<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-
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