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<br />
<br />In terms of local runoff in the metropolitan
<br />area of Denver, especially with respect to the
<br />smaller tributaries of the South Platte River,
<br />the May 5-6 storm was fairly commonplace.
<br />Residents can expect similar occurrences at
<br />fairly frequent intervals. Peak-flow recurrence
<br />intervals ranged from only 3 years at some sites
<br />to 20 years or more at others (Ducret and Han-
<br />sen, 1973, tables 3,4). The flooding of the South
<br />Platte itself was essentially a 50-year event.
<br />In terms of geologic effects, such as scour
<br />and especially mass wastage, the storm was un-
<br />usual- probably because of the thorough sat-
<br />uration of the gtound that resulted from the
<br />moderate rate of sustained precipitation. In
<br />nearly 25 years of residence in the area, the
<br />author has not seen its equaL
<br />The rainfall and flooding that accompanied
<br />the storm had widespread geologic and hydro-
<br />logic effects, brought about through erosion,
<br />sediment transport and deposition, and slope
<br />failure; By "geologic effects" are meant the
<br />quasi-permanent changes, however small, in the
<br />character or form of the rocks, soils, and physi-
<br />cal geography that together make up the land-
<br />scape. Many of these effects were of a small
<br />scale compared to the gross geologic setting of
<br />the area, but they provided dramatic evidence
<br />of the irresistibility of geologic processes, and
<br />they accounted for 'a substantial part of the
<br />damage to the works of man. Thus, the deposi-
<br />tion of a few inches of sediment on a few acres
<br />of ground, or the erosion of a few cubic yards
<br />of river bank, might be of little obvious note in
<br />the geologic scheme of things but might dam-
<br />age or destroy cropland or a section of highway
<br />worth many thousands of dollars. Similarly, the
<br />undercutting of the footing of a single bridge
<br />pier might destroy the whole bridge and neces-
<br />sitate the expenditure of many thousands of
<br />dollars for replacement or repair. A small land-
<br />slide might jeopardize the integrity of a struc-
<br />ture worth a large sum of money. Geologic
<br />processes set into motion by the storm thus
<br />caused extensive damage to lands and engineer-
<br />ing works throughout the Greater Denver area
<br />and the South Platte River flood plain. Dam-
<br />aged roads, bridges, dams, canals, and the like,
<br />were the chief losses in the public sector.
<br />
<br />PURPOSE OF THE REPORT
<br />Because of the rapid present and projected
<br />growth of Greater Denver, problems related to
<br />storm drainage are likely to increase in scale
<br />and frequency in the future. Natural processes,
<br />once set into motion, may be very difficult to
<br />control, but man can, with foreknowledge, pre-
<br />vent or avoid many direct encounters, He can
<br />sometimes circumvent the effects of a major
<br />storm, and he can take steps to minimize the
<br />causes and effects of inundation. The first step
<br />is to assess and attempt to understand the
<br />causes of past storm damage.
<br />This report, therefore, presents a capsule
<br />summary of the principal effects of the storm
<br />of May 5-6, 1973, in the hope that a broader
<br />public understanding of the effects of a past
<br />storm will lead to a greater awareness of the
<br />potential effects of future storms and to more
<br />effective measures to forestall storm damages
<br />in the future,
<br />PLACE NA;\IES
<br />Dozens of local place names are variously
<br />mentioned in this report, Most of these names
<br />are plotted on the location map (fig. 1), but to
<br />so identify all of them on a map of this size is
<br />not feasible. Such features as roads and street
<br />intersections are readily located from city maps
<br />and directories published by private and public
<br />agencies, including the U.S. Geological Survey's
<br />topographic maps at a scale of 1: 24,000 (1 inch
<br />to 2,000 feet), which are available at map-sales
<br />outlets. Oil-company road maps of Denver and
<br />vicinity are available at most gasoline service
<br />stations and are very useful references. One or
<br />more of these sources can be consulted to locate
<br />place names closely enough for most purposes.
<br />ACKNOWLEDGMENTS
<br />Much information in this report was pro-
<br />vided by Geological Survey colleagues who in-
<br />vestigated flood effects in various parts of the
<br />Greater Denver area immediately after the
<br />storm; included were Roger B. Colton, Eugene
<br />R. Hampton, Robert M, Lindvall, John O. Ma-
<br />berry, Edward E. McGregor, and Howard E.
<br />Simpson. Local newspaper accounts have been
<br />freely utilized, also. David J, Varnes kindly
<br />loaned the original drawing from his diagram
<br />showing the nomenclature of a landslide
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