Project: Unstable Slopes Criteria - Empirical Evaluation of Shallow Landslide Susceptibility, Frequency, and Runout by Landform

Washington Administrative Code (WAC) Section 222-16-050(1)(d)(i) lists the five rule-identified landforms (RIL) and directs the reader to Section 16 of the board manual where the RIL and their criteria are described in detail. Those five RIL are utilized by DNR’s FPA approval process to determine if timber harvest has the potential to deliver sediment or debris to a public resource or in a manner that would threaten public safety (WAC 222-10-030(2)(b), SEPA policies for potentially unstable slopes and practices). The Unstable Slope Criteria Project will evaluate the degree to which the landforms described in the unstable slopes rules and board manual identify potentially unstable areas that are likely to impact public resources or threaten public safety.

Current RIL definitions and criteria are based on landforms and processes that are inferred to yield relatively high landslide densities, are influenced by forest management, and are likely to have a probable significant adverse impact (WAC 222-10-030(2)(c)). They were developed from field observations, regional research, and watershed analysis data collected from various sources and methods. Observations of storm-induced landslides that have occurred since the current rules were implemented have shown that a sizable proportion of delivering hillslope landslides may originate from terrain that does not meet RIL criteria. Likewise, while models have been built that predict maximum runout potential, there are no explicit criteria for assessing delivery to public resources or risk to public safety.

The studies included in the Unstable Slopes Criteria Project use lidar-based landslide inventories and landform mapping, and assessment of vegetation and precipitation history to calculate shallow landslide susceptibility. The susceptibility and runout project objective is to develop methods to use lidar differencing to map shallow landslides and runout areas across the landscape accurately and to derive the topographic elements from digital surface models associated with location of the landslide. The landslide locations can then be associated with different landforms and terrain elements and the relative density of slope failures by landform can be calculated. The landslide density will be associated with measures of storm magnitude to develop a relationship between the landslide rate and the storm return interval.