ASSESSING MASS WASTING POTENTIAL ON STATE-MANAGED FORESTLANDS ON THE TAHUYA BLOCK, MASON COUNTY, WASHINGTON

Forest practices activities such as timber harvesting and road construction have the potential to accelerate erosion. In steep terrain, accelerated erosion often comes in the form of mass wasting which can detrimentally affect water quality and fish habitat. To prevent management-related mass wasting and avoid adverse environmental impacts, natural resource managers often utilize landslide hazard assessments when planning forest practices activities.

We assessed the mass wasting potential across approximately 50,000 acres of State-managed forestland and some surrounding private lands in northeastern Mason County on the Kitsap Peninsula. The purpose of the assessment was to identify and map areas with a moderate or high risk of mass wasting. We developed a detailed landslide inventory using five years of aerial photographs, LiDAR, and geologic maps. The landslide inventory served as the basis for developing landslide hazard zones which are unique in terms of their slope gradient, slope form, mass wasting process, and sensitivity to forest practices. The products include a 1:24,000 scale map of observed landslides, a 1:24,000 scale map of landslide hazard zones, and a report detailing the project approach and findings. These products are used by Department of Natural Resources staff and staff from cooperating agencies and organizations to identify proposed forest practices that will require a site-scale investigation to further assess mass wasting potential.

Aerial photograph and LiDAR analysis of the area did not accurately represent shallow-rapid landslide hazards associated with inner gorge landforms. For instance, field reconnaissance within a single inner gorge revealed 14 mappable landslides that were not visible in aerial photographs and were too small (average 0.05 acres) and shallow to detect using LiDAR. These shallow-rapid landslides occurred between trees and as a result, did not disturb the canopy.

Field investigations combined with information derived from historic aerial photographs and LiDAR facilitated the development of accurate, high-quality landslide hazard maps. The maps are an essential tool in assessing the potential for management-related mass wasting on State-managed forest lands.