Instabilité gravitaire | EPFL Graph Search

vignette|Cônes d'éboulis sur la rive nord de Isfjorden en Norvège. L’instabilité gravitaire est également connue sous le nom de processus de pente, ou mouvement de masse rocheuse. Ce processus géomorphologique implique des mouvements de roches (sol, substrat, régolithe...) le long de l'inclinaison d'une surface topographique sous l'effet de la gravité. Derrière ce nom, se cachent différents processus comme la solifluxion, les glissements de terrain, les laves torrentielles, les avalanches, les effondrements... Les dimensions de ce type de processus peuvent varier de plusieurs ordres de grandeur et peuvent se mettre en place sur Terre mais également sur Mars, Vénus, Titan, etc. Lorsque la force de gravité excède les forces de résistance des matériaux, une rupture dans le sol va alors provoquer cette instabilité gravitaire. La stabilité de la pente est ainsi contrôlée par la cohésion, la friction interne et l'état de contrainte. La pente la plus forte que le matériau sans cohésion puiss

The geomorphometry of endorheic drainage basins: implications for interpreting and modelling their evolution

Jean-Marc Dorsaz, Andrea Rinaldo

Endorheic basins are catchments with no hydrological connection with marine environments. They cover 20% of the Earth's surface, and are mostly located in arid regions. Their drainage networks converge to lakes, salt flats or alluvial plains, whose dynamics are strongly driven by precipitation, evapotranspiration and groundwater discharge, among other factors. Integrated surface drainage and the creation of whole drainage systems typical of open basins are commonly restricted in these regions. Interestingly, the fluvial basin morphology of endorheic basins has not been extensively studied, and a variety of quantitative morphological descriptors used in open basins have not been utilized in the geomorphic analysis of endorheic basins. The objective of this study is to better understand the basin morphology of endorheic river basins by using well-known geomorphological properties and their variations across scales. For three basins in northern Chile we computed the following descriptors and the corresponding relevant scales: the cumulative distribution of contributing area, the horizontal shape of the basins (i.e. Hack's law, normalized Euclidean length, and sinuosity of the streams), slope-area relationship, Horton's ratios and drainage density. We detected several properties typically found in open basins, but certain features which seem to be unique to closed basins were also identified. In particular, we found that horizontal and vertical geomorphic features seem to be linked, which suggests that an independent treatment of these features may not be appropriate for closed basins. Similar results were found regardless of the basin area, which illustrates the relevant effects of features that are specific to these particular regions. It is expected that our findings will improve both the geomorphic assessment of these basins and hydrological modelling of surface water and groundwater. Copyright (c) 2013 John Wiley & Sons, Ltd.

