H43G-01 13:40h
Experimental Insights on Friction at the Base of Geophysical Flows
We have studied in the laboratory granular mass flows of rock fragments using a high-speed video camera at 2000 frames per second. These flows were generated using beds of pumice fragments positioned on a rough rotating disk whose angular velocity is controlled by a motor. These pumice fragments have an irregular shape and an average sieve diameter equal to 8.75 mm. Our experimental results suggest the validity, on average, of the same Coulomb's relationship between shear and normal forces at the base of granular mass flows irrespective of their Savage number value (the Savage number represents the ratio between grain collision stresses and gravitational grain contact stresses). In Coulomb's law the shear stresses do not depend on the shear rate. We expect the results of our analysis to be valid, for example, in pyroclastic flows, debris flows and rock avalanches. Therefore, our experiments suggest that geophysical flows do not behave as Bagnoldian grain flows and they do not behave as Bingham fluids with a yield strength. Our experiments suggest also that, in natural pyroclastic flows, relatively large pumice fragments can be totally eroded by collisions and turned into the fines that form the overriding ash clouds. In our experiments, the amount of this ash increases when the value of the Savage number increases.
H43G-02 13:55h
Measurement of vertical and basal shear forces from debris flows at the Illgraben observation station, Switzerland
The water content of debris flows influences the bulk behavior of the flow, yet it has rarely been directly measured in the field. In December 2003 we expanded our Illgraben debris flow observation station to directly measure vertical and shear forces as debris flows pass. The Illgraben catchment produces several debris flows per year, ranging from classical granular or stony debris flows with many large boulders to muddy debris flows with few boulders. The new device consists of a 4m wide by 2m long steel plate installed flush with the channel bed, immediately upstream of the brink of a check dam. The plate is equipped with transducers to measure both vertical and shear forces. An overhead radar sensor allows estimation of the flow depth. The first two successfully-measured debris flows (flow depths up to 1.5m, particles up to several m in diameter) were characterized by multiple surges. The vertical and shear forces suddenly increased with surge arrival and then stabilized after passage of the front. Qualitatively, the force data are in agreement with video images showing the arrival of individual surges and large boulders rolling across the plate. Using the data immediately following the passage of each surge, we estimate bulk densities ranging from 1500 to 2200kg/m3 for very watery and granular surges, respectively. Unfortunately our attempts to measure basal fluid pore pressure have only yielded results for fluvial sediment transport events but not yet for debris flows.
H43G-03 14:10h
Role And Behaviour Of Clay Minerals In Alpine Debris Flows
The role of clay minerals is generally considered as negligible in alpine debris flow studies and experiments. This assumption is discussed on the basis of field and experimental data. In two neighbouring alpine catchments, with similar morphology and contrasted geology, the physical, mineralogical and chemical properties of the fine earth ($<$2mm fraction) of soils, debris flow deposits, and parent materials were compared. The clay minerals (clay and silt size) were extracted and characterized for mineralogy and cation exchange capacity (CEC). Their critical coagulation concentrations (CCC) were determined in laboratory experiments. The soil and surface solutions were sampled and monitored for chemical composition, which were compared to the experimental CCCs. The chemical equilibration time between soil solution and solid phase was tested on saturated paste extracts. It was observed that the fine earth of the debris flow deposits was not representative of the triggering materials in terms of fabric, density, particle size, particle shape and mineralogy. Results show that the particles were rounded, crushed and segregated during the flow. The parent materials were composed of platy particles with a small bulk density suggesting an edge-to edge fabric. It contained phyllosilicates of silt and clay size, coated with transformed organic matter. The deposits were composed of rounded particles with a large bulk density suggesting a close-packed fabric. Compared to the parent materials, the deposits contained a larger amount of large mono-crystalline particles of either quartz or calcite with round shape, and less small platy particles of phyllosilicates. The CEC of the fine earth and of the clay-size and silt-size particles was consequently about two times higher in the parent materials than in the deposits. This was interpreted as the results of two main phenomena, namely (i) the leaching of the suspended colloidal particles out of the deposits during the depositional stage, and (ii) the crushing of the larger non-colloidal particles during the flow. The CCCs of the extracted clay minerals were close to CCC values reported in the literature for similar minerals. The solutions reached equilibrium with the solid phases within 20min of equilibration time in the laboratory. The field solid phases and surface waters were at equilibrium with the solid phases. There concentrations were only slightly higher than the CCCs, thus allowing for flocculation of the minerals. However, it is most probable that the concentrations of the soil and surface waters drop largely below the CCC values during snow-melt or heavy rainfall. Furthermore, the CCCs were determined in the lab without shaking energy. During the flow in the field, the phyllosilicates will be more easily dispersed because additional shaking energy sharply increases the CCC.
