WASHINGTON'S LANDSLIDE HAZARD ZONATION PROJECT – OVERVIEW AND STATUS REPORT

Landslides in forested basins deliver sediment and debris and threaten public safety in Washington State. The Landslide Hazard Zonation (LHZ) Project is an effort to understand and map the past and present conditions that create sensitivity to forest practices. The project is federally funded because it is related to implementation of the Endangered Species Act and the Clean Water Act.

The LHZ Project objective is to create GIS data layers of landslides and landslide hazard zones mapped in forested basins on state and private lands. Although the mapping is accomplished by remote means with limited field work, the LHZ products are deemed highly useful in predicting the occurrence of hazardous areas. The maps are used by the Department of Natural Resources (DNR) as a screening tool for Forest Practices applications and by forest land managers to assist in landscape planning.

An explicit protocol was developed for the project to ensure consistency in data collection and results. The methodology is specific to Washington State but is similar to other such projects worldwide.

Two 1:12,000-scale reconnaissance-type maps and an explanatory report are created for each watershed. The first map and accompanying data set is a Landslide Inventory produced using historical air photographs and various state-of-the-art remotely sensed data. The second map shows the Landforms and their respective Hazard Ratings derived from the landslide inventory. Hazard ratings are based on the number and area of landslides per unit acre per year during which they were detected. Accompanying the maps is a descriptive report that includes detailed landform descriptions and hazard ratings, etc. The products receive intensive reviews to guarantee their high quality.

Progress on the project is impressive. To date, over 35,000 individual landslides have been entered into the database with 95 watersheds complete. Over 1,100,000 acres have been mapped in the LHZ project, while an additional 2 million acres have been compiled from other studies.

Hazards differ for shallow and deep-seated landslides. Seventy percent of the landslides are recognized as shallow-rapid and the rest are considered deep-seated. Modeled landslides show ≈13% of the total land base as unstable versus mapped landslides which indicate that ≈16% of the land base is unstable.