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<br />BULLETIN OF THE ASSOCIATION OF ENGINEERING GEOLOGISTS <br /> <br />310 <br /> 100 <br />~ 90 <br />z <br />" <br />~ 80 <br />Iii <br />~ 70 <br />u <br />~ <br />iI' 60 <br />~ <br />Ii 50 <br />0 <br />~ <br />~ <br />~ <br />~ 40 <br />z <br />w <br />U <br />Z <br />0 30 <br />u <br />~ <br />~ 2JJ <br />~ <br />" <br />w <br />~ <br /> 10 <br /> <br />?~ <br />"~'1. <br />,~, <br /> <br /> <br />o <br />o 10 W ~ ~ 00 W N M M 100 <br />VOLUME OF WATER PER UNIT VOLUME OF <br />WATER-SEDIMENT MIXTURE. IN PERCENT <br />Figure 1. Relation between sediment concentration and volume <br />of water per unit volume of the water~sediment mixture (from <br />Beverage and Culbertson, 19(4). <br /> <br />scribing historic mountain flooding. The literature <br />review (for example, Follinsbee and Sawyer, 1948) <br />and suggestions by other engineers and hydrologists <br />directed us to seven sites of reported or believed <br />outstanding floods in small mountain channels <br />draining less than 10 km' (4 mi') in area and above <br />2,300 m (7,500 ft) in altitude. Gaging-station records <br />and onsite reconnaissances indicate that five of <br />these, and perhaps other floods reported in small <br />mountain basins, may not be waterfloods but de. <br />bris flows, a form of mass movement. Debris <br />flows are known as a mass-movement interme- <br />diate between landsliding and water flooding, <br />with mechanical characteristics different from <br />either of these processes (Johnson, 1970). A <br />debris flow is a form of rapid mass movement <br />of a body of granular solids, water, and air. De- <br />bris flows are distinguished from mudflows on the <br />basis of particle size. Debris flows have 50 per- <br />cent of the solids larger than sand, whereas mud. <br />flows are a form of earthflow consisting of material <br />that is wet enough to flow rapidly, and contains <br />mostly sand., silt., and clay-sized particles (Varnes, <br />1978). The distinction between waterfloods and de. <br />bris flows is important, because mitigating proce. <br />liures for waterfloods may be ineffective for debris <br />flows, and indirect methods of measuring cpeak dis. <br /> <br />charges are not valid for debris flows. The resulting <br />indirect-discharge estimates may give excessive <br />flood discharges for small basins in the Rocky <br />Mountains if the evidence left by the flood was as- <br />sumed to have been caused by water, when actually <br />it was caused by a debris flow. <br /> <br />MECHANICS OF DEBRIS FLOWS <br />VERSUS WATERFLOODS <br /> <br />Large sediment concentrations alter the fluid <br />characteristics of flowing water. As fine-grained <br />material is added to water, fall velocity of particles <br />decreases (Graf, 1971). Five percent silt by weight <br />in flowing water dampens eddy currents, decreasing <br />turbulence (Lane, 1940), One method for differen- <br />tiating waterfloods from mudflows is shown in Fig. <br />ure 1, where sediment concentrations of 80 percent <br />or greater by weight are considered the dividing <br />point between waterfloods and mudflows. Johnson <br />(1970) reports 60-90 percent solids by weight in <br />sampled debris flows. Curry (1966) measured an <br />average 91 percent solids by weight for debris flows <br />in Mayflower Gulch, Colo" and Waldron (1967) reo <br />ported sediment concentrations of 20 to 79 percent <br />by weight (57.4 percent average) in the Rio Reven- <br />tado debris flows of Costa Rica. Sokolovskii (1968, <br />p. 405) indicates that mudflows contain approxi. <br />mately 500 to 1,500 kg of sediment per cubic <br />meter of water (31-94 Ibs/ft3) (50-151 percent sol. <br />ids by weight), Thus, during a debris flow, more <br />solids than water can be moved, and water ac- <br />tually can be a very small percentage of the total <br />flow. Classification of flows using sediment load reo <br />quires that a sample be obtained during the flow. In <br />remote, small mountain watersheds where water- <br />floods and debris flows occur quickly, such sam. <br />pIing usually is not possible, Geomorphic and sedi. <br />mentologic criteria are thus a more practical and <br />meaningful way to ascertain process, <br />Increasing concentrations of fine.grained sedi- <br />ment affect flow characteristics by increasing the <br />viscosity and density of the flow, Debris flows can <br />have viscosities as much as 1,000 poises, compared <br />with 0,01 poise for pure water at 20"C (Campbell, <br />1975), and densities of 2,0-2,5 gmlcm3 (Johnson, <br />1970). Large concentrations of fine sediment con. <br />tribute strength to the flow, The strength (K) <br />must be exceeded before any deformation or flow <br />occurs. Johnson (1970) derives the following rela- <br />tion for estimating strength of debris flows: <br /> <br />K ~ Ty sino: (I) <br /> <br />.' <br /> <br />- <br />