Resolution No. 4814 - Laserfiche WebLink

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8.0 LANDSLIDES 8,i Landslide Overview and Definitions The term "landslide" refers to a variety of slope instabilities that result in the downward and outward movement of s~ope-forming materials, including rocks, soils and artifidal fill. Four types of landslides are distinguished based on the types of materials involved and on the mode of movement. These four types of ~andslides are illustrated in Figures &1 to 8.4 from the Regional All Hazard Mitigation Master P~an for Benton, Lane, Lincoln, and Linn Counties (Phase One, Technical Appendix, 1998) which includes a more technical discussion of these four types of landslides. Rockfa~ls are abrupt movements of masses of geologic materials (rocks and soils) that become detached from steep slopes or cliffs. Movement occurs by free-fa~L bouncing and roiling. Fa~s are strongly influenced by gravity, weathering, undercutting or erosion. Rotational Slides are those in which the rupture surface is curved concavely upwards and the slide movement is rotationa~ about an axis parallel to the slope. Rotational slides usually have a steep scarp at the upslope end and a bulging "toe" of the slid material at the bottom of the slide. Roads constructed by cut and fill along the side of a slope are prone to slumping on the fill side of the road. Rotational slides may creep slowly or move large distances suddenly. Translational Slides are those in which the moving material slides along a more or less fiat surface. Translational slides occur on surfaces of weaknesses, such as faults and bedding ptanes or at the contact between firm rock and overlying loose soils. Translational slides may creep slowly or move large distances rather suddenly. Debris Flows (mudflows) are movements in which loose soils, rocks and organic matter combine with entrained water to form slurries that flow rapidly downsiope. All of these types of landslides may cause road blockages by dumping debris on road surfaces or road damages if the road surface itself slides downhill. Utility lines and pipes are prone to breakage in slide areas. Buildings impacted by slides may suffer minor damage from small settlements or be completely destroyed by large ground displacements or by burial in slide debris. Also, as evidenced by 1997 winter storms in Oregon, landslides may also result in casualties. There are main factors that determine susceptibility (potential) for landslides: 1 ) slope, 2) soil/rock characteristics, 3) vegetative cover, and 4) water content. Steeper slopes are more prone to all types of landslides. Loose, weak rock or soil is more prone to landslides than is more competent rock or dense, firm soils. Slopes Public Review Draft: August 6, 2004 8-1