H43G-04 14:25h
A Physical-Based Model of Postfire Hillslope Dry Ravel
Dry ravel is a gravity-induced downslope surface movement of soil grains, aggregates, and rock materials that commonly occurs on steep hillslopes after disturbances such as wildfires. A quantitative analysis and model of dry ravel is needed to understand the behavior and effects of dry ravel movement. In this study, we developed a physical-based model based on classic mechanical laws and particle flow theory. The model predicts source locations, movement path, and deposition areas of dry ravel produced after wildfires. The short-term dry ravel process is computed with theoretical calculations, and the long-term effects are described with both theoretical calculations and empirical probabilistic characterization. We assessed the sensitivity of the input coefficients, and provide a suitable range for these input coefficients. To calibrate and validate the model, we calculated dry ravel estimates from a wildfire area in the San Dimas Experimental Forest in southern California, and compared with experimental results. The comparison shows that the model is suitable for analyzing dry ravel on steep slopes, with easily-weathered parent material.
H43G-05 14:40h
An Approach for Assessing Post-Wildfire Debris-Flow Hazards Applied to Basins Burned by the 2003 Old and Grand Prix Wildfires in Southern California
The increased incidence of catastrophic wildfires in the western United States and the encroachment of development into fire-prone ecosystems have created a critical need for methods to quantify potential hazards posed by debris flows produced from burned watersheds. Debris flows are one of the most hazardous consequences of rainfall on recently burned hillslopes. Statistical models developed to estimate the probability and magnitude of post-wildfire debris-flow activity were used to generate debris-flow hazard maps for basins burned by the Grand Prix and Old fires in October 2003 in southern California. The probability maps are based on the application of a logistic multiple-regression model that describes the percent chance of debris-flow production from an individual basin as a function of areal burned extent, soil properties, basin gradients, and storm rainfall. The peak discharge maps are based on application of a multiple-regression model that estimates debris-flow peak discharge at a basin outlet as a function of basin gradient, areal burn extent, and storm rainfall. In response to 25-year, 10-year, and 2-year recurrence rainstorms, conditional probabilities of debris-flow occurrence ranged between 0 and 99%, and estimated peak discharges ranged between 10 and 190 m3/s. These maps identify those basins that are most prone to large debris-flow events and provide information for mitigation and evacuation planning. Evaluation of the response of the December 25, 2003, rainstorm that triggered debris flows and flooding from the recently burned basins allows for an assessment of the effectiveness of the mapping approach and provides guidance for refinement of the models.
H43G-06 14:55h
Using Controlled Landslide Initiation Experiments to Test Limit-Equilibrium Analyses of Slope Stability
Most studies of landslide initiation employ limit equilibrium analyses of slope stability. Owing to a lack of detailed data, however, few studies have tested limit-equilibrium predictions against physical measurements of slope failure. We have conducted a series of field-scale, highly controlled landslide initiation experiments at the USGS debris-flow flume in Oregon; these experiments provide exceptional data to test limit equilibrium methods. In each of seven experiments, we attempted to induce failure in a 0.65m thick, 2m wide, 6m$^{3}$ prism of loamy sand placed behind a retaining wall in the $31\deg$ sloping flume. We systematically investigated triggering of sliding by groundwater injection, by prolonged moderate-intensity sprinkling, and by bursts of high intensity sprinkling. We also used vibratory compaction to control soil porosity and thereby investigate differences in failure behavior of dense and loose soils. About 50 sensors were monitored at 20 Hz during the experiments, including nests of tiltmeters buried at 7 cm spacing to define subsurface failure geometry, and nests of tensiometers and pore-pressure sensors to define evolving pore-pressure fields. In addition, we performed ancillary laboratory tests to measure soil porosity, shear strength, hydraulic conductivity, and compressibility. In loose soils (porosity of 0.52 to 0.55), abrupt failure typically occurred along the flume bed after substantial soil deformation. In denser soils (porosity of 0.41 to 0.44), gradual failure occurred within the soil prism. All failure surfaces had a maximum length to depth ratio of about 7. In even denser soil (porosity of 0.39), we could not induce failure by sprinkling. The internal friction angle of the soils varied from $28\deg$ to $40\deg$ with decreasing porosity. We analyzed stability at failure, given the observed pore-pressure conditions just prior to large movement, using a 1-D infinite-slope method and a more complete 2-D Janbu method. Each method provides a static Factor of Safety (FS), and in theory failure occurs when FS $\leq$ 1. Using the 1-D analysis, all experiments having failure had FS well below 1 (typically 0.5-0.8). Using the 2-D analysis for these same conditions, FS was less than but closer to 1 (typically 0.8-0.9). For the experiment with no failure, the 2-D FS was, reassuringly, $>$ 1. These results indicate that the 2-D Janbu analysis is more accurate than the 1-D infinite-slope method for computing limit-equilibrium slope stability in shallow slides with limited areal extent.
