HR: 14:10h
AN: H43G-03    [Abstracts]
TI: Role And Behaviour Of Clay Minerals In Alpine Debris Flows
AU: * Boivin, P
EM: pascal.boivin@ird.fr
AF: Laboratory of Soil Science, LPE-ISTE-ENAC, Swiss Federal Institute of Technology, Lausanne, 1015 Switzerland
AU: * Boivin, P
EM: pascal.boivin@ird.fr
AF: LGIT, LGIT, Universit‚ J. Fourier, BP53, Grenoble Cedex, 38041 France
AU: Bardou, E
EM: eric.bardou@bluewin.ch
AF: Laboratory of Soil Science, LPE-ISTE-ENAC, Swiss Federal Institute of Technology, Lausanne, 1015 Switzerland
AU: Pfeiffer, H
EM: Hans-Rudolf.Pfeifer@cam.unil.ch
AF: Centre d'Analyse Min‚rale, Universit‚ de Lausanne, BFSH 2, Lausanne, 10015 Switzerland
AB: 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.
DE: 8159 Rheology--crust and lithosphere
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
DE: 0400 Biogeosciences
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting