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<br />314 <br /> <br />BULLETIN OF THE ASSOCIATION OF ENGINEERING GEOLOGISTS <br /> <br /> <br />- :.-.~;>>;~t$;~~~ <br /> <br />Figure 4. Characteristic composition of a debris-flow levee of <br />Lucky Gulch near Dotsero, Colo., area 2 in Figure 2. Diameters <br />of the largest boulders in the matrix of fine-grained material are <br />approximately I meter (3.3 ft). Note the concentration of boul- <br />ders at the top of the deposit. Hammer in left center of photo- <br />graph is 0.3 meter (I ft) long. Right channel bank. looking south- <br />wes!. Photo taken August 1978. <br /> <br />the central part of a debris flow may not deform, <br />and it is carried by the flow as a rigid plug. This is <br />shown by the velocity distributions of waterfloods <br />and debris flows in Figure 5. On a small tributary <br />of South Halfmoon Creek near Leadville, trees as <br />much as 0.3 m (I ft) in diameter were destroyed <br />in the center of the debris fan, but trees on the edge <br />of the fan were only scarred. Superelevation of mud <br />lines on upstream sides of surviving trees dimin- <br />ished from values of 343 mm (13,5 in) on trees 1.5 <br />m (5 ft) from the edge of the flow, to only 152 mm <br />(6 in) on trees at the edge of the flow, This obser- <br />vation is consistent with the interpretation that the <br /> <br />01_ <br />~I~ <br />cl~ <br />Bo <br />//I~"-. <br />, " <br /> <br />Bingham ~igid plugl I \ <br />/r--;:r----T-----\---'\ <br />! l i \ \ <br />~ I l' ' \ \ <br />3 ! / ! \ \ <br />" I / I \ \ <br />! / i \ II <br />I ' \ <br />I \ <br /> <br />CHANNEL WIDTH <br />Figure 5. Velocity distributions of Newtonian and Bingham <br />fluids (from Johnson, 1970). <br /> <br /> <br />Figure 6. Upstream view of debris-flow channel resulting from <br />passage of rigid debris plug, South Halfmoon Creek tributary <br />near Leadville, Colo., area 3 in Figure 3. Channel is approxi- <br />mately 10 meters /33 ft) wide and 3 meters (10 ft) deep. View is <br />east up the channel. Photo taken July 1978. <br /> <br />flow was a Bingham fluid whose point velocity de- <br />creases rapidly toward the edges of the flow (Figure <br />5), This rigid plug forms a U.shaped channel as <br />shown by the channel of the tributary to South Half. <br />moon Creek in Figure 6 (Johnson, 1970), This chan- <br />nel form also can occur following waterfloods <br />where more coarse material is supplied to the chan. <br />nel than can be transported (Scott and Gravlee, <br />1968), <br />When the debris flow stops, the strength of the <br />material allows the formation of steep fronts and <br />sides. creating levees and steep terminal lobes, A <br />lobe of a debris flow which damaged the eastern <br />part of Telluride, Colo" in 1914 is shown in Figure <br />7. Note the concentration of coarser material at the <br />front of the now. Laminar flow and little turbulence <br />allow the preservation of brittle, fragile, weathered <br />rock fragments in fresh flows. <br />Boulder fronts or flat.topped gravel berms ex- <br />tending across the flood channels with the coarsest <br />material at or near the surface formed during the <br />dam.break flood of 1964 on the Rubicon River, Cal. <br />if. (Scott and Gravlee, 1968), Krumbein (1942) reo <br />ferred to similar features formed by floodwaters in <br />the Arroyo Seco. Calif" as "boulder jams." These <br />boulder fronts may have formed as slip faces of <br />large dune or delta bedforms, or they may represent <br />the front of a subaqueous viscous flow in which the <br />bedload moved as a churning mass. These boulder <br />fronts may be differentiated from the terminal lobes <br />or snouts of debris flows because they lack the fine. <br />grained matrix and occur below high-water marks <br />on the valley sides, <br /> <br />.' <br />