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Shape and Condition of Slope <br />Geomorphic (Slope Form) <br />The form and condition of a slope can affect its stability. Geomorphic factors affecting slope <br />form include height and steepness, as well as vegetation and underlying geology. Increased <br />steepness and slope height generally correlate with reduced stability. Many of the landslide - <br />prone slopes along the corridors are more than ten stories (100 feet) in height and quite steep <br />(35-45 degrees slope gradient). This steep orientation exceeds the long-term stability of the <br />relatively weak sediments that comprise the slopes, and such slopes or segments of slopes are <br />often referred to as being in an "oversteepened condition." Increased slope height and the lack of <br />vegetative cover, especially conifers, increase the amount of rainfall that reaches the slope <br />surface. Vegetation generally contributes to how well the near -surface soils hold together and <br />thus helps resist surface erosion. Bare slopes tend to be more prone to erosion than well - <br />vegetated slopes. Large trees, however, can also be a detriment to localized slope stability, <br />where they root on steep slopes underlain by dense soils. For this reason, the presence and type <br />of vegetation and its contribution or detraction from stability needs to be evaluated on a site - <br />specific basis by qualified professionals. <br />Whether water infiltrates into the ground or runs off is influenced by the permeability <br />(porousness) of the geologic substrate, its degree of saturation (affected by antecedent <br />conditions) and precipitation intensity. The compact (solid) and fine-grained nature of some of <br />the underlying geologic units within the corridor limits infiltration and increases the likelihood of <br />saturating and weakening the near -surface, loosened soils. Within the corridor, this condition <br />commonly results in the separation and rapid transport of relatively thin, slab -like portions of the <br />slope, known as debris avalanches. Concentrated surface water runoff within drainages and <br />swales can further lead to channel -confined slope failures, involving the rapid transport of highly <br />fluidized debris, known as debris flows. More than 80 percent of the documented landslides <br />between 1914 and 2001 were shallow landslide types (debris avalanches and debris flows) <br />(Shannon & Wilson, 2001). Figure 5 illustrates how precipitation and groundwater can influence <br />the occurrence of deep-seated landslides. <br />12 <br />