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<br />or retaining walls, tilted poles or fences, and small soil <br />ripples or ridges (FIgUre 10). <br /> <br /> <br />CURVED TREE TRUNKS <br />\ <br /> <br />Figure 10. Creep. or the imperceptible slow move- <br />ment of shallow surficial soils. The process is <br />reflected in the tilting of telephone and power <br />poles, fences out of alignment. am/ed tree <br />trunks. and a ribbed appearance of the slope due <br />to soil ripples. <br /> <br />Debris flow <br />A debris flow is a form of rapid mass movement in <br />which loose soils, rocks, and organic matter combine <br />with entrained air and water to form a slurry that then <br />flows downslope. Genetally speaking, five conditions <br />must be present for a debris flow to occur: 1) steep <br />slopes, 2) loose rock and soil materials, 3) clay minerals, <br />4) saturated soils, and 5) rainfall- or snowmelt-generated <br />runoff of sufficient intensity and duration to initiate slope <br />movement. <br />Debris-flow areas are associated with steep gullies. <br />Individual debris flows can usually be identified by the <br />presence of debris fans at their termini (FIgUre 11). <br /> <br /> <br />,/' <br />/ t <br /> <br />, <br />..//' <br /> <br />.4>>/ <br /> <br />j <br /> <br />i <br /> <br />DEBRIS FAN <br /> <br />Figure 11. Debris fan fanned by debris flows. <br /> <br />14 <br /> <br />Debris ava nche <br />varnes p. 18) classifies the debris avalanche as a <br />variety of rapid to extremely rapid debris flow. In <br />comparing bris avalanches to debris flows he says: <br />In debris s, progressMo failure is more I1Ipid. and <br />the who mass, either because it is quite wet or because <br />it is on a s1oPe, liquefies, at least in part, flows, and <br />twnbles . commonly along a stream channel, and <br />_ wen beyond the foot of the slope. Debris <br />avalanche are generally long and narrow and often 1eave a <br />serrate 0 \tsbaped scar tapering upbiII at lbe head. . . . in <br />contrast lbe horseshoe-shaped scarp of a slump. <br /> <br />Earthflow <br />Earthflows a characteristic "hourglass" shape <br />(FIgUre 12). A bowl or depression forms at the head <br />where the s ope material liquefies and runs out. The <br />flow itself is elongated and usually channelized and <br />spread out the toe. Flows generally occur in fine- <br />grained ma rials or clay-bearing rocks on moderate <br />slopes and der saturated conditions. However, dry <br />flows of ular material are also possible. <br /> <br /> <br />Figure 12. earthflow (modified from Varnes. <br />1978). <br /> <br />Mudflow <br />A mudflow "an earthf10w consisting of material that is <br />wet enough to flow rapidly and that contains at least 50 <br />percent san -, silt-, and clay-sized particles" (Varnes, <br />1978, p. 18). <br /> <br /> <br />ACTIVITIES AND NATURAL <br /> <br />TRIGGERING LANDSLIDES <br /> <br />Both na and man-induced changes in the environ- <br />ment can er landslides. The geologic history, as <br />well as acti ties associated with human occupation, <br />directly de . es, or contributes to the conditions <br />that lead to lope failure. The basic causes of slope in- <br />stability are . Iy well known, based on a large number <br />of well-d ented case studies. The cause can be in- <br />