Abstract Slope movements, including types of landslides and extremely slow soil creep ( Varnes, 1978 ), occur throughout the United States and within many national parks. The collection of vital signs of regional landslide information, referred to as monitoring, is not only scientifically useful, but is beneficial for assessment of landslide hazards and risk, which is in turn important for regional operations and planning. Different types of slope movement, such as fall, topple, slide, spread, and flow, can occur in a variety of materials and degrees of slopes. Specific types of landslides ( Fig. 1 ), such as rockfall, earth slump, and debris flow, can occur depending upon the types of geologic materials and movement ( Cruden and Varnes, 1996 ). A landslide can be caused by one or more of several factors, of which geological, morphological, physical, and human factors are the most common. The term landslide trigger refers specifically to an external stimulus, such as intense rainfall, rapid snowmelt, earthquake, volcanic eruption, or stream or coastal erosion. These stimuli initiate an immediate or near-immediate landslide movement by rapidly increasing shear stresses or porewater pressures, by ground acceleration due to seismic activity, by removing lateral support, by reducing the strength of slope materials, or by initiating debris-flow activity. Most landslides with recognized triggers are caused by precipitation: rainfall, snow meltwater, or combinations of both. In rock masses, rain and meltwater penetrate joints and produce hydrostatic pressures. In soils, the increase of pore-water pressures reduces shear resistance ( Schuster and Wieczorek, 2002 ).

Abstract Despite having a low recorded historical incidence of landsliding, the Sierra Nevada has undergone large prehistoric and historical rockfalls and rockslides that could be potentially catastrophic if they occurred today in the more densely populated parts of the region. Several large documented rockfall and rockslides have been triggered either by strong seismic shaking or long periods of unusually wet weather; however, in several instances no obvious triggering event can be identified. The glaciated topography of the higher elevations of the Sierra Nevada has produced many relatively small falls and slides within relatively hard, massively jointed, granitic rocks; however, where exposed to weathering for long periods after glaciation, the oversteepened rock slopes are prone to uncommonly large falls and slides. At lower elevations on the nonglaciated slopes of the Sierra Nevada, rockslides commonly occur within more weathered granitic rocks, where the strength of the rock mass is typically affected by joint weathering and alteration of the intact rock to saprolite. Historical large rockfalls and rockslides in the Sierra Nevada have created additional secondary natural hazards, including debris flows and floods from the breaching of landslide dams that can be as hazardous as the initial rockfalls and rockslides.

Abstract Classifications of flowing sediment-water mixtures have, in the past, been based primarily on relative, qualitative differences in the style and rate of movement as well as on morphology and sedimentology of deposits. A more quantitative and physically relevant classification is presented here, based on thresholds in rheologic behavior. The classification is constructed on a two-dimensional matrix in which flows are located according to deformation rate (mean velocity) and sediment concentration, with composition of the mixture constant. Three major rheologic boundaries are crossed as sediment concentration increases from 0 (clear water) to 100 percent (dry sediment): (1) the acquisition of a yield strength—the transition from liquid “normal streamflow” to plastic “hyperconcentrated streamflow”; (2) an abrupt increase in yield strength coinciding with the onset of liquefaction behavior—the transition to “slurry flow”; and (3) the loss of the ability to liquefy—the transition of “granular flow.” These three rheologic boundaries shift according to particle-size distribution and composition of the mixture. Processes controlling flow behavior depend on deformation rate (velocity). Rate-independent frictional and viscous forces dominate at lower velocities and in finer grained mixtures; rate-dependent inertial forces dominate at higher velocities and in coarser grained mixtures. As velocity increases, grain-support mechanisms change from low-energy varieties (buoyancy, cohesion, structural support) to progressively higher energy mechanisms (turbulence, dispersive stress, fluidization). Existing nomenclatures of geologic flow phenomena can fit within this rheologic classification. The morphology and sedimentology of flow deposits commonly can be used to deduce rheologic behavior, but caution needs to be exercised in inferring processes from deposits.

Abstract Japanese concepts of modeling debris flow are thoroughly reviewed in this chapter. Many Japanese models, ranging from highly theoretical non-Newtonian fluid models to the very simple empirical relations of Bingham and Bagnold, are evaluated in terms of accuracy, generality, and practical usefulness, and are compared with a generalized viscoplastic fluid model described herein. Most debris flow formulas and criteria presently used in Japan are closely related to those developed by Takahashi on the basis of Bagnold’s “dispersive” pressure concept. Although the generality of Bagnold’s model is still at issue, Japanese scientists apparently have accepted Takahashi’s debris flow formulas and criteria. For example, Takahashi’s velocity profile for (steady) uniform debris flow in wide channels is only valid for grain-intertia regime. Applying Takahaski’s solution to modeling other flow regimes than the grain-inertia may thus have to adjust the value of Bagnold’s numerical constant in order to better fit the computed velocities to the measured ones. This and many other aspects of debris-flow modeling concepts in Japan are critically examined. An appraisal of the present status of Japanese research in debris flow modeling helps determine the direction of future efforts in debris flow research.

