SAN DIEGO — The Sacramento District U.S. Army Corps of Engineers and its consultants combined LiDAR (light detection and ranging) technology with traditional geomorphic and paleoseismic trenching techniques to discover the previously unrecognized Polaris Fault Zone in the North Lake Tahoe Basin. A paper about this discovery, authored by the Corps, Kleinfelder, and AMEC Geomatrix, recently won the Outstanding Paper Award at the 30th annual United States Society on Dams (USSD) 2010 Conference in Sacramento, Calif.
Martis Creek Dam, located in the Truckee Basin north of Lake Tahoe, Calif., is one of the highest-risk dams in the Corps’ inventory. Built in 1972, the dam has a history of excessive seepage during high-reservoir levels, which has prevented the project from fulfilling its flood-control and water-storage objectives. As part of a risk-informed hazards evaluation of Martis Creek Dam, the Corps, Kleinfelder, and AMEC Geomatrix relied heavily on high-resolution LiDAR data to conduct these seismic studies.
The Corps purchased LiDAR data from the Truckee-Donner Public Utility District and used the data to generate contour maps of terrain downstream of and surrounding Martis Creek Dam. The Corps also bought ortho-rectified aerial photography for the same grids. Comparing imagery and LiDAR data allowed engineers and scientists to map vegetation contrasts and fault-related geomorphology, such as lineaments, scarps, and sag ponds.
This geomorphic analysis resulted in the recognition of a previously unknown and through-going lineament. The suspected feature was then trenched and logged to evaluate the presence and age of faulting, confirming it as a Quaternary fault, now known as the Polaris Fault, located between the spillway and dam embankment.
Quaternary geologic mapping and geomorphic analysis of the overall Polaris Fault Zone is ongoing and will be used to identify candidate sites for future paleoseismic trenching that may provide better age dating, and to categorize the fault of the dam’s left abutment. Trenching should also enable the team to compute slip rate estimates and better constrain the displacement direction, magnitude, and frequency of movement.
“What we have discovered here is that LiDAR gives us the ability to strip away the vegetation and manmade features to see the bare-earth model. Without this, the Polaris Fault was previously unrecognizable. It is truly a powerful tool for seismologists,” said Bruce Hilton, principal engineering geologist with Kleinfelder and co-author of the paper that received the USSD award.
The LiDAR-based study is outlined in detail in the paper “Use of Hi-Resolution LiDAR in Discovering the Polaris Fault, Martis Creek Dam, Truckee, California.” The USSD honored the authors — Lewis E. Hunter, Ronn S. Rose, Bruce R. Hilton, William McCormick, and Todd Crampton — with the Outstanding Paper Award at the 30th annual USSD 2010 Conference. The USSD committee selected the paper, one of more than 100 presented during the conference, based on the technical content and quality of the paper, as well as the oral presentation.