Showing papers by "Richard M. Iverson published in 1997"

The physics of debris flows

Abstract: Recent advances in theory and experimen- tation motivate a thorough reassessment of the physics of debris flows. Analyses of flows of dry, granular solids and solid-fluid mixtures provide a foundation for a com- prehensive debris flow theory, and experiments provide data that reveal the strengths and limitations of theoret- ical models. Both debris flow materials and dry granular materials can sustain shear stresses while remaining stat- ic; both can deform in a slow, tranquil mode character- ized by enduring, frictional grain contacts; and both can flow in a more rapid, agitated mode characterized by brief, inelastic grain collisions. In debris flows, however, pore fluid that is highly viscous and nearly incompress- ible, composed of water with suspended silt and clay, can strongly mediate intergranular friction and collisions. Grain friction, grain collisions, and viscous fluid flow may transfer significant momentum simultaneously. Both the vibrational kinetic energy of solid grains (mea- sured by a quantity termed the granular temperature) and the pressure of the intervening pore fluid facilitate motion of grains past one another, thereby enhancing debris flow mobility. Granular temperature arises from conversion of flow translational energy to grain vibra- tional energy, a process that depends on shear rates, grain properties, boundary conditions, and the ambient fluid viscosity and pressure. Pore fluid pressures that exceed static equilibrium pressures result from local or global debris contraction. Like larger, natural debris flows, experimental debris flows of ;10 m 3 of poorly sorted, water-saturated sediment invariably move as an unsteady surge or series of surges. Measurements at the base of experimental flows show that coarse-grained surge fronts have little or no pore fluid pressure. In contrast, finer-grained, thoroughly saturated debris be- hind surge fronts is nearly liquefied by high pore pres- sure, which persists owing to the great compressibility and moderate permeability of the debris. Realistic mod- els of debris flows therefore require equations that sim- ulate inertial motion of surges in which high-resistance fronts dominated by solid forces impede the motion of low-resistance tails more strongly influenced by fluid forces. Furthermore, because debris flows characteristi- cally originate as nearly rigid sediment masses, trans- form at least partly to liquefied flows, and then trans- form again to nearly rigid deposits, acceptable models must simulate an evolution of material behavior without invoking preternatural changes in material properties. A simple model that satisfies most of these criteria uses depth-averaged equations of motion patterned after those of the Savage-Hutter theory for gravity-driven flow of dry granular masses but generalized to include the effects of viscous pore fluid with varying pressure. These equations can describe a spectrum of debris flow behav- iors intermediate between those of wet rock avalanches and sediment-laden water floods. With appropriate pore pressure distributions the equations yield numerical so- lutions that successfully predict unsteady, nonuniform motion of experimental debris flows.

Debris-flow mobilization from landslides

Abstract: ▪ Abstract Field observations, laboratory experiments, and theoretical analyses indicate that landslides mobilize to form debris flows by three processes: (a) widespread Coulomb failure within a sloping soil, rock, or sediment mass, (b) partial or complete liquefaction of the mass by high pore-fluid pressures, and (c) conversion of landslide translational energy to internal vibrational energy (i.e. granular temperature). These processes can operate independently, but in many circumstances they appear to operate simultaneously and synergistically. Early work on debris-flow mobilization described a similar interplay of processes but relied on mechanical models in which debris behavior was assumed to be fixed and governed by a Bingham or Bagnold rheology. In contrast, this review emphasizes models in which debris behavior evolves in response to changing pore pressures and granular temperatures. One-dimensional infinite-slope models provide insight by quantifying how pore pressures and granular temperatures c...

