LANDSLIDE HAZARD MAPS FOR WASHINGTON MARINE SHORES –TOOLS FOR PUBLIC AND PRIVATE DECISION MAKING

The Washington Division of Geology and Earth Resources (Washington’s State Geologic Survey) developed 1:12,000-scale reconnaissance landslide inventories and landform maps that provide relative landslide hazard interpretations for the marine shoreline areas of Thurston and Mason Counties, Washington. These maps are intended as tool for local level land use decisions and provide a wealth of information about specific landslides to help document the rationale for relative slope hazard assignments. Aside from providing baseline information for private landowners and their scientific consultants, these maps provide counties and cities specific and scale-appropriate data and interpretations on which to base the critical areas ordinances required by Washington’s Growth Management Act. The data are made publicly available for free from the Washington Department of Natural Resources website at http://www.dnr.wa.gov/ResearchScience/Topics/GeologicHazardsMapping/Pages/landslides.aspx.Marine shoreline areas are among the most valuable lands in Washington in terms of both ecology and property values. They also harbor a disproportionate share of landslide hazards. To help manage these hazards, our surveys defined shoreline hazard areas to include all areas for which we believe landslide activity would be reasonably expected to meaningfully affect shorelines. Landslides were inventoried using LiDAR, aerial photographs from multiple years, and extensive field observations, primarily from boats. All documented landslides were digitized in GIS, and these GIS files will be available from DNR’s website. Aside from a wide range of landslide attributes that are described by coded values, many extensive comments about individual slides are provided. Due to the overwhelming number of landslides along the marine shores, not all documentable landslides were tallied. Instead, the landslides we tallied were deemed sufficient to justify relative hazard ratings that we assigned to various landforms. Landform maps categorize all shorelines according to a set of landforms deemed relevant to relative slide hazards based on the observed landslide frequencies within each landform. GIS-based data dissemination permits users to display the maps in ways that emphasize either hazard levels or other attributes.