HR: 13:55h
AN: H43G-02    [Abstracts]
TI: Measurement of vertical and basal shear forces from debris flows at the Illgraben observation station, Switzerland
AU: * McArdell, B W
EM: mcardell@wsl.ch
AF: Swiss Federal Research Institute WSL, Department of Natural Hazards, Birmensdorf, CH-8903 Switzerland
AB: 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.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting