PRELIMINARY LANDSLIDE COMPILATION MAP BASED ON SITE SPECIFIC RECONNAISSANCE ASSOCIATED WITH TIMBER HARVEST PLANS, HUMBOLDT COUNTY, CALIFORNIA

A primary limitation of regional landslide mapping is the subjective nature of topographic and aerial photographic interpretation. For the past 5 years, extensive ground based mapping has been conducted at site specific levels for individual timber harvest plans within the ownership of the Scotia Pacific Company. The associated engineering geologic evaluations provide an interpretation of the activity status and mode of failure of individual landslides. This information is reviewed, and at times modified, by regulatory agencies before the Timber Harvest Plan is approved. This process ultimately results in field verified, site specific landslide mapping of large portions of managed landbase. The Scotia Pacific Company Geology Department recently began to compile, at a scale of 1:12,000, the extensive landslide mapping associated with approved Timber Harvest Plans. This work also involves the preparation of a database to accompany the map. Each landslide is assigned a number that corresponds to specific attributes in the database. Attributes include the specific THP name and file number, the responsible firm, drainage subbasin, activity status, mode of failure, dimensions, and a field for miscellaneous notes. The many practical applications of the compilation maps and database include resource planning and allocation, prioritization of landslide prevention and mitigation measures, streamlining the required review of pertinent mapping and assisting foresters in the initial layout of timber harvest plans. This poster presents the preliminary compilation map of Timber Harvest Planning-level landslide mapping to date for the ownership within the Van Duzen Watershed in Northern California.