Abstract The thousands of debris flows that mobilized from shallow slides in the San Francisco Bay region during the rainstorm of January 3-5,1982, left evidence of the range in soil textures susceptible to mobilization and of differences in completeness and speed of mobilization. These differences in mobilization are related to a broad range in the ratio of saturated water content to liquid limit, which we have used as an approximate index of mobilization potential. To understand such differences in mobilization, we have explored the transformation from slide to flow, using relations among inplace void ratio, void ratio needed for flow from the slide scar, and the steady-state line. These relations define two principal means of direct transformation from slide to flow: contractive soil behavior, which commonly results in liquefaction, and dilative soil behavior, which in many cases probably results in partial mobilization of the slide mass. These means of mobilization determine the completeness of mobilization of slides and the time required for mobilization; they also influence the thickness and lumpiness of deposits, as well as the travel distance of debris flows. These relations permit means of mobilization to be predicted in both an approximate and a precise manner through soil testing.

Abstract Very large rock avalanches, involving more than about 10 6 m 3 of rock debris, exhibit anomalously low coefficients of friction. Consequently they travel much farther than conventional slope-stability criteria predict. Such long-runout landslides ( sturz-strom ) include the catastrophic Elm (1881), Frank (1903), and Sherman Glacier (1964) events. Attempts to explain this behavior have considered water or air lubrication, local steam generation, or even the formation of melt layers within the rock debris. Discovery of deposits of such landslides on Mars and the moon, however, appears to rule out the fundamental involvement of volatiles or atmospheric gases in the flow mechanism. It appears that large, high-frequency pressure fluctuations due to irregularities in the flow of the debris may locally relieve overburden stresses in the rock mass and allow rapid pseudoviscous flow of even dry rock debris. If the avalanche volume is large enough, the rate of production of this vibrational (acoustic) energy exceeds its loss rate, and sustained motion is possible. Small-scale laboratory experiments have verified theoretical predictions of the rheology of such acoustically fluidized debris. This rheology is consistent with the rate and pattern of observed large rock avalanches. Although much work remains to be done, acoustic fluidization is the most plausible explanation of the fluidity of large, dry debris avalanches.

Abstract There were two lahars that reached the Toutle River during the eruption of Mount St. Helens, Washington, on May 18,1980. The North Fork lahar was much larger than the South Fork lahar, had a much more rectangular hydrograph shape, and was much more destructive. Hydrographs (graphs of discharge versus time) constructed for both lahars demonstrate that differences between the lahars existed as close to the lahar sources as measurements were made, indicating that differences in processes that initiated the lahars must have been responsible for observed disparities between the lahars. The South Fork lahar was apparently generated when a laterally directed pyroclas-tic cloud triggered slab snow avalanches, and then rapidly incorporated and melted the snow. The North Fork lahar was generated from a small portion of avalanche debris in which ice was comminuted to an abnormally small size. This ice melted rapidly and saturated the host avalanche debris, which then liquefied during a long harmonic tremor event. The North Fork lahar differed greatly from the South Fork lahar because of significant dissimilarities between the pyroclastic cloud and harmonic tremor sequence that were directly responsible tor the characteristics of each lahar. Because differences in lahar characteristics were ultimately responsible for the contrast in destructiveness, I have concluded that the process of initiation is an extremely important factor controlling downchannel destruction. The importance of initiation must be accounted for in the quantitative analysis of lahar hazard.

Abstract Development within the mountains of coastal British Columbia has recently increased the exposure of people and facilities to debris flows. Attempts to specify weather conditions under which debris flows are apt to occur—such as threshold precipitation—appear not to work because of the highly contingent nature of the flows. Debris must exist in unstable position in or near the channel, and conditions prior to the flow may strongly condition the necessary trigger to mobilize it. Events have been observed in the following circumstances: locally concentrated rainfall with high antecedent moisture and no snowmelt (the “classical case”); uniformly distributed, moderate rainfall with snowmelt; low intensity rainfall and heavy snow-melt; and heavy rainfall onto deeply frozen, but thawing, ground. A weather-based warning threshold for the British Columbia coast would be fairly complex. At present, such a system would include the substantial probability of issuing nuisance predictions of nonoccurring events. There is an indication that the incidence of debris flows has increased since 1980. Reasons why this might be so are investigated. Aside from the occurrance of four very wet years since then, no clear meteorologic correlation can be made.

