<br />BULLETIN OF THE ASSOCIATION OF ENGINEERING GEOLOGISTS
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<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)
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