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<br />COSTA AND JARRETT-DEBRIS FLOWS
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<br />100 200 kiLOMETERS
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<br />EXPL~.~Al"lUrr.
<br />l. ked Dirt Credo tributary near Top<:lnos, Colo.
<br />2. Lucky Gulch Dear DOlsefO, Colo.
<br />3. Soulh Halfmoon Creek mbulary near Leadville,
<br />Colo.
<br />4. Soulh h,lrk Dutch Creek Ifll>ulllry ne." Redstone,
<br />Colo.
<br />5. East River tflhulal)' n"ar ('resled Ruu... Colo.
<br />6. Skyroclet uukh III Ouray. Colo.
<br />7. Comet Creek III Telluride, Colo.
<br />
<br />Figure 2. Location of areas of debris-flow investigations.
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<br />the correct process. OUf investigations of historic
<br />catastrophic waterfloods in small mountain drain-
<br />age basins indicate that preserved water.flood land.
<br />forms consist of irregularly spaced and discontin.
<br />uous longitudinal boulder bars with a coarse sand
<br />and gravel matrix below a surface armoring.
<br />Based on our experience, the geomorphic char-
<br />acteristics of a stream channel during and following
<br />the passage of a debris flow are shown in Figure 3A
<br />and, during and following the passage of a water.
<br />flood, in Figure 3B. In a debris flow, the strength
<br />(K) and buoyant forces resulting from the density
<br />of the transporting fluid can move very large boul-
<br />ders on shallow slopes. When the debris flow stops,
<br />the resulting deposit consists of large boulders in a
<br />matrix of fine.grained debris, forming a pebbly
<br />mudstone-like deposit in many places. Boulders are
<br />supported in a matrix containing substantial
<br />amounts of silt and clay. Because of the small dif.
<br />ference in density between boulders and the matrix
<br />material in the debris flow, buoyant forces and dis-
<br />persive stresses (Bagnold, 1954) may concentrate
<br />coarse boulders at the top of the deposit, forming
<br />reverse grading (Fisher, 1971). These characteris-
<br />tics are found in old debris-flow deposits in the
<br />channel of Lucky Gulch. a tributary to Sweetwater
<br />Creek (Figure 4), Following waterfloods. vegetation
<br />
<br />313
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<br />
<br />Atleca debris 110'>\'
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<br />
<br />During a walerflood
<br />
<br />
<br />Alter a waterflood
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<br />Figure 3. Typical channel cross sections: A, during and after
<br />passage of a debris flow; and B, during and after a wa(erflood.
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<br />adjacent to the channel and on alluvial fans is
<br />scarred, high water marks usually remain, and
<br />ground litter is disrupted below high water marks,
<br />Evidence of a large discharge of water can be found
<br />in the channel or on floodplains downstream, Grav-
<br />el and sand splays can occur, but no levees exist,
<br />and deposits are better sorted than debris-flow de.
<br />posits, Water-flood deposits can be poorly to weak.
<br />Iy cross.stratified, with gradational boundaries. Se.
<br />dimentological criteria are commonly used to
<br />differentiate debris flow and water-flood alluvial
<br />conglomerates (Shultz, 1980).
<br />Water-flood deposits, commonly poorly sorted.
<br />are generally better sorted than debris-flow deposits
<br />(Table 2), Average sorting coefficients for debris
<br />and mudflows range from 3,9 to 11,5; those of wa.
<br />terftoods in mountainous regions range from 1.8 to
<br />2.7.
<br />Laminar flow of a Bingham fluid indicates that
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