Abstract In early November 1977, a storm system that formed in the Gulf of Mexico moved northeastward into the Appalachian Mountains. It produced intense (as much as 102 mm/hr) and heavy (200-300 mm) rainfall that set off debris avalanching in steep terrain of the Pisgah National Forest, North Carolina. Antecedent rainfall during September and October was 177 percent of normal and the wettest on record for these 2 months. The storm began on 2 November, and rainfall was relatively continuous and even (20-50 mm/day) for the next 3 days. The long-duration rainfall was capped by intense convective downpours the night of 5-6 November when debris avalanching occurred. Peak intensities measured at 15 gauges near Asheville, North Carolina, ranged from 21 to 102 mm/hr, with nearly half exceeding 75 mm/hr. Return intervals for peak intensity rainfall in the range of 75 to 102 mm/hr are 50 to 200+ yr. Total storm rainfall for these gauges ranged from 35 to 250 mm, with peak 24-hr rainfalls of 30 to 180 mm. Rainfall intensities for 1-, 3-, 6-, 12-, and 24-hr periods at a gauge near one avalanching site were 69,137,159,164, and 180 mm, respectively. Development of the storm was monitored by GOES infrared satellite imagery in real time, and flash flood warnings were issued. Debris avalanching and high stormflow produced peak stream flows with return periods ranging from 20 to 100+ yr. The largest debris avalanches occurred on steep slopes (70% +), started at high elevations (900-1,100 m) in shallow residual soils (less than 1 m deep), had tracks commonly greater than 700 m, and carried a volume of material averaging 2,500 m 3 per avalanche.

Abstract Rainfall intensity and duration of storms has been shown to influence the triggering of debris flows. After examining storm records of the San Francisco Bay region, documenting when debris flows occurred, and measuring piezometric levels in shallow hillside soils, continuous high-intensity rainfall was found to play a key role in building pore-water pressures that trigger debris flows. Debris flows in 10 storms between 1975 and 1984 in a 10-km 2 area near La Honda, California, were examined, and their rainfall records compared to the records of other storms to determine the antecedent conditions and the levels of continuous, high-intensity rainfall necessary for triggering debris flows. No flows were triggered before 28 cm of rainfall had accumulated each season, which suggests that prestorm soil-moisture conditions are important. After this sufficient antecedent rainfall, a threshold of rainfall duration and intensity—which accounted for triggering at least one debris flow per storm within the study area—was identified. The number of debris flows increased in storms with intensity and duration characteristics significantly above this threshold. By studying where debris flows initiated in storms of different intensity and duration, debris flow susceptibility was found to depend on soil thickness and hillside concavity and steepness. Moderate intensity storms of long duration triggered complex soil slump/debris flows in thick soils on concave slopes below large drainage areas, whereas high-intensity storms of short duration caused complex soil slide/debris flows in thinner soils without respect to size of drainage area. From these observations, an empirical model based on geology, hydrology, and topography is proposed to account for the triggering of debris flows at selective sites by storms with different combinations of intensity and duration once the antecedent and intensity-duration thresholds are exceeded.

Abstract Debris flows following Are are a common, but poorly understood, problem in southern California. Research to date suggests that they result from greatly accelerated rates of surface erosion by both wet and dry processes during the days and weeks following a fire. Significant amounts of hillslope debris are delivered to stream channels during the fire by a process called dry ravel. An important feature of postfire erosion is the rapid development of extensive rill networks on hillslopes. These rill networks are linked to a layer of water-repellent soil that forms a few millimeters below the ground surface during the fire. These rill networks result from numerous, tiny debris flows that occur on the hillslopes during the early storms. The rill networks form rapidly, often in a matter of minutes, and provide an efficient means for transporting surface runoff to stream channels. This helps explain why postfire debris flows often occur during very small storms and after short periods of rainfall.

Abstract This chapter presents a method by which morphometric criteria can be used to obtain a rapid first-approximation of potential debris flow hazard on alluvial fans in the Canadian Rocky Mountain Front Ranges. Geomorphic and sedimentologic evidence indicates that many fans are affected by debris flow processes. Such fans generally are steeper than 4° and have small, steep first- or second-order drainage basins with Melton’s ruggedness number ( R ) more than 0.25 to 0.3. Fans not prone to debris flows are dominated by fluvial processes and have gentler slopes in less rugged third-order or higher drainage basins. This morphometric approach should have wide applicability for continuously graded basins in unglacierized regions.

Abstract The central and southern Appalachian region experiences intense rainfall events that trigger episodes of debris slides and debris flows. High rainfalls may be preceded by wet periods, normal conditions, or droughts, and still result in rapid mass movements. Most slides and flows occur in existing hillslope depressions and move downslope. The bedrock-soil contact is the most common movement interface, although slippage and flowage are also common in deep soils. Lithologic, structural, soil, vegetative, and land-use influences on mass movements are identifiable in some areas, yet not apparent in others. Better data on precipitation thresholds, movement mechanisms, and slide and flow precursors are urgently needed. Accelerating tourism growth rates and development of mountainous areas are accompanied by greater losses of human property and life caused by slope failures. The dangers of rapid debris slides and flows threaten increasing numbers of people in developing areas.