H44A-01 16:00h
The Effect of Bottom Curvature on Mudflow Dynamics: Theory and Experiments
A one-dimensional mathematical model of mudflow dynamics is formulated assuming mud to behave rheologically as a Herschel-Bulkley fluid. Novel features of the present contribution regard the propagation of mud-flows over a curvilinear bottom. A set of nonlinear partial differential equations is found to govern the unsteady, non-uniform flow of such fluids. Following the approach of {\it Huang and Garc{\`{\i}}a} [1998], depth-integrated forms of the continuity equation, as well as shear-integrated and plug-integrated forms of the momentum equations are derived. A perturbation analysis is also carried out under the assumption of 'slender' flow. The leading order problem takes the form of a kinematic wave equation. A free oscillating numerical solution of this hyperbolic conservative equation is obtained using the flux corrected transport (FCT) method. Characteristics and run out distances of the mud-flow are studied. The effects of bottom curvature on mud-flow dynamics is analyzed. Experiments are performed releasing a given volume of Kaoline and water mixture from a reservoir into an upward concave wide channel. Comparison between the numerical solution and the experimental data are generally satisfactory, though suggesting limitations typical of any approach which neglect convective accelerations.
H44A-02 16:15h
Experimental Study of Surface Erosion by Granular Flows
Field studies suggest that in steep landscapes mass flows of coarse grained material may be the primary agents responsible for cutting canyons. No process-based theory for bedrock incision by such flows exists, and the infrequency of such events makes them impractical to study in the field. Stock (2003, Ph.D. UC Berkeley) has suggested that in the case of debris flows, the wear rate arises primarily from particle collisions with the bed. Therefore, the stresses on the bed should be correlated with the inertial stresses in the flow. Here, we explore the relationship between wear rate of synthetic bedrock and inertial stresses in granular flows by making measurements in a 60 cm diameter, 15 cm wide vertically rotating (horizontal axis) drum. The debris composition consists of varying amounts of gravel, water, and fines, from dry granular flows to muddy slurries. The shear rate is estimated from the difference between the surface and bottom velocities of the flow divided by the flow depth. We estimate Bagnold and Savage numbers for each experimental material and conditions range from inertially dominated to more viscous states. We measure erosion by differencing the initial and final mass of the imbedded erodible rock sample. For no-slip conditions, we observe that erosion rate increases with higher shear rates and larger grain diameters. Experiments with observable slip at the bed have a lower shear rate, but high erosion rates. To explore this further, we vary boundary roughness, affecting the amount of sliding that occurs at the bed of the flow. For no-slip conditions, the wear appears to occur at the front of the flow, when the faster-moving surface particles overtake the flow front and impact the bed. These results suggest that inertial stresses, which scale with shear rate and grain diameter, are correlated with erosion, but any sliding will alter the amount of wear. Consequently, bed roughness and its effect on bottom slip significantly affects bedrock wear. Results from these experiments are being used to guide the design of future experiments in a larger 4-meter diameter, 0.75 meter wide vertically rotating drum. These experiments will more accurately simulate the scale and behavior of natural debris flows.
H44A-03 16:30h
Modeling of Potential Lahars Motivated by Landslides in Crater Lake of Baitoushan Volcano
Collapse and landslides, occurred in many volcanic craters, could result in more dangerous disasters if they encounter with lake water. Baitoushan volcano, an intraplate stratovolcano in the Changbaishan volcanic field on the border between China and North Korea, contains a crater lake approximately 4.45km in length along its long axis, about 200m in average depth. The lake is surrounded by steep rock walls that rise up to 300 $\sim$400m above its surface. Several horseshoe-shaped scars in the walls indicate that some large-scale landslides had happened since the last enormous explosive eruption one thousand years ago. At present the walls still have the possibilities of rockfalls and landslides. If they entered the lake with high speed, could produce life-threatening waves. The waves would transmit through the lake surface and release flood instantly to the downstream from the outlet in the north of the crater, which transform into lahars mixing with the loose pyroclastic deposits along the hillslope, and destroy almost everything where they passed by. Our work aims to model the potential lahars motivated by the landslides of Baitoushan volcano and to predict inundation areas and travel time of flows. For this purpose the heights of waves triggered by those historic landslides have been estimated based on declivities' volumes and landslide speeds. HEC-RAS, a program of surface water model developed by U.S. Army Corps of Engineers, is utilized to simulate the potential lahars using the range of wave height as the boundary condition. The research results indicate that the lahars, on the one hand, might spend about eighty minutes reaching the nearest town at the foot of the volcano along Erdaobaihe River valley, and on the other hand possibly flood into the nearby rivers if the flux is remarkably large.