Debris-flow initiation experiments using diverse hydrologic triggers

Abstract: Controlled debris-flow initiation experiments focused on three hydrologic conditions that can trigger slope failure: localized ground-water inflow; prolonged moderate-intensity rainfall; and high-intensity rainfall. Detailed monitoring of slope hydrology and deformation provided exceptionally complete data on conditions preceding and accompanying slope failure and debris-flow mobilization. Groundwater inflow and high-intensity sprinkling led to abrupt, complete failure whereas moderate-intensity sprinkling led to retrogressive, block-by-block failure. Failure during ground-water inflow and during moderate-intensity sprinkling occurred with a rising water table and positive pore pressures. Failure during high-intensity sprinkling occurred without widespread positive pore pressures. In all three cases, pore pressures in most locations increased dramatically (within 2-3 seconds) during failure. In some places, pressures in unsaturated materials rapidly "flashed" from zero to elevated positive values. Transiently elevated pore pressures and partially liquefied soil enhanced debris-flow mobilization. INTRODUCTION Most subaerial debris flows mobilize from landslides triggered by shallow ground-water flow in hillslopes. Several field investigations have attempted to record the shallow ground-water conditions that initiate debris flows (e.g. Harp et al. 1990; Johnson and Sitar 1990; Montgomery et al. 1990); however, no study has yet obtained high-resolution hydrologic data for the relatively infrequent, atypical hydrologic conditions that trigger debris flows. As an alternative, we created artificial slope failures under carefully controlled experimental conditions with intensive monitoring of hydrologic and deformation responses. Comparable control in the field would be difficult to achieve. Our experiments focused on monitoring soil moisture and pore-water pressure responses under three hydrologic conditions believed to trigger debris flows: localized ground-water inflow from adjacent bedrock or soil (e.g. Mathewson et al. 1990); prolonged moderate-intensity rainfall (e.g. Cannon and Ellen 1988); and bursts of high-intensity rainfall (e.g. Campbell 1975). I Hydrologist, U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025, U.S.A. (Phone: 415-329-4891, E-mail: mreid@usgs.gov) 2 Hydrologists, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661, U.S.A.

Hydraulic modeling of unsteady debris-flow surges with solid-fluid interactions

Abstract: Interactions of solid and fluid constituents produce the unique style of motion that typifies debris flows. To simulate this motion, a new hydraulic model represents debris flows as deforming masses of granular solids variably liquefied by viscous pore fluid. The momentum equation of the model describes how internal and boundary forces change as coarse-grained surge heads dominated by grain-contact friction grade into muddy debrisflow bodies more strongly influenced by fluid viscosity and pressure. Scaling analysis reveals that pore-pressure variations can cause flow resistance in surge heads to surpass that in debris-flow bodies by orders of magnitude. Numerical solutions of the coupled momentum and continuity equations provide good predictions of unsteady, nonuniform motion of experimental debris flows from initiation through deposition.

Response of Flexible Wire Rope Barriers to Debris-Flow Loading

Abstract: In June 1996, an experimental study at the U.S. Geological Survey Debris-Flow established the degree to which flexible wire rope harriers could contain rapidly-moving, staged debris flows consisting of water-saturated, poorly-graded gravelly sand. Six tests were conducted with four different barrier designs. All debris flows had volumes of about 10 cubic meters, masses of about 20 metric tons. and impact velocities of 5 to 9 meters per second. The study demonstrated that flexiblewirea_ozetafLers can effectively mitigate or even completely contain small debris flows.

First beam-commissioning results from the pep-iib-factory high energy ring: intensity effects.

Abstract: In two beam commissioning periods, clear evidence for intensity-related effects has been seen at the PEP-II High Energy Ring. In the presence of one unshielded bellows, beam currents appeared limited to about 60 mA and pronounced vertical beam motion was seen, which scraped parts of the beam out of the ring. With all bellows shielded, beam currents up to 300 mA were obtained, limited by a loss of longitudinal control at the highest beam current. Tune shift with intensity was observed, and there is evidence for an increase in bunch motion along an injected bunch train.

Status and early commissioning of the PEP-II High Energy Ring

Abstract: The High Energy Ring of the PEP-II B-Factory has been constructed in the PEP tunnel. It is now beginning beam commissioning. This report will address the status of the ring systems and our experience in commissioning the systems as well as the first beam running.