Landslide Hazard Map of Kirkland, WA

Abstract

Landslides are one of the most significant geohazards in the Puget Sound region and have a history of causing extensive damage to development (Baum, Harp, and Highland, 2007). Mapping landslide hazard areas is imperative in reducing the risk landslides pose to people and property. Given the general importance of landslide mapping, the City of Kirkland, WA is interested in such an assessment with respect to their annexation of 7.5 square miles in the northern portion of the City. In particular, landslide mapping is necessary to update the City’s Growth Management Act products for the annexed lands.

The Puget Sound region has been repeatedly glaciated over the last 2.4 million years, resulting in a series of glacial deposits of clay, sand, and till. The Vashon stade of the Fraser glaciation reached the Puget Lowland 17,400 calendar years before present and retreated 16,400 calendar years before present (Troost and Booth, 2008). Fraser-age deposits of glacial outwash sand overlying relatively impermeable glacial silt and clay deposits are particularly prone to failure, especially in saturated conditions, because the clay layers can create perched aquifers (Baum et al., 2007). Landslides tend to be associated with the rainy season in the Puget Lowland (October – May) when groundwater accumulates, adding weight to slopes and causing elevated pore pressures (Shannon and Wilson, Inc., 2007). Coastal landsliding is also common in the area, as wave action erodes and destabilizes the base of slopes (Shipman, 2001).

Protocols developed by Oregon Department of Geology and Mineral Industries (DOGAMI) (2009, 2012) provide an effective model for addressing hazards and risks of landslides in Kirkland, WA. These protocols are used to produce a digital inventory of landslide deposits and to identify areas susceptible to shallow landslides. Using geographical information systems (GIS) in conjunction with aerial imagery, high resolution lidar, and field reconnaissance to identify landslides, landslide flanks and deposits are digitized as polygons, while scarps are digitized as lines. Data for each landslide, including type of movement, geology, and slide depth, is included in the attribute table. Geology, slope, and data from the landslide inventory are inputs to the DOGAMI shallow landslide susceptibility model. Factor of safety is calculated for each geologic unit, and slopes with critical factor of safety values are highlighted to produce a hazard map. Factors of safety between 1.25 and 1.5 are considered moderately hazardous, and factors of safety less than 1.25 are identified as highly hazardous. The factor of safety map is filtered to eliminate manmade features like ditches, small retaining walls, and short road cuts, and then a buffer is applied to include areas adjacent to high hazards.

The resulting landslide hazard map can be used to inform City of Kirkland citizens and staff about highly hazardous areas. Future City planning may use this map as a reference for public utilities, infrastructure, and land use. This map may also aid in mitigating hazards to reduce future losses, and increasing public awareness of risk.