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<br />faci I itate its movement. The rates of movement for <br />landslides vary from tens of feet per second to fractions of <br />inches per year. lands II des can occur as react I vated 0 I d <br />sl Ides or as new sl ides in areas not previously experiencing <br />them. Areas of past or active landsl Idlng can be recognized <br />by their topographic and physical appearance. Areas <br />susceptible to landslides but not previously active can <br />frequently be identified by the similarity of geologic <br />materials and conditions to areas of known landsl ide <br />activity. <br /> <br />Consequences <br /> <br />landslides in the U.S. are estimated to cause more than $1 <br />billion a year In property damage. according to the <br />Transportation Research 80ard of the National Academy of <br />Sciences. K.allroads, highways, homes, and entire communities <br />are lost to landsl Ides which demol Ish and/or bury them. In <br />Colorado the 19th century mining camp of Brownsvl lie just <br />west of Sf Iver Plume Is burled beneath a rain triggered <br />I ands II de that became a debr I s f low, I tis now under <br />Interstate 70. Landsl Ides occur commonly throughout <br />Colorado, and the annual damage is estimated to exceed three <br />million dollars to buildings alone. <br /> <br /> <br />These two photos show the headscarp (a) and toe (b) of a <br />landsl ide In a residential subdivision south of Denver. <br />Fortunately, this 51 ide occurred during uti I ity placement and <br />lot grading prior to construction of homes. Geologic <br /> <br />26 <br /> <br />Aggravating Circumstances <br /> <br />Landsl Ides are one of the primary natural processes shaping <br />the land. Man's activities that frequently cause significant <br />increases In landslide activity include: <br />1} excavation of a steep slope or <br />the toe of an existing land- <br />sl ide# thus removing support of <br />the upslope mass, <br /> <br />2) addition of material to the <br />head (top) of a landslide which <br />pushes the slide material <br />downslope, <br />3) addition of moisture to the <br />landslide mass, increasing the <br />weight and decreasing the <br />strength. <br /> <br />The activities which tend to increase landslide potential <br />include excavation for highways and houses, and earth fl I Is <br />for highways and houses, lawn watering or surface drainage <br />diversions, and changes In water Infiltration rates. <br />Alteration of surface land use such as roadcuts and water <br />Impoundments, which allows more water into the subsurface of <br />a slide-prone slope, is a major contributing factor in <br />landsl ides. <br /> <br /> <br />Investigations resulted In alteratIons In the development <br />plan to prevent future damage. Note the very gentle slopes <br />at the toe of the sl ide. Such earth movements are common In <br />Colorado. <br />