H41B-0292 0800h
Experimental Investigation of Unconfined Granular Flows Along an Incline : When do Levees Form?
Natural granular flows such as rock avalanches, debris flows or pyroclastic flows are known to spontaneously channelize through the formation of levees along their sides. We report in this paper the results of laboratory experiments aimed at understanding this phenomenon in a simple flow geometry. The experiment consists in releasing granular material on a rough inclined plane at a constant mass flow rate. The resulting unconfined granular coulee has the shape of a finger descending the slope. Using image analysis, we measure the time evolution of both the flow morphological characteristics (width and thickness) and the surface velocity field for different values of the mass flow rate and the slope angle. Surface velocity measurements reveal the presence of static zones along the sides of the flow. Our main observation is that although the input mass flow rate remains constant during the whole experiment, the coulee never reaches a stationary state. On the contrary, the flow morphological characteristics constantly evolve with time : the granular flow slowly enlarges while its thickness decreases. If the flow is maintained for a sufficiently long time, it finally distabilizes leading to the formation of a wavy pattern along the sides of the coulee. As a result of this non-stationary flow dynamics, the final deposit morphology strongly depends on the duration of the flow. If the flow duration is short, the deposit exhibits a classical levees/channel morphology. However the levee thickness progressively decreases when the flow duration is increased. In the case of a very long flow, the levees eventually vanish. Systematic comparisons between numerical and experimental modelisations, especially on the transient dynamics of the levees/channel morphology might provide new constraints on the frictionnal constitutive behaviour of avalanches.
H41B-0293 0800h
Dramatical Impact Of Low Amounts of Swelling Clays On The Rheology Of Alpine Debris Flows
Field observations show that the role and amount of swelling clays in the complex hard suspensions of alpine debris flow type were underestimated (see Boivin et al., this session). This work aims at exploring to which extent the swelling clay content influences the global rheology of a flow of rock grains from which the size spectrum extends from clays to gravel. We made a sample from calibrated materials with a grain size distribution similar to that of a viscoplastic debris flow (Bardou et al., 2003). Four replicates were made with the same grading curve. The clay content of the samples was 2% dry weight only, and different 2:1 swelling clay to 1:1 clay ratio were used. The swelling clay ratio (SCR) was calculated as the percentage of 2:1 clay in the clay fraction of the bulk samples. The 1:1 clay was (industrial) kaolinite and the 2:1 clay was a natural soil smectite. The smectite content in the bulk sample ranged from 0% to 2% dry weight, corresponding to SCR ranging from 0 to 80%. The four prepared samples were sheared in the large-size apparatus fully described in Tattersall and Banfill (1983). This apparatus is based on the measure of the torque necessary to rotate an impeller immersed in the sample. The impeller has the form of an "H" and moves in a plane according to two parallel axes. The observed behaviour were very contrasted. The sample with SCR=0 was poorly sensitive to changes in the solid concentration, in contrast to the three samples with SCR$>$0. Moreover, a small change in the SCR of the clay fraction induced a dramatic change of the behaviour of the mixture. For SCR=0, only little changes in the rheological parameters of the bulk samples were observed with respect to changes in the solid concentration. On the contrary the rheological parameters of the bulk samples with SCR$>$0, apparently followed a power law according to solid concentration. These tests carried out in the laboratory accord with observations realised on natural debris flow material. Although these results can't be upscaled to a quantitative assessment of the effects of changes in the SCR in a dense granular suspension they have, however, has a direct implications for debris flow rheological studies. Even though weighting for a very little part of the materials, the swelling clays cannot be neglected in the analysis of such mixtures. Moreover, colloid properties of these clays are very sensitive to factors such as electrolyte composition, clay surface coatings and shaking energy. Therefore, the electrolyte used in test the materials, and chemical equilibration time within electrolyte and solid phase, should be carefully selected with respect to field conditions. References Bardou, E., Ancey, C., Bonnard, C. and Vulliet, L., 2003. A typological approach of debris flow useful for hazard assessement in the alpine area. In: D. Rickenmann and C. Cheng-lung (Editors), 3rd DFHM, Davos. Bardou, E., Bowen, P., Boivin, P., Banfill, P.F.G. submitted Effect of the Clay Type on the Rheology of an Heterogeneous Dense Granular Material. Implication for the Study of Alpine Debris Flow. Boivin, P., Bardou E., Pfeifer, H.-R.: Role And Behaviour Of Clay Minerals In Alpine Debris Flows. This session Tattersall, G.H. & Banfill, P.F.G. 1983: The rheology of fresh concrete. Pitman, London.
