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<br />
<br />all to generate a damaging flood, Even a small,
<br />normally dry watercourse is capable of inflict-
<br />ing damage, especially if its path is obstructed
<br />by manmade structures. One must remember
<br />that a ravine is the product of erosion by run-
<br />ning water, that streams erode most effectively
<br />during occasional flood stages which greatly
<br />exceed normal flow, and that the lack of appre-
<br />ciable runoff in an innocuous-looking ravine does
<br />not imply that great volumes of water may not
<br />occupy the watercourse during a heavy storm.
<br />Oversteepened slopes contribute greatly to
<br />storm damage, By far, the majority of slope
<br />failures that we observed immediately after the
<br />May 5-6 storm - slumps, earthflows, mudflows,
<br />and rockfalls - were consequences of manmade
<br />alterations, They occurred where slopes had
<br />been oversteepened, either by cutting or by fill-
<br />ing, beyond the bearing capacity of the material
<br />in the slopes, If similar failures are to be pre-
<br />vented in the future, careful stability analyses
<br />must precede slope modifications. Expert pro-
<br />fessional judgment is needed to evaluate such
<br />slope problems in hilly parts of Greater Denver
<br />before construction work is started, Sound engi-
<br />neering practices must go hand in hand with
<br />cutting or filling operations in hillside situations.
<br />Loose fills dumped into gullies or onto hill-
<br />sides at the angle of repose of dry material are
<br />foredoomed to eventual failure; mute testimony
<br />was provided by some such failures after the
<br />May 5-6 storm. As a word of caution, slope
<br />modifications should be held to a minimum in
<br />geologic settings where the risk of slope failure
<br />is high. In general, the risk is highest in areas
<br />where bedrock or overburden contains a high
<br />percentage of plastic shale or clay, especially
<br />clay having high swell-shrink properties, and
<br />areas where appreciable moisture is likely to
<br />enter the ground either naturally or artificially,
<br />
<br />Much hilly terrain in the western part of the
<br />Greater Denver area has a high risk potential
<br />for slope failure if modified by cutting or filling.
<br />Much of this terrain, however, also has natural
<br />attributes that would be highly desirable in
<br />scenic easements, green belts, or natural parks
<br />and that can contribute, therefore, to an in-
<br />creased quality of living in the area. With fore-
<br />sight and planning, potential liabilities to the
<br />community can become genuine assets.
<br />
<br />REFERENCES CITED
<br />
<br />Ducret, G. L., Jr., and Hansen, W. R., 1973, Storm of
<br />)'Iay 5-6, 1973, in the Denver ~ietro Area - Fre-
<br />quency and Effect: Urban Drainage and Flood
<br />Control District, Denver, Colo., 40 p., 6 figs.,
<br />5 tables.
<br />Follansbee, Robert, and Sawyer, L. R., 1948, Floods in
<br />Colorado: U.S. Geol. Survey '"Vater-Supply Paper
<br />997, 151 p,
<br />Gardner, M. E., Simpson, H. E., and Hart, S. S., 1971,
<br />Preliminary engineering geologic map of the Golden
<br />quadrangle, Jefferson County, Colorado: U.S. Geol.
<br />Survey Misc. Field Studies Map MF-308.
<br />l\'1atthai, H. F., 1969, Floods of June 1965 in South
<br />Platte River basin, Colorado: U.S. Geol. Survey
<br />'Vater-Supply Paper 1950-B, p. B1-B64.
<br />Nilsen, T. H., 1972, Preliminary photointerpretation
<br />map of landslide and other surficial deposits of the
<br />Mt. Hamilton quadrangle and parts of the Mt.
<br />Boardman and San Jose quadrangles, Alameda and
<br />Santa Clara Counties, California: U.S. Geol. Sur-
<br />vey Misc. Field Studies Map MF-339.
<br />Simpson, H. E., 1973, Map showing landslides in the
<br />Golden quadrangle, Jefferson County, Colorado:
<br />U.S. Geol. Survey Misc. Geol. Inv. Map I-761B.
<br />Van Horn, Richard, 1954, Landslides near Golden, Colo-
<br />rado: Engineers Bull., v. 38, no. 12, p. 6-15.
<br />1972, Surficial and bedrock geologic map of the
<br />Golden quadrangle, Jefferson County, Colorado:
<br />U.S. Geol. Survey Misc. Geol. Inv. :Map I-761A.
<br />Varnes, D. J., 1958, Landslide types and processes, in
<br />Eckel, E. B., ed., Landslides and engineering prac-
<br />tice: Natl. Acad. Sci.-Natl. Research Council Pub.
<br />544, Highway Research Board Spec. Rept; 29,
<br />p. 20-47,
<br />
<br />v u.s. GOVERNMENT PRINTING OFFICE, 19730-543-579/156
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