precise Automated Monitoring for Deformation of Dams Using The Global Positioning System (GPS),” Proceedings of Tailings Dam 2000, Association of State Dam Safety Officials and U.S. Committee on Large Dams, Las Vegas, Nev., March 28, 2000.
Barry Hillman, president of Condor Earth Technologies, Inc., oversees the company’s development of real-time, GPS-based survey, mapping, navigation, and monitoring systems, including the 3D Tracker system described in this article. David Rutledge, program manager for Condor’s infrastructure monitoring division, led the development of the 3D Tracker system. Steven Meyerholtz serves as senior engineer responsible for the dam safety instrumentation program in the Seattle District of the U.S. Army Corps of Engineers. Cory Baldwin is a senior systems engineer at Condor and is responsible for deploying GPS monitoring systems.