H41B-0294 0800h
Sediment Supply and Transport Processes in the Initiation Area of Debris Flows, Ohya Landslide, Japan
Debris flows in mountain streams and ravines cause severe natural hazards due to their high velocity, large volumes, and immense destructive power. In catchments with high sediment supplies and steep channels, debris flows are generally caused by channel mobilization. In these catchments, there is a possibility that changes in quality and quantity of channel deposits influence the initiation of debris flows, because channel deposits constitute the main material of the flow as well as determine the hydrological initiation conditions. However, temporal changes in quality and quantity of deposits in steep channels have not been evaluated because of difficulties related to field observations. Here we examine changes of channel deposits in the initiation zone based on field observations in upper Ichinosawa catchment of the Ohya landslide in Japan. In spring 1998, a monitoring system was installed consisting of video cameras, ultrasonic sensors, capacitive water depth probes and water pressure sensors to discriminate the occurrence of debris flows and assess the rainfall-runoff processes of the catchment. Channel deposits of upper Ichinosawa catchment were periodically photographed to estimate their volume and diameter. Such measurements indicate that sediment supply by freeze-thaw increases the volume of channel deposits and decreases the diameter of bed surface material, whereas occurrence of debris flows decrease the volume of channel deposits and increase the diameter of bed surface material. Influence of other sediment supply processes (landslide and surface erosion) and bedload and suspended sediment transport on changes in volume and diameter of channel deposits is small compared with freeze-thaw and debris flow processes. Physical analyses indicate that sediment supplies and transport processes are largely influenced by gradients of hillslopes and channels. Field observations show that volume and diameter variations in channel deposits cause changes in rainfall-runoff processes. These seasonal changes in channel deposits may be important for estimating volume and timing of debris flows.
H41B-0295 0800h
Prediction of Mass Wasting, Erosion, and Sediment Transport With the Distributed Hydrology-Soil-Vegetation Model
Erosion and sediment transport in a temperate forested watershed are predicted with a new sediment module linked to the Distributed Hydrology-Soil-Vegetation Model (DHSVM). The DHSVM sediment module represents the main sources of sediment generation in forested environments: mass wasting, hillslope erosion and road surface erosion. It produces failures based on a factor-of-safety analysis with the infinite slope model through use of stochastically generated soil and vegetation parameters. Failed material is routed downslope with a rule-based scheme that determines sediment delivery to streams. Sediment from hillslopes and road surfaces is also transported to the channel network. Basin sediment yield is predicted with a simple channel sediment routing scheme. The model was applied to the Rainy Creek catchment, a tributary of the Wenatchee River which drains the east slopes of the Cascade Mountains, and Hard and Ware Creeks on the west slopes of the Cascades. In these initial applications, the model produced plausible sediment yield and ratios of landsliding and surface erosion , when compared to published rates for similar catchments in the Pacific Northwest. We have also used the model to examine the implications of fires and logging road removal on sediment generation in the Rainy Creek catchment. Generally, in absolute value, the predicted changes (increased sediment generation) following fires, which are primarily associated with increased slope failures, are much larger than the modest changes (reductions in sediment yield) associated with road obliteration, although the small sensitivity to forest road obliteration may be due in part to the relatively low road density in the Rainy Creek catchment, and to mechanisms, such as culvert failure, that are not represented in the model.
H41B-0296 0800h
Landslide Susceptibility Index Determination Using Aritificial Neural Network
The occurrence of landslide is the result of the interaction of complex and diverse environmental factors. The geomorphic features, rock types and geologic structure are especially important base factors of the landslide occurrence. Generating landslide susceptibility index by defining the relationship between landslide occurrence and that base factors using conventional mathematical and statistical methods is very difficult and inaccurate. This study focuses on generating landslide susceptibility index using artificial neural networks in Southern Japanese Alps. The training data are geomorphic (e.g. altitude, slope and aspect) and geologic parameters (e.g. rock type, distance from geologic boundary and geologic dip-strike angle) and landslides. Artificial neural network structure and training scheme are formulated to generate the index. Data from areas with and without landslide occurrences are used to train the network. The network is trained to output 1 when the input data are from areas with landslides and 0 when no landslide occurred. The trained network generates an output ranging from 0 to 1 reflecting the possibility of landslide occurrence based on the inputted data. Output values nearer to 1 means higher possibility of landslide occurrence. The artificial neural network model is incorporated into the GIS software to generate a landslide susceptibility map.
H41B-0297 0800h
A Simple Expedient Method for Interpreting the Landscape With Digital Imagery Tools: The Secret Srings Landslide as an Example
The volcanic borderland between California and Oregon is a remote, pristine area, with limited geologic/environmental information available. This landscape is dominated by the 300-500 m Klamath River gorge, which is incised through Cascade volcanic rocks. Weak pyroclastic rocks exposed in the gorge facilitated the development of massive landslides, the largest of which is the Secret Spring Landslide (Williams 1949), (Hammond 1983), (US Forest Service 1983, 1996), (Hazlett, et al, 1997). It is 2- 4 km wide, 5 km long, and located south of the river, about 11 km upstream from Copco Reservoir. Three dimensional images (from 30 m DEM's) viewed on a monitor screen were used to stratify the landscape around the landslide in an iterative, intuitive process, as follows: a) Observe feature in an image; b) Inspect feature on air photos (stereoscopically) and in the field; c) Examine image again, modifying viewing parameters (scale, relief, sun angle, tilt, etc.) and make new interpretations. This process allows discrimination and delineation of various terrains, e.g., talus slopes., glacial features, landslide scarps, more active to less active landslides, fractures, faults, lineations, evidence of river blockage, abandoned stream channels, etc. Manipulation of viewing parameters is critical for interpretation. Modifying the parameters challenges the interpreter to reexamine the processes involved in shaping the terrain. Focusing on the regional view allows integration of innumerable details of the terrain to define each local view. For the digital imagery work "Hawkeye" digital software developed by M. Vannier was used, (copyright) 18 Software, Portland, OR (18software.com).
H41B-0298 0800h
A Combined Structural Geology and GIS Approach to Rockslides: an Example from Western Norway
The western coast of Norway presents an ideal area to study active rockslide development due to the recent post-glacial uplift. This study presents the preliminary results of a combined GIS-structural geology approach to the examination of a potentially catastrophic rockslide in the Romsdalen area of western Norway, a mountainous area, despite being well populated, that is particularly vulnerable to rockslides. Svarttinden is a 1600m high mountain lying on a 12-1300m plateau 1km from the southern edge of the Romsdalen Valley. Recent landslide activity from the mountain side under investigation is evinced by the presence of a debris fan, which has been previously dated at c.5000BP. The rockslide removed in the region of 5 millions m3 of rock material. The purpose of this study was to determine the cause of the previous slide and evaluate the likelihood of further rockslides from the same mountainside by applying GIS and structural geology. Preliminary investigations have shown that the mountain is dissected by a north-south trending, steeply-dipping brittle fault. This has acted as a transfer fault, delimiting the western extent of the palaeo-rockslide. The palaeo-rockslide failed along a single, flat-lying (30-35°) down-slop dipping brittle fault. Remnants of a fault breccia up to 20cm are found on this surface. Evidence exists for shearing on this structure and we consider this a major fault plane (MFP), along which the rockslide has occurred. SEM examination of the microstructures present in this fault gouge will be presented. The western half of this mountain, which lies to the east of the major north-south transfer fault, is underlain by the same low-angle fault gouge. The volume of the rock mass above this MFP is approximately 7 millions m3. Several other low-angle structures are present above the MFP, further weakening the rockmass. Up to several metres of down-slope displacement is observed on these structures. High angle tension fractures are abundant in the mountainside above the MFP, detaching down onto it. These structures increase in frequency and displacement downslope. The low-angle fault planes lie sub-parallel to a local, shallowly north-dipping foliation in the gneissic host-rocks and appear to be localized along fold discontinuities within the gneisses. These folds appear to have acted as a significant 'locking mechanism' for movement along the failure planes as evidence is seen for fault tip-zones buttressing against the high angle southern limbs of these folds and reverse high angle fault structures in the fold axial planes, representing local vertical extension as opposed to downslope shearing. Local ramp structures in the MFP led to the increased frequency of high-angle tension fractures. This suggests that the geometry of the MFP is probably a significant factor in changing the degree of fracturing of the potential rockslide rockmass and therefore may have an affect on the continuity of the rockmass prior to failure. To estimate the volume above the MFP a potential sliding surface was inferred in 3D from field observations and the concept of "sloping local base level" (SLBL). Using a digital terrain model, the SLBL permits to define a surface above which the rocks are assumed erodible (Jaboyedoff 2004). Then the spatial distribution of the shear stress on the sliding plane and the energy of propagation of blocks can be estimated and introduced in a GIS for hazards assessment and zoning. References Jaboyedoff, M., Baillifard, F., Couture, R., Locat, J., and Locat, P. 2004: Toward preliminary hazard assessment using DEM topographic analysis and simple mechanic modeling.