Wiley-Blackwell2013

Streambank erosion hazard mapping

John Beck

Streambank erosion hazard mapping has received much less attention than flood inundation mapping in the past due to the complexity of the task as well as bank protection works that have reduced bank erosion and unfortunately, the ecological functions of our watercourses at the same time. Damages due to streambank erosion in some flooding contexts are greater than the flood water damages (Loat and Petrasheck, 1997). For these reasons, streambank erosion hazard mapping should be an integral part of flood hazard mapping and methods must be developed to accomplish it. This research proposes a methodology for mapping streambank erosion hazards based on the directives of the Swiss Federal Office for Water and Geology (now within the Swiss Federal Office for the Environment). It permits the calculation of bank failure widths and their probability as opposed to future channel migration paths. This research also investigates the input data necessary for streambank erosion hazard modeling. Geomorphological mapping must be the first step to streambank erosion hazard mapping as it permits the identification of the sediment movements in the catchment. After this step, modeling of streambank erosion can be undertaken. Geofluvial models that combine hydraulic sediment transport and geotechnical modeling are well suited for streambank erosion modeling. The model CCHE1D is such a model and was adapted for the calculation of the streambank erosion hazard on an 8 kilometer reach of the Lower Venoge River, Switzerland. CCHE1D performs one dimensional hydraulic calculations. A shear stress correction function based on channel curvature distributes mean boundary shear stress appropriately to outer and inner bank toes and the phase lag of maximum toe shear stress compared to the apex of the bend curvature is ensured by a convolution of upstream shear stresses. Tension cracking was added to the slab failure algorithm due to its significant effect on bank failure widths. After a bank failure, the cross section shape does not change which allows the flow conditions to remain the same and in turn allows the probability of failure for the modeled bank profile to be evaluated. To gain a better understanding of streambank erosion on the Lower Venoge River, detailed erosion and flow depth monitoring were done on two 1 kilometer river reaches from November 2003 through September 2005. These measurements showed the mass failures to be mainly soil falls and cantilever failures. Measured bank erosion was linearly related to the product of maximum discharge and flood volume. Bank and bed sediment data were also collected for the study reach. Eighty-two cross sections were surveyed in 2004 in the 8 kilometer study reach. A new cross section tool was developed to properly reproduce scour holes in bends. It calculates transverse position and distance, graphs the cross section to allow identification of bank definition points, linearly interpolates, calibrates bed topography parameters based on surveyed cross sections, and interpolates with respect to channel curvature. Hydrological modeling allowed for the generation of input hydrographs for the period January 1979 - February 2005. This information was combined with historically based low probability floods to construct three 300 year discharge series. Flow and erosion measurements allowed for the calibration of roughness and critical shear stress parameters, respectively, in the CCHE1D model. Where detailed erosion measurements were not available, past channel migration served as a guide for estimating the critical shear stress. Calibrated critical shear stress was poorly correlated with measured bank properties indicating the necessity of measuring critical shear stress. A regression equation involving the percentage of fine sand – large silt and the fraction of non-vegetated bank explained 75% of the variation. The three 300 year discharge series were simulated with CCHE1D. Bank failures series were output for each of the banks of the 1149 computational nodes in the study reach. Empirical frequency was used to determine the bank failure width of a given probability. These bank failure widths and their probabilities were used to calculate a streambank erosion danger. To further qualify this danger, it was mapped with a bar proportional to the mean annual simulated erosion rate. The extreme failure width for the entire reach was determined by multiplying the maximum simulated failure width by a safety factor. The erosion hazard in straight reaches is too high showing that the shear stress reduction due to the highly vegetated banks needs to be taken into account better in the shear stress correction function. This research has demonstrated the feasibility of streambank erosion hazard mapping, although the quantity of input data necessary is prohibitive. Data acquisition methods must be researched and improved to reduce costs, and research must be continued to improve understanding of bank failure processes to be included in geofluvial models.

EPFL2006

Laboratory Investigation on Seismic Response of Rock Fractures Filled with Granular Materials

Wei Wu

Filling materials may exist in all scales of rock fractures, not only influencing seismic wave attenuation, but controlling rock mass instability. The objective of this study is to experimentally investigate the seismic response of rock fractures filled with granular materials. This experimental study simplifies the complex interaction between seismic waves and filled rock fractures and considers the seismic-induced compressive responses of a filled single fracture and a set of filled parallel fractures and the seismic-induced frictional slip on a filled fracture. The seismic-induced compressive response of a filled single fracture is investigated by a split Hopkinson rock bar (SHRB) test. The SHRB test simulates the P-wave propagation across the filled fracture and verifies the analytical solutions predicted by the displacement and stress discontinuity model. The seismic wave attenuation across the filled fracture is due to wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The specific fracture stiffness and the specific initial mass of the filling materials are two key parameters in interrelating the physical, mechanical and seismic properties of the single fracture filled with dry granular materials. The SHRB test is modified to study the seismic-induced compressive responses of a set of filled parallel fractures and to verify the analytical solutions predicted by the modified recursive method based on the displacement and stress discontinuity model. The seismic wave attenuation across the filled parallel fractures is due to multiple wave reflections between the filled fractures, as well as wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The dynamic compaction of the filling materials induces the decreases of loading rate and dominant frequency when the P-wave propagates across each filled fracture in a fracture set. The seismic-induced frictional slip on a filled fracture is performed by a dynamic-induced direct-shear (DIDS) test. A generated plane P-wave propagates as a dynamic shear load and induces the frictional slip along a layer of filling materials. The dynamic shear stress is non-uniformly accumulated in the filling materials. The shear stress at the trailing edge controls the dynamic triggering of frictional slip owing to the highly compacted filling materials. The dynamically triggered frictional slip is quickly arrested by the re-connection of sand contacts after the instantaneous disturbance, while the statically triggered frictional slip is unrecoverable during a short duration and contains a main slip and a few short slips before and after the main slip. The seismic response of rock fractures filled with granular materials is decomposed into the seismic-induced compressive responses and the seismic-induced frictional slip in this study. Both seismic-induced responses are strongly related to the dynamic response of the filling materials. The analytical models and the experimental experiences are possible to be numerically combined to study random wave propagation across rock fractures filled with granular materials and to be applied to estimate seismic wave attenuation and rock mass instability.

EPFL2013