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Eugene -Springfield Area Natural Hazards Mitigation Plan 2. Hazard Descriptions 2.7.1 Causes and Characteristics of the Hazard The term "landslide" refers to a variety of slope instabilities resulting in the downward and outward movement of slope -forming materials including rocks, soils, and artificial fill. The IMS-60 Study evaluated two categories of landslide types: • Shallow Landslides: failure plane is 15 feet or less below ground surface; • Deep Landslides: failure plane is greater than 15 feet below ground surface. The IMS-60 Study found some areas are more susceptible to shallow landslides, whereas other areas are more susceptible to deep landslides, and some are susceptible to both. Maps showing the areas susceptible to shallow and deep landslides are included in Section 3. The Eugene -Springfield area is susceptible to four types of landslides (Figure 2-7) which may occur as either shallow or deep landslides: • Rockfalls are abrupt movements of masses of material (rocks and soils) detached from steep slopes or cliffs. Movement occurs by free -fall, bouncing, and/or rolling. Falls are strongly influenced by gravity, weathering, undercutting, and/or erosion. • Rotational slides are those in which the rupture surface is curved concavely upwards and the slide movement is rotational about an axis parallel to the slope. Rotational slides usually have a steep scarp at the upslope end and a bulging "toe" comprised of the slope material at the bottom of the slide (Figure 2-8). 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 flat surface. Translational slides occur on surfaces of weaknesses, such as faults and bedding planes or at the contact between firm rock and overlying loose soils. Translational slides may creep slowly or move large distances suddenly. Flows are plastic or liquid in nature and the slide material breaks up and flows during movement. This type of landslide occurs when a landslide moves downslope as a semi -fluid mass, scouring or partially scouring rock and soil from the slope along its path. A flow landslide is typically rapid - moving and tends to increase in volume as it moves downslope as it scours out its channel. Though immediate damage is limited to where the slide occurs, landslides can have far reaching repercussions if infrastructure or water ways are involved. Historic 2-44 January 2020