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Vantage to Pomona Heights Chapter 3 230 kV Transmission Line Project FEIS Affected Environment PAGE 3-293 within the six-mile wide study area—the class B Hanson Creek Fault, running approximately three miles south of the Saddle Mountains and the class B Frenchman Hills Structure Fault to the north of the Action Alternatives. All of these faults are shown on the Geohazards Map in Appendix A. Liquefaction occurs when soils lose shear strength and deform during an earthquake, acting like quicksand which is capable of causing great damage to structures in the area. Liquefaction typically occurs in areas of loose sandy soils that are saturated with water, such as low-lying coastal areas, lakeshores, and river valleys. Liquefaction susceptibility maps have been prepared for each county in the state of Washington, including Yakima, Grant, Benton, and Kittitas counties (WDGER 2010a). These maps provide an estimate of the likelihood that soil will liquefy as a result of earthquake shaking based on the physical characteristics of the soil (e.g., grain texture, compaction, and depth of groundwater). Liquefaction susceptibility maps depict the relative hazard in terms of low, low to moderate, and moderate to high liquefaction susceptibility (Appendix A - Geohazards Map). The risk of liquefaction is extremely low to low throughout most of the Project study area. The few exceptions include: 1) alluvium in some drainages and outburst flood deposits where small areas of low to moderate and moderate to high susceptibility occur; 2) moderate-to-high susceptibility in two alluvium deposits—one large deposit along the Columbia River on Route Segment NNR-8 and one small deposit on Route Segments NNR-2and NNR-3) low-to-moderate susceptibility within landslide deposits—several small deposits along Route Segment NNR-6, one large deposit along Route Segment MR-1, and one small deposit along Route Segment NNR-7. Additionally, the mapped landslides that are described below are mapped as moderate to high areas of susceptibility. Landslides are the downward and outward movement of earth materials on a slope through sliding or flowing along a slope failure plane. The slope failure can be a result of one or more of the following: ground saturation; ground shaking; removal of the ‘toe’ of the feature; and loading the upslope end of the feature. Historical landslides in the Project study area have been identified by the WDGER (WDGER 2010b). Historical and Quaternary landslide deposits are shown on the Geohazards map in Appendix A. These landslide features are located along Route Segments 1b, 1c, and 2d and include several small deposits along Route Segment NNR-6 and one small deposit along Route Segment NNR-7. These features are of unknown age, but aerial photo review indicates they are not currently active. While stable Quaternary landslide features are constructed upon regularly, to maintain the stability of these features, field review and determination of Best Management Practices (BMPs) for these two areas would be prudent. The northern slopes of the Saddle Mountains along Route Segment 3c are subject to rockfall and sluffing due to the steep slopes. While large mass-wasting events are improbable in their current undisturbed condition, large-scale modification of the existing slope conditions (e.g., access roads.) should be avoided. Steep terrain is considered a geologic hazard and a local critical area. For the purposes of this FEIS, steep terrain is defined as slopes ranging from 15 to 30 percent and very steep terrain is defined as slopes greater than 30 percent. Potential impacts to these areas are addressed in Section 4.15 of this FEIS. Both Yakima County and Grant County consider slopes 40 percent or greater to be high risk [Yakima County Code, Section 16.08.02(3)(a)(1); Grant County Code, Section 24.08.500(c)(8)]. However, this FEIS provides a more conservative analysis than required by local codes and classifies steep slopes (high risk) as 30 percent or greater. Kittitas County is currently revising their critical areas ordinance and it is expected to be updated in 2017. 3.15.2.3 Soils The soil types present in the Project study area can be generally divided into three groups: