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<br />COSTA AND JARRETI-DEBRIS FLOWS
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<br />DEBRI$FLOW
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<br />_3 South Halfmoon Creek
<br />Tributary near Leacl.
<br />~lile. Colo.
<br />.4 Sout" Fork Dulch Creek
<br />Tnbutary "ear RedstD"e,
<br />Colo.
<br />eSEastR,verTf1butarynell'
<br />Crestad Butle. Colo.
<br />e7 Cornel Creek at Telluride,
<br />Colo. (196geyenll
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<br />DRAINAGE AREA. IN SaUARE KILOMETERS
<br />EXPLANATION
<br />WATERFLOODS WITH LARGE SEDIMENT LOADS
<br />S,g Thompson Ri...."'1ributa.."s(1976 floodl
<br />&A Dark Gulch al Glen Comfo.t, Colo.
<br />&8 Noel's Ora.... al Gle" Comfor1, Coin,
<br />"C O......il's GulCh "liar Glen Haven. Colo.
<br />.a.D Big ThompSontributarv below Loveland
<br />HeightS,Colo.
<br />&E North Fork Bill Tt>ompson Rivertflbuta.v
<br />""e' Glen Haven. Colo.
<br />&F LOng GulCh nellT Drake. Cola
<br />
<br />10
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<br />_2 LuCkY Gulch nea' DOlse.o. Cola
<br />_6 Skyrocket Gulch at Ouray_ COlo.11923 event}
<br />
<br />Figure 14. Magnitude of 500-year floods for Colomdo Rocky
<br />Mountains above 2,300 meters (7,500 ft) elevation (McCain and
<br />Jarrett, 1976). Numbers next to symbol represent sites in Fig~
<br />ure 2.
<br />
<br />and squares in Figure 14 represent slope-area dis-
<br />charge estimates for waterfloods with large sedi-
<br />ment loads, These include two sites we investigated
<br />(sites 2 and 6, Figure 2) and selected tributaries in
<br />the Big Thompson River basin following a cata-
<br />strophic flood in 1976 (Grozier and others, 1976).
<br />Analysis based only on the available rainfall data
<br />from the 1976 Big Thompson storm would not ex-
<br />plain the magnitude of the peak-discharge estimates
<br />made by the U,S, Geological Survey, Reconstructed
<br />flood peaks based only on rainfall analysis for the
<br />Big Thompson storm were 50 percent smaller than
<br />some of the peak-discharge estimates made in the
<br />
<br />"
<br />
<br />~
<br />
<br />319
<br />
<br />field by the U,S, Geological Survey (Miller and oth-
<br />ers, 1978), Because of this discrepancy, investigators
<br />tried to devise a storm pattern and time distribution
<br />of rainfall that would agree more with the slope-
<br />area estimates. In this analysis, it was assumed that
<br />the entire flow volume was water, and that the
<br />streamflow estimates were accurate. However, the
<br />extensive channel erosion in many small tributaries
<br />indicates that a large but unknown proportion of the
<br />flood volume was sediment. In one small basin
<br />(point D in Figure 14), an estimated 55,000 metric
<br />tons (60,600 tons) of material were eroded from the
<br />channel network and transported out of the basin,
<br />This was 1,000 to 3,000 times the average annual
<br />sediment load from the basin, and does not include
<br />materials eroded from hillslopes (Andrews and Cos-
<br />ta, 1979). If some volume of sediment is subtracted
<br />from the slope-area estimates, the resultant peaks
<br />are in better agreement with the original rainfall
<br />analysis, Also, slope-area estimates may be too
<br />great because high-water marks may have been left
<br />before major channel erosion. However, these re-
<br />fined storm analyses, assuming the flood peaks
<br />were all water, increased the estimated rainfall in-
<br />tensity and volume for this storm (Miller and oth-
<br />ers, 1978).
<br />Whether some sediment was moved in the Big
<br />Thompson basin as debris flows is uncertain, Any
<br />evidence of mass movement would have been de-
<br />stroyed or severely modified by the waterflood,
<br />This is not the same problem as mistaking a debris
<br />flow for a waterflood, because in the Big Thompson
<br />event the depth of the waterflood following any de-
<br />bris flow was large enough to overtop any debris-
<br />flow evidence.
<br />
<br />IMPORTANCE OF DIFFERENTIATING
<br />BETWEEN DEBRIS FLOWS AND
<br />WATERFLOODS
<br />
<br />The investigations of reported waterfloods in
<br />small mountain watersheds of Colorado have indi-
<br />cated that some guidelines or criteria are necessary
<br />to help scientists responsible for investigating such
<br />phenomena to differentiate between waterfloods
<br />and debris flows, The San Juan Mountains of Col-
<br />orado have long been known as an area of frequent
<br />mass movements (Howe, 1909), Yet, some flood-
<br />insurance reports covering parts of this area indi-
<br />cate that mudflows and debris flows are not a prob-
<br />lem in this region.
<br />Conversely in a floodplain study of the Uncom-
<br />pahgre River, which drains the San Juan Mountains
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