UTILIZATION OF LIDAR DERIVED DEMs IN ASSESSING GEOLOGIC HAZARDS NEAR EARTHEN DAMS IN CALIFORNIA, USA

The U.S. Army Corps of Engineers – Sacramento District is using LiDAR-derived digital elevation models (DEM) and hillshade maps as integral tools in analyses of geologic and hydrologic hazards. LiDAR imagery is being applied to further define and characterize known active faults, identify and characterize previously unknown faults on regional and local scales, and aid floodplain mapping downstream of Corps structures. Off-the-shelf LiDAR data originally acquired to support hydrologic modeling at Martis Creek Dam (Nevada County) revealed several linear features 2.5 km southeast of the dam that project toward the dam. Additional data now reveal a northwest-trending right-lateral fault zone that extends ~35 km and passes within 100 m of the dam’s left abutment. LiDAR surveys have also been flown along the Kern Canyon Fault (KCF), which runs beneath the right abutment of the Isabella Auxiliary Dam (Kern County). The fault had previously been believed inactive. However, analysis of the LiDAR data helped target fruitful areas for detailed geomorphic mapping and paleoseismic trenching that have documented Holocene fault rupture. The success of these two surveys in identifying previously unknown faults or in redefining a known fault as active has been critical in demonstrating that LiDAR technology is an important tool for evaluating geologic hazards in dam safety reevaluations. These demonstrations are playing an important role in justifying and planning additional surveys along the western foothills of the Sierra Nevada Mountains. Two such surveys are planned to support seismic hazard studies of Hidden Dam (Madera County) and Success Dam (Tulare County). These data will be used for regional characterization of seismic sources as well as for assessing spillway geologic conditions. Geomorphic structures suggested by 10m USGS DEMs, but beyond the extent of current published maps will be investigated to further evaluate potential seismic hazards to Corps structures. Lower resolution imagery, but consistent with FEMA guidelines, are also being collected downstream of these structures for floodplain mapping, These data form the topographic baseline for modeling flooding events, estimating risks downstream, and for estimating economic benefits of the project.