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.

/pdf/the-physics-of-debris-flows-1ncsxf8awr.pdf

The physics of debris flows

Landslide hazard evaluation: a review of current techniques and their application in a multi-scale study, Central Italy

A model for the topographic influence on shallow landslide initiation is developed by coupling digital terrain data with near-surface through flow and slope stability models. The hydrologic model TOPOG (O'Loughlin, 1986) predicts the degree of soil saturation in response to a steady state rainfall for topographic elements defined by the intersection of contours and flow tube boundaries. The slope stability component uses this relative soil saturation to analyze the stability of each topographic element for the case of cohesionless soils of spatially constant thickness and saturated conductivity. The steady state rainfall predicted to cause instability in each topographic element provides a measure of the relative potential for shallow landsliding. The spatial distribution of critical rainfall values is compared with landslide locations mapped from aerial photographs and in the field for three study basins where high-resolution digital elevation data are available: Tennessee Valley in Marin County, California; Mettman Ridge in the Oregon Coast Range; and Split Creek on the Olympic Peninsula, Washington. Model predictions in each of these areas are consistent with spatial patterns of observed landslide scars, although hydrologic complexities not accounted for in the model (e.g., spatial variability of soil properties and bedrock flow) control specific sites and timing of debris flow initiation within areas of similar topographic control.

/pdf/a-physically-based-model-for-the-topographic-control-on-3babhrnl49.pdf

A physically based model for the topographic control on shallow landsliding

Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.

Halloysite clay minerals — a review

https://www.geo.uzh.ch/~chuggel/riskcourse/dai_landslide_risk_ass_enggeol02.pdf

Landslide risk assessment and management: an overview

A real-time system for issuing warnings of landslides during major storms is being developed for the San Francisco Bay region, California. The system is based on empirical and theoretical relations between rainfall and landslide initiation, geologic determination of areas susceptible to landslides, real-time monitoring of a regional network of telemetering rain gages, and National Weather Service precipitation forecasts. This system was used to issue warnings during the storms of 12 to 21 February 1986, which produced 800 millimeters of rainfall in the region. Although analysis after the storms suggests that modifications and additional development are needed, the system successfully predicted the times of major landslide events. It could be used as a prototype for systems in other landslide-prone regions.

https://science.sciencemag.org/content/sci/238/4829/921.full.pdf

Real-Time Landslide Warning During Heavy Rainfall

Mobilization of debris flows from soil slips, San Francisco Bay region, California

We describe two different GIS-based approaches to the delineation of debris-flow hazard. The first is empirically based, and uses logistic regression to predict sites of rainfall-induced shallow landslides that initiate debris flows in San Mateo County, California. The second is both empirically and process based, and uses multiple physically-based simulations of debris flows to evaluate hazard downslope from initiation sites in Honolulu, Hawaii. The two approaches use fundamentally different data to delineate hazard in fundamentally different ways, and are described here as contrasting examples of approaches to hazard delineation.

Statistical and Simulation Models for Mapping Debris-Flow Hazard

The goal of this investigation is to determine threshold levels of intensity and duration of storm rainfall necessary to initiate debris flows on the steep hillslopes of the Honolulu District. These rainfall thresholds could find a future application in a debris-flow warning system for Honolulu, similar to a prototype warning system developed several years ago and now operating in the San Francisco Bay region in northern California. Previous studies suggest that debris flows are triggered when heavy rainfall, infiltrating into the shallow subsurface, becomes impounded and creates elevated pore pressures that weaken the slope materials. Based on this idea, a simple numerical model was developed to study the interaction between heavy rainfall and pore-pressures on steep hillslopes. In order to calibrate this model to the ground conditions within the Honolulu District, detailed recordings of both rainfall and shallow pore pressures have been made at two remote sites on steep hillslopes. These recordings confirm the presence of temporarily elevated pore pressures in response to heavy rainfall and show a correlation between the peak porepressure response and the maximum amounts of rainfall recorded for periods of 3 to 6 hours. An extensive historical data base of rainfall amounts and debris flow occurrences has been compiled for 17 large storms in the Honolulu District over the past 33 years. Plots of these historical data display well defined, lower bound relationships between the number of debris flows reported during storms and the peak rainfalls recorded over periods of 1 to 6 hours. Two threshold levels were derived from these lower bound relations: a "safety" threshold below which the likelihood of damaging debris flows is very low, and an "abundant" threshold level above which rainfall is likely to cause many debris flows and thus pose a hazard to life and property.

/pdf/development-of-rainfall-warning-thresholds-for-debris-flows-5ds5ir9at6.pdf

Development of rainfall warning thresholds for debris flows in the Honolulu District, Oahu

The Wasatch Front north of Salt Lake City has a recurrent history of debris flows. Two distinctly different types of climatic events—summer thunderstorms and rapid spring snowmelts—have triggered debris flows. Heavy autumn rains, exceptional winter snowpacks, and cool and extended springs followed by sudden warming characterized weather during 1983 and 1984. As a result of these climatic events, large amounts of water were rapidly added to the landscape, resulting in several hundred landslides, many of which subsequently mobilized into debris flows and (or) hyperconcentrated floods. Reconnaissance during June of 1983 revealed many hillsides with recently active landslides that had moved slightly but had not mobilized, or had only partially mobilized, as debris flows. The presence of these partly-detached landslides prompted an investigation of the potential for future debris flows. Wieczorek and others (1983) developed a technique for evaluating the potential for debris flows and hyperconcentrated floods to reach the canyon mouths, based on three assumptions: 1) debris flows would occur in response to a large influx of water to the soils, either from a thunderstorm or rapid snowmelt; 2) the partly-detached landslides were the most likely sources for future debris flows; and 3) debris-flow material incorporated from channels would be proportional to that from landslide sources. The climatic and debris-flow events of 1984 were sufficiently similar to those of 1983 that we could evaluate these assumptions. Slightly fewer debris flows occurred in 1984 than in 1983, consistent with a slightly lower snow-pack water content and slightly slower rate of melting. Debris flows occurred near the rapidly receding snowline, apparently as a result of the large influx of water into the soils there. On an areal basis, partly-detached landslides identified in 1983 were statistically significant as sources for debris flows in 1984. In the few documented cases, the contribution of material scoured from channels was variable but very high and appeared significantly influenced by the geomorphic condition of the stream channel.

Stephen D. Ellen

Papers

Real-Time Landslide Warning During Heavy Rainfall

Mobilization of debris flows from soil slips, San Francisco Bay region, California

Statistical and Simulation Models for Mapping Debris-Flow Hazard

Development of rainfall warning thresholds for debris flows in the Honolulu District, Oahu

Debris Flows and Hyperconcentrated Floods Along the Wasatch Front, Utah, 1983 and 1984