H41B-0299 0800h
Utilizing Tritium and CFC-12 to Determine Groundwater Sources in an Unconfined Aquifer Within the Abalone Cove Landslide, Palos Verdes, California
The Abalone Cove landslide occupies 80 acres of an ancient landslide complex on the Palos Verdes peninsula, and was re-activated in 1979. The uphill portion of the ancient landslide complex has remained stable in historic times. Water infiltration into the slide is a short term catalyst for mass movement in the area, so it is important to determine the sources of groundwater throughout the slide mass. Water may enter the slide mass through direct percolation of recent precipitation, inflow along the head scarp of the ancient landslide or by rising through the slide plane from a deeper aquifer. The objective of this contribution is to use geochemical tracers (tritium and CFC-12) in combination with numerical modeling to constrain the importance of each of these sources. Numerical models were constructed to predict geochemical tracer concentrations throughout the basin, assuming that the only source of water to the slide mass is percolation of recent precipitation. Predicted concentrations were then compared to measured tracer values. In the ancient landslide, predicted and measured tracer concentrations are in good agreement, indicating that most of the water in this area is recent precipitation falling within the basin. Groundwater recharged uphill of the ancient landslide contributes minor flow into the complex through the head scarp, with the majority of this water flowing beneath the ancient slide plane. However, predicted tracer concentrations in the toe of the Abalone Cove landslide are not consistent with measured values. Both CFC-12 and tritium concentrations indicate that water is older than predicted and communication between the slide mass and the aquifer beneath the slide plane must occur in this area. Infiltration of this deep circulating water may exert upward hydraulic pressure on the landslide slip surface, increasing the potential for movement. This hypothesis is consistent with the observation that current movement is only occurring in the area in which tracers indicate communication with the deeper aquifer.
H41B-0300 0800h
The Effect of Deep Ground Water on the Initiation of Kumanodaira Landslide in Central Japan
The hydrometric and isotopic analyses were carried out to study the groundwater flow processes at the Kumanodaira landslide scars in Gunma Prefecture, central Japan. Kumanodaira landslide, which occurred on 8th and 9th in June 1950, consists of two slope failure events and killed 50 people; total 2 mm of rainfall was monitored for two days before the disaster, however, 320 mm/day of rainfall had been monitored nine months before. Precipitation and stream flow have been monitored in four small catchments (K1, K2, K3, and K4: Kumanodaira landslide occurred in K4) in Kumanodaira area to elucidate the mechanism of Kumanodaira landslide. To realize the data use of antecedent precipitation, effective rain was defined as R$_{w}$=$\Sigma$a$_{1i}$R$_{1i}$; in which R$_{w}$ is the effective rain, a$_{1i}$ is the reduction coefficient of i hour before (a$_{1i}$=0.5$^{i/T}$: T is a half value period), and R$_{1i}$ is the one hour precipitation of i hour before. Two wells were also installed in K4; the Well A located relatively downslope of the catchment reaches the soil/bedrock interface, and Well B located upslope penetrates under the soil/bedrock interface. These data show that the rainfall-runoff response in K4 is much slower than that in the other catchments. The groundwater in shallow soil (Well A) was found to be corresponded to the effective rain with half value period of 2400 hours, whereas the groundwater in the bedrock (Well B) never corresponds to the effective rain with any half value periods. The stable isotopic ratio ($\delta^{18}$O) of rainfall, surface water, and groundwater samples from each catchment were analyzed. This analyses show that only $\delta^{18}$O of the Well B differs considerably from that of the others, implying that the water by heavy rainfalls during the summer season would supply to the Well B. Therefore these results suggest that groundwater flows as "piston flow" in the bedrock, responding several months after the rainfall. This caused significant difference in residence time from soil water, and outflow of the bedrock groundwater should triggered Kumanodaira landslide.