H44A-04 16:45h
Mega-landslides in eastern Tibet: Implications for landscape and river profile evolution, and the interpretation of tectonics from topography
Field work within the Yalong and Dadu Rivers, two major tributaries of the Yangtze River that dissect the eastern margin of the Tibetan Plateau, indicates that a high frequency of large, deep-seated landslide events, both modern and ancestral, have led to extensive and prolonged river damming throughout each of these deep river gorges. These observations highlight a strong feedback between hillslope processes and channel morphology that is prevalent throughout this landscape. The eastern margin of the Tibetan plateau is a rapidly evolving landscape adjusting to regional and localized surface uplift, climate changes, and large-scale river re-organization through river capture. The impact of mega-landslides introduces another important influence contributing to the transient state of river profiles in the region, further complicating the interpretation of profiles extracted from topographic data and the application of river incision models. The role of mega-landslides must be considered in trying to understand how river profiles can be used to highlight important aspects of the tectonic and geomorphic evolution of the plateau margin. Landslides exert a first order control on river profile and valley morphology, and often lead to the formation of strath terraces and bedrock gorges unrelated to long-term bedrock river incision rates. Profile knick-points are consistently related to stretches of river dominated by boulder deposits delivered to the channel by large landslides. Landslide deposits typically armor the bed and force river channels to steepen and narrow. When particularly large, landslides dam the valley and trigger upstream aggradation. River incision sets the pace of landscape lowering, but in eastern Tibet, landslides appear to be significantly inhibiting river incision. We are currently analyzing samples collected in the field to gather age information for various landslides and gain a better understanding of the impact, past and present, of mega-landslides in eastern Tibet river gorges. We are dating various features related to impoundment, such as lake sediments, fill terraces, in-place landslide boulders, using three quaternary dating methods: cosmogenic radio nuclides (CRN), optical stimulated luminescence (OSL), and carbon 14 (14C). This will help in determining the age, and perhaps duration, of individual landslide dam interruptions. Preliminary data indicates that landslide deposits can linger within the fluvial system, inhibiting bedrock incision, for $>$40,000 years.
H44A-05 17:00h
Earthquake Induced Landslides in the Old City of Zefat, N Israel; History, Current Hazard and the Influence of 2000 Years of Human Habitation
The old quarter in the city of Zefat (Northern Israel) has a long history of repeated damage from earthquakes, probably due to earthquake-induced landslides (EILS). In this study we examine field and historical evidence for static and dynamic slope instability in Zefat and evaluate the current EILS hazard in light of its long (ca. 2000 years) human habitation that profoundly changed the natural geotechnical conditions. The city of Zefat is located on the slope of an elevated region bordering the seismogenic Dead Sea transform. The bedrocks are mainly marls, chalks and limestones of Santonian to Eocene times, but most of the city is built on more than 10 meters of anthropogenic talus/strata of two types: (1) talus-like: inclined ($>$$30\deg$) layers of pebbles and pottery embedded in unconsolidated earth fill and (2) ruins filled and covered by earth. Meter-scale slumps and mudflows are common in this material during the rainy season. We evaluate the EILS hazard with GIS based Newmark analysis using as base layers D.T.M with 625m-sq grid cell size and a geotechnical map. Each exposed geotechnical unit was assigned mechanical properties (cohesion, angle of internal friction, density) reflecting its lithology and slope stability properties. For calibration of the assigned mechanical model, we first analyzed the M=6, October 1759 and M=7, January 1837 earthquakes, both with distance to fault rapture zone (R) of about 15km. Sites of slope failure in these earthquakes were mapped using field evidence and historical chronicles of damaged synagogues. The M=5 earthquake of August 1984 (R=50km) with no EILS was also analyzed. Calibration was satisfied when calculated slope failure was comparable to the mapped historical reports and field observations of slope instability. To evaluate the current hazard, 20 theoretical earthquakes in the magnitude range of 4-7 and epicentral distances of 10-120km were analyzed, using the calibration described above. We found that M=7, M=6 and M=5 earthquakes will induce landslides in Zefat in R of more than100km, 50km and 10km respectively. Most slope failure will occur in the anthropogenic talus. Structures and lifelines founded in the talus will probably be heavily damaged. Anthropogenic talus is common in Near-East cities with long habitation history. Understanding its seismic behavior will reduce damage in future earthquakes.
