GEOLOGIC HAZARD AND OTHER DERIVATIVE MAPPING FOR CITY OF KIRKLAND, WA

In Washington State, cities are legally required to map and update their “Critical Areas” and “Geologically Hazardous Areas”. The City of Kirkland, with its recently annexed 7 sq mi, undertook such an effort in part because of recent tragic landsliding near Oso, WA. GeoMapNW, a group of faculty, graduate and undergraduate students at the University of Washington, produced a series of maps and databases that the City could use to update zoning, development, and planning efforts. The digital map package, at scales of 1:6,000 and 1:12,000, consists of a high-resolution geological map, a liquefaction hazard map, a seismic hazard map, a depth to groundwater map, a map of springs, a groundwater infiltration potential map, a landslide inventory, and a landslide susceptibility map. The mapping effort was accomplished using a series of queries from a newly updated database of geotechnical borings and wells, new field mapping including visiting construction sites, modeling, and geomorphic analyses of recently acquired LiDAR. City personnel were intimately involved with the mapping effort resulting in more robust, detailed, and useful products. In addition, interaction with and outreach to citizens provided additional information about the locations of springs and landslides.

Findings include significantly more land mapped as: higher potential for infiltration, susceptible to shallow landslides, and susceptible to liquefaction during earthquake shaking. As with other high-resolution geological maps in the region, less till was seen at the ground surface resulting in more coarse-grained deposits beneath the topsoil layer. Landslide mapping, following Burns and others (2012), overestimates the area susceptible to shallow landsliding due to the abundance of steep, but short, landscaping and transportation land modifications. And lastly, the potential for active faulting in the area was addressed.