GEOLOGICAL HAZARDS MITIGATION APPROACH FOR A SMALL TOWN IN WASHINGTON

The State of Washington established the Growth Management Act (RCW 36.70A) as a means to control development on land with geological hazards as well as conservation and protection of environmentally sensitive areas. These lands are designated as “critical areas”. All counties and cities in the state are required to first establish critical areas ordinances then periodically review, evaluate, and, if necessary, revise their critical areas ordinances. The objective of regulations applied to geological hazard critical areas is to protect the public from threats to safety and to protect public and private property from natural hazards.

In 2007, the City of Stevenson, a small, growing municipality located along the Columbia River approximately 40 miles east of the Portland, adopted an ordinance amending critical areas protection. The city encompasses land with significant landslide hazards and a recent history of damaging slope failures and debris flows.

PBS Engineering + Environmental (PBS) was retained to develop a geological hazards map of the City and update the geological hazards section of the Critical Areas Ordinance (CAO). The mapping was primarily based on interpretation of images created from LIDAR data in combination with aerial photographs and a detailed topographic map. The effort included review of a 1977 slope hazards map and geologic literature. Due to budgetary constraints field confirmation of interpreted landslides was not performed. PBS also prepared a technical memorandum that explains the methodology and limitations, and as well as a summary of hazards by geographic area.

Three hazard categories are defined in the CAO: 1) High Hazard – for sites with active or historically active landslides; and/or discreet inactive landslides; 2) Medium Hazard – with inactive landslide complexes; debris flows; and/or potentially unstable slopes; and 3) Unknown Hazards – where hazards are not designated. The hazard map was developed to be used as a reference document only. Specific hazard categories must be determined through detailed geotechnical assessments or geotechnical investigations as prescribed in the ordinance. The ordinance includes assessment requirements and performance standards.