H43G-07 15:10h
Regulation of Landslide Motion by Dilatancy and Pore-pressure Feedback
A new mathematical model demonstrates how diverse styles and rates of landslide motion can result from regulation of basal Coulomb friction by dilation or contraction of water-saturated basal shear zones. Normalization of the model equations shows that feedback due to coupling between landslide motion, shear-zone volume change, and pore-pressure change depends on a dimensionless parameter $\alpha$, which in turn depends on the dilatancy angle $\psi$ and the intrinsic time scales for pore-pressure generation and dissipation. If soil in the shear zone contracts during slope failure, then $\alpha$ $<$ 0 and positive pore-pressure feedback and runaway acceleration are inevitable. If the shear-zone soil dilates, then $\alpha$ $>$ 0 and negative feedback permits slow, steady landslide motion to occur while rain infiltration supplies positive pore pressure. Predicted steady-state slip velocities v obey v = -(K/$\psi$)p , where K is the shear-zone hydraulic conductivity and p is the normalized (dimensionless) negative pore pressure generated by dilation. If pore-pressure buildup due to rain infiltration continues unabated, or if shear-zone soil density approaches a steady state (i.e., if $\psi$ decays to zero) as landslide motion proceeds, then slow stable motion eventually gives way to unbounded acceleration. Furthermore, if basal Coulomb friction decreases as a function of slip rate, landslide motion can exhibit stick-slip instabilities. Such instabilities occur only in a particular region of a two-dimensional parameter space defined by $\alpha$ and a dimensionless friction decay constant, r. Within the $\alpha$ - r parameter space, changes from stable to unstable sliding are manifestations of a Hopf bifurcation, which explains why seemingly similar landslides can exhibit disparate behavior.
H43G-08 15:25h
Quantifying topographic, structural, and lithologic controls on large landslides
The occurrence of large landslides often reflects material properties that favor slope instability. Uncertainty associated with analyses of individual bedrock landslides is often significant and we lack the ability to test and apply mechanistic slope stability criteria at the regional scale. In the Oregon Coast Range (OCR), deep-seated landslides within the gently folded Tyee Formation (Eocene deltaic-submarine ramp sediments) have been recognized, but their relationship to bedrock properties is poorly constrained. We developed an automated algorithm that uses the topographic signature (specifically the relationship between curvature and gradient) of deep-seated landslides to map their distribution. In contrast to steep and highly dissected terrain frequently identified as characteristic of the OCR (which exhibits steep, planar sideslopes and highly curved, low-gradient ridgetops and valleys), terrain prone to large landslides tends to have low values of both drainage density and curvature, and gradients that cluster between 0.16 and 0.44. Our analysis indicates that the distribution of failure-dominated terrain in our 10,000-km2 study area is influenced by systematic variations in sedimentary facies and bedrock structure. The fraction of terrain altered by large landslides (>100,000 m3) varies from 5% in the sand-rich (delta-slope and proximal ramp facies) southern section of our study area to ~25% in the north (distal ramp facies), coincident with an increase in the thickness of siltstone beds and a decrease in the sandstone:siltstone ratio. Structural controls are superimposed on facies-related variations as deep-seated landslides are frequently found on slopes whose downslope aspect corresponds to the bedrock dip direction. For 1516 strike and dip measurements in our study area, we calculated the fraction of proximal terrain (< 2.5 km) altered by deep-seated landsliding. In the sand-rich southern region, the proportion of proximal slide-dominated terrain increases modestly with bedrock dip; an increase in dip angle from 0° to 16° corresponds to a change from 1% to 9% in the fraction of local terrain shaped by landslides. In the silt-rich northern region, terrain altered by deep-seated landsliding is pervasive and an increase in dip from 0° to 16° corresponds to a change in the fraction of slide-prone terrain from 10% to 28%. This pattern likely reflects a bedrock-driven change in the mechanical requirement for instability; the increased frequency of weak, siltstone innerbeds in the northern region favors instability even where bedrock dips gently. Given structural and lithologic data, our calibrated model enables us to predict the occurrence of large landslides and their potential impacts.