UTILITY OF LIDAR SURVEY FOR LANDSLIDE MAPPING OF THE HIGHWAY 299 CORRIDOR, HUMBOLDT COUNTY, CALIFORNIA

Geologists with the California Geological Survey have prepared geologic and landslide maps along highway corridors throughout California as part of a cooperative project with the California Department of Transportation (Caltrans). For each highway corridor, we prepare a geologic map and a landslide inventory map. Several of the corridors are in the densely wooded Coast Ranges of northern California. Mapping landslides in heavily forested terrain often requires an extraordinary effort to recognize landslides by field mapping, or in most cases where the time and money for mapping is limited, the acceptance of maps that are less complete and less accurate that those in un-forested land. The Highway 299 corridor in Humboldt County has very dense forest and a high level of landslide hazard. We anticipated that this corridor would be difficult and time-consuming to map accurately at the scale requested by Caltrans.

We contracted for a LiDAR survey of the corridor because of the anticipated difficulties in preparing large scale, accurate and complete landslide maps using aerial photographs and field work within the time available. The LiDAR survey was processed into a “bare-earth” digital elevation model with a ten-foot grid size. We processed the DEM into shaded relief, contour, and slope maps to help in mapping the landslides. Geomorphic features suggesting landslides are evident in each view of the dem and can be used in combination to map landslides in considerable detail. The shaded relief map provides a view similar to an aerial photo, and landforms can be interpreted from the patterns of sunlight and shadow. Additionally, if the lighting is not well oriented to highlight a particular feature, the direction and angle of the “lighting” can be changed. The slope map assists in this interpretation by highlighting those slopes that are steeper or less steep than the surrounding slopes. Steeper slopes may represent a landslide scarp, when adjacent to a gentler side-hill bench. Contour maps help to highlight the same features of steeper and gentler slopes, in a more familiar format that geologists have been using to interpret landslides for decades. The level of detail of the maps derived from the DEM show that LiDAR surveys can result in more detailed landslide maps, which may take less time to prepare, especially in heavily forested terrain.