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<br />accurately meaStlred ilt many channel locations. <br /> <br />(7) The distinct V-ohape of the debris-flow channels <br /> <br />was modified by subsequent flood waters that i~cised <br /> <br />a V-shaped notcll intel the channels. <br /> <br />These features are charact.eristic of debris flcH<ls in general and can <br /> <br />be described satisfactorily if the mechanics of the flows are understood. <br /> <br />Johnson (1970) proposed a detailed mechanical description of how debris <br /> <br />flows move and are able to transport large boulders. H" indicates flows <br /> <br />are able to transport boulders near their upper surfaces because they possess <br /> <br />.-.. <br />a finite shear stength as well as the ability to flo',", as a visc'ous fluid, <br /> <br />Large boulders are often transported near the toP" of the debris flows, <br /> <br />10 contrast to boulder transport i.n floods wilich tak,es pl"ce near the bottom <br /> <br />of channels (Figure 4). A certain percentage of clay must be present in <br /> <br />the mud in order to give the material strength and enable it to transport <br /> <br />boulders. In addition, the sizes of boulders transported appear to be <br /> <br />limited only by channel size and the available material. <br /> <br />Inspection of <br /> <br />deposits in the channels and on the fans at Glenwood Springs showed that <br /> <br />some boulders weighing 5 to 10 t (4500 to 9000 kg) were transported on <br /> <br />or near the upper surfaces of flows more than 5 it (1.5 m) deep. <br /> <br />Th i s <br /> <br />particular mode of transport is vE~ry important to consider if structural <br /> <br />protection from flows is planned because rock impact to a structure could <br /> <br />occur several feet above ground leve 1.. <br /> <br />Regardless of whether a previous <br /> <br />channel exists, debris flows can move across unobstructed fan surfaces <br /> <br />- 21 - <br />