H44A-06 17:15h
Potential of the seismic monitoring for the understanding of gravitational instabilities
The macroscopic deformation of rocks, for scales ranging from that of laboratory sample (cm) to that of the rock massive and earth crust, is associated to local irreversible processes (cracks/faults propagation and shearing). These fast movements involve acoustic wave's propagation, which can be observed by remote sensing. Thus, the seismic monitoring during the strain progression can help to a better understanding of rock behaviour and can lead to the determination of failure precursors. Despite of this strong comprehensive potential, this observational tool has not been often used for the study of gravitational instability. Here we present seismic monitoring data's concerning rock slopes instability of size ranging from 103 m3 (cliffs of Mesnil-Val in north-western France, and Valabres south-eastern France) to 50.106 m3 (Deep seated instabilities of ``La Clapiere'', and ``Les Ruines de Sechilienne'', both located in the French Alps). In a general manner, these results show that recordable seismicity exists in the frequency range of 1 Hz to 10 kHz, associated with the rock material deformation. In the case of deep seated instability, we observed large fluctuations of the seismic activity which appear to be related to the seasonal variations of the displacement velocity measured at the slope surface. At the lower scale of the two cliffs studied here, we observed behaviours that could be proposed as candidate for precursors of collapse. For the first case, we observed a huge increase of the seismic activity few hours before a collapse. The seismic rate increase was several magnitude orders larger than the mean seismic activity what permits us to propose this increase as a precursor of the collapse. In the second case, the location of seismic sources permits to identify active deformation zones corresponding to rock mass discontinuities directly observable on the cliff surface. These results indicate the zones on which the failure may occur in the future. This should be verified in the case of a collapse occurs, if any. Overall, these results show the strong potential of the seismic monitoring for reaching a better knowledge of the mechanisms of rock masses deformation and of gravitational instabilities. The determination of operational failure precursors basing on the seismic monitoring is not yet reached but is a major target of this research.
H44A-07 17:30h
Powder Snow Avalanches - Results From Vall\'ee de la Sionne, Winter 2004
Powder snow avalanche surfaces can be effectively tracked using videogrammetry. 3-dimensional surface maps were generated from two digital video recordings of an artificially released avalanche, captured from different places. From these maps the avalanche volume and front velocity is inferred. Several different types of sensor are mounted on a mast in the avalanche path. These include impact pressure sensors, optical velocity sensors and a pitot air pressure sensor. In this paper the air pressure data and the information it gives about the air flow both outside and inside the avalanche is described in detail. A simple dipole approximation is used to estimate the speed and size of the avalanche and a comparison with the videogrammetric analysis can be made. In addition the internal structure of the avalanche is considered and its implications for modelling discussed.
H44A-08 17:45h
Energy and dissipated work in snow avalanches
Using the results of large scale avalanche experiments at the Swiss Vallee de la Sionne test site, the energy balance of several snow avalanches is determined. Avalanches convert approximately one-seventh of their potential energy into kinetic energy. The total potential energy depends strongly on the entrained snowcover, indicating that entrainment processes cannot be ignored when predicting terminal velocities and runout distances. We find energy dissipation rates on the order of 1 GW. Fluidization of the fracture slab can be identified in the experiments as an increase in dissipation rate, thereby explaining the initial and rapid acceleration of avalanches after release. Interestingly, the dissipation rates appear to be constant along the track, although large fluctuations in internal velocity exist. Thus, we can demonstrate within the context of non-equilibrium thermodynamics that -- in space -- granular snow avalanches are irreversible, dissipative systems that minimize entropy production because they appear to reach a steady-state non-equilibrium. A thermodynamic analysis reveals that fluctuations in velocity depend on the roughness of the flow surface and viscosity of the granular system. We speculate that this property explains the transition from flowing avalanches to powder avalanches.