H41B-0301 0800h
Dynamic Triggering of Landslides
Dynamic stress generated by earthquakes is one of the major causes for triggering landslides. There are several techniques available to simulate the ground motion associated with these earthquakes and how it generates failure. These methods, however, are primarily based on triggering by shear failure; the role of tensile failure in earthquake triggered landslides is not clear. We have developed a method to investigate the dynamic stress associated with ground motion and show how this can be used to asses the role of generating shear and tensile failure in the initiation of slope instability. In this project, a 1D elastic finite element model is used to produce the dynamic stress generated by a given ground motion for a 2D infinite slope. This model allows us to test increasingly complex scenarios of an incoming wave with arbitrary time dependence. From the total stress tensor, the principal stress components are computed and displayed on the traditional Mohr circle. At the point of intersection with a failure envelope, stress has exceeded strength, causing the slope to fail. The location of intersection of the Mohr circle provides the orientation of the failure plane on the slope, as well as the failure mechanism (shear versus tensile). The Mohr-Coulomb failure envelope is utilized to determine shear failure situations. The Griffith failure criterion is a modified extension of the Mohr-Coulomb envelope and allows us to model for either shear or tensile failure. This resulting information is graphed pictorially giving us a view of how the incoming wave generates shear or tensile failure as it hits the failure plane. This project provides valuable insight into the generation of slope failure and the behavior over time that is important in the dynamic triggering of landslides.
H41B-0302 0800h
The Internal Structure and External Morphology of Submarine Landslides: A Causative Link
The internal structure of submarine landslide deposits has been described in detail by relatively few workers, and thus our understanding of them is very incomplete. General internal characteristics include zones of brittle and ductile deformation; normal and listric-normal faults and fault zones, reverse and thrust faults, folds, cleavages, and mixed or homogenized horizons and zones. The large variety of structures internal to landslide deposits implies that stress and strain are partitioned in a very complicated fashion, often varying greatly over short vertical and horizontal distances and differently within the various materials making up the mass-failure. Unraveling the structural makeup of these deposits, and linking this to facies variations within them allow us to help determine details of the stress and strain distribution and hence the mechanics of their movement and deposition. Fieldwork has lead to the following preliminary findings. 1) Slide horizons are characterized by coarser-grained facies than the mass-transported material, implicating elevated pore-fluid pressures as a primary lubricant in these mass-movements. 2) Various cleavages bearing consistent relationships to the direction of movement of the slide body, and the orientation of the slide horizons can be present in the landslide deposits. On the microscopic scale, these cleavages take the form of grain-rotation, grain-translation, and brittle fracture of grains or aggregates of grains. 3) The internal partitioning of strain is very complicated, on all scales, often leaving highly deformed strata immediately laterally adjacent to apparently internally undeformed strata. Some of these relationships can be interpreted in terms of failure systematics, which include both retrogressive failures and single event failures, and some are due to strain localization along lines of weakness in the deposit. 4) Outcrop-scale structures mimic seismic-scale structures, and include brittle and ductile deformation. Additionally, high-resolution 3D seismic data and field map relationships indicate that there is a direct physical relationship between the upper morphology, the internal structure, and the basal morphology of many submarine landslide deposits. This implies that the stresses `felt' by the base of a landslide are projected into the internal structure of the deposit, and directly influence the morphology of the resulting deposit (and hence the post-depositional sea-floor).
H41B-0303 0800h
Debris Flow Architecture and Processes in Offshore Trinidad: Implications for basin fill in tectonically active margins
The eastern continental margin of Trinidad is situated along the tectonically active oblique converging southeastern boundary of the Caribbean and South American plates and proximal to the Orinoco Delta. Factors that have contributed to gravitational instabilities in the shelf edge include high sedimentation accumulation rates, high frequency sea-level fluctuations during the Quaternary, frequent earthquakes and the abundance of methane hydrate. This volatile mix of factors favor the formation of episodic gravity induced deposits that have affected thousands of square kilometers of the deep marine environment. Debris flows are the predominant type of gravity induced deposits in the area. Multiple episodes of debris flow occurrence have been identified using nearly 10,000 square kilometers of three-dimensional seismic data that cover the entire eastern margin. Units can reach up to 250 meters in thickness and occur over 100's of kilometer square areas. Maps that have been generated for the uppermost flow show significant basal scour, up to 33 meters deep generated during passage of the flow. Scours also show divergent patterns in map view indicating changes in the flow conditions. Flow scour erosional shadows around prominent seafloor mud volcanoes preserving evacuated strata on the downslope side of these obstructions. Internal architecture shows high amplitude discontinuous and chaotic seismic facies, and stacked thrust imbricates association with compressional bends in the flow path. The scale and occurrence frequency of these features suggest that they may form a significant threat to submarine installations and possibly generate tsunamigenic waves that can threaten shipping and coastal communities.