25th Annual Spring Seminar
Development and Risk in Landslide Sensitve Areas
ASCE Seattle Section Geotechnical Group and
University of Washington Department of Civil Engineering
Seattle, Washington - April 5, 2008

LANDSLIDE HAZARDS AND CRITICAL AREA ORDINANCES

IN THE CENTRAL PUGET SOUND AREA

By

Donald W. Tubbs

Landslide hazard mapping within the central Puget Sound area, and the development of criteria for landslide hazard mapping, preceded the development of Critical Areas Ordinances. Regional landslide hazard mapping in this area began in the early 1970’s. The landslide hazard criteria used in the 1978 King County Sensitive Areas Map Folio and incorporated into the 1979 King County Sensitive Areas Ordinance still forms the basis of most such mapping in the area, though the criteria has evolved and been expanded over the years.

The early landslide hazard mapping was based on existing geologic maps which were mainly available at scales ranging from 1:24,000 to 1:48,000, and had contour intervals of 20 and 25 feet. As a result, the criteria developed for the mapping did not address minimum slope heights. The maps were intended to represent potential landslide initiation areas, and did not address adjacent areas (e.g. upslope or downslope) that might be affected by the landslides. These limitations have generally been addressed within CAO’s by specifying minimum slope heights and by setback and buffer requirements.

The 1990 Growth Management Act provides a suggested list of criteria that may be used to identify landslide hazard areas, and a 1995 amendment requires counties and cities to include “best available science” in developing critical areas policies and development regulations. Criteria for identifying landslide hazard areas within specific cities and counties are specified in their respective Codes, which can usually be found (sometimes along with hazard maps) on their websites, or may be found at the Municipal Research and Services Center of Washington website: www.mrsc.org.

It is the landslide hazard criteria specified in a specific CAO, rather than any existing landslide hazard map, that determines whether an individual site falls within a CAO landslide hazard area. It is therefore necessary to perform sufficient site investigation to determine whether an individual site falls within a landslide hazard area. The scope of the investigation required to make that determination is often specified in the CAO.

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