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<br />COSTA AND JARRETT-DEBRIS FLOWS <br /> <br />321 <br /> <br />I <br /> <br />of between 168 and 207 mm (6.6-8.2 in) occurred <br />(Mears, 1979). This value exceeds the world's <br />greatest point rainfall for a duration of 10 minutes <br />(Linsley and others, 1975, table 3-16). If a water <br />discharge of 6 m'!s (200 ft'!s) or less is assumed, as <br />suggested by our field work, the 10-minute rainfall <br />intensity would be approximately 10 mm (0.4 in), <br />Assuming the latter estimate is correct, as support- <br />ed by the evidence presented, outstanding rainfall <br />volumes are not necessary to generate debris flows <br />on small mountain streams. Debris flows can be <br />started by moderate, not record-breaking rainfalls. <br />Furthermore, if debris flows in small mountain <br />channels are incorrectly identified as waterfloods, <br />estimated associated rainfall intensities and vol- <br />umes will be too great. If these excessive rainfaIl <br />estimates are transposed to other watersheds for <br />use in hydrologic planning, design runoffs will be <br />excessive. <br /> <br />CONCLUSIONS <br /> <br />Investigation of reported outstanding floods on <br />small mountain watersheds in Colorado indicates <br />that debris flows have been misidentified as water- <br />floods in five out of seven cases. This suggests that <br />debris flows in other small mountain streams also <br />may have been misidentified as waterfloods, Dam- <br />ages associated with flooding in steep mountain <br />tributaries can be due to mud and debris flows as <br />well as waterfloods. Sufficient geomorphic and <br />sedimentologic evidence remains in the basin for <br />many years following a debris flow to ascertain that <br />a debris flow did occur. Evidence for a debris flow <br />includes: <br /> <br />", <br /> <br />1. High-water marks on the channel side of levees <br />occur well above the elevation of undisturbed <br />adjacent forest litter. <br />2, There is a lack of downstream evidence of ex- <br />cessive discharge. <br />3. There may be a lack of evidence of rill erosion <br />resulting from excess intense precipitation in <br />watersheds, <br />4, Levees of poorly sorted debris border the chan- <br />nels and terminal lobes mark the end of flow, <br />5, Large rocks found in levees and on debris fans <br />may be surrounded by a fine-grained matrix <br />forming a pebbly mudstone-like deposit similar <br />to glacial till, <br />6, The largest rocks are concentrated near the sur- <br />face and edges of deposits. Inverse grading may <br />occur. <br /> <br />~. <br /> <br />7, Small vegetation on channel borders or margins <br />may divert or disrupt flow, <br />8. Deposits are more poorly sorted than water- <br />flood deposits, <br /> <br />It is not one, but an assemblage of these character- <br />istics that are required to identify a debris flow from <br />a waterflood, <br />In many basins, numerous older levees with ma- <br />ture vegetation prove that debris flows similar to <br />the ones we studied have occurred several times in <br />the recent geologic past. They are not exceptional <br />events as would be suggested if interpreted as wa- <br />terfloods. Failure to identify the geologic processes <br />correctly has led to large errors in the significance <br />of the hazards, both in terms of design waterfloods <br />and design rainfall in small mountain basins, Debris <br />flows can be initiated by the combination of mod- <br />erate, not record-breaking rainfalls, accumulations <br />of poorly sorted debris, and steep slopes, and be <br />mistaken as waterfloods. <br /> <br />ACKNOWLEDGEMENTS <br /> <br />We would like to thank Edmund D, Andrews, <br />John G. Elliott, and Christina V. Smitt for help in <br />the field collecting data, and Robert Brennan, Rob- <br />ert W, Fleming, and Garnett P. Williams for their <br />review and helpful comments on the manuscript. <br />This report is part of the Foothills Flood Project <br />conducted by the Colorado District, Water Re- <br />sources Division, U.S, Geological Survey. Publi- <br />cation has been approved by the Director, U,S. <br />Geological Survey. <br /> <br />REFERENCES <br /> <br />A & S CONSL'LTA"lTS, 1978, Floodplain InformatiolJ Report, <br />Uncompahf?re Rh'er, Ouray to Dallas Creek, Ouray Count.v, <br />Colorado: Denver, Colorado Water Conservation Board, <br />35 p. <br />ANDREWS, E. D. ","ll} COSTA, J. E., 1979, Stream channel <br />changes and estimated frequency of a catastrophic flood, <br />Front Range of Colorado: Geolof?ical Socie(v of America <br />Abstracts with Program.I', Vol. 11, No.7, pp. 379. <br />BAGNOLD, R. A., 1954, Experiments on a gravity-free disper- <br />sion of large solid spheres in a Newtonian fluid under shear: <br />Royal Society (London) Proceedings, See. A, VoL 225, pp. <br />49-63. <br />BEATY, C. B., 1974, Debris flows. alluvial fans and a revitalized <br />catastrophism: Zeitschrijtjilr Geomorphologie, Vol. 21, pp. <br />39-51. <br />BENSON, M. A. AND DALRYMPLE, T., 1967, Generalfield and <br />office procedure.I' for indirect dischar{{e measurements: <br />U.S. Geological Survey, Techniques of Water-Resources <br />Investigations, Book 3, Chap. A-I, 30 p. <br />BEVERAGE, J. P. AND CULBERTSON, J. K., 1%4, Hypercon- <br />centrations of suspended sediment: Journal Hydraulics Di- <br /> <br />-... .- <br />