FIELD MEASUREMENTS OF DEBRIS FLOW PROPERTIES AND CHANNEL-BED ELEVATION CHANGE

Field observations of debris flow activity have been collected at the Illgraben catchment (10.4 km²) in the Swiss Alps starting in the year 2000. After the debris flows pass the fan apex, they presently travel along a 2 km channel and nearly all of the debris flows reach the Rhone River at the end of the fan. Data include flow depth, front velocity, rainfall, and shear and normal force from a force plate on the channel bed. In addition, data are available from an array of force plates and geophones installed on a vertical concrete wall which is oriented parallel to the direction of the flow. Additional observations of channel-bed erosion are available from sensors buried in the channel bed in 2008 as well as from photographs and video images since 2000. A comparison of the debris flow data with the erosion observations suggests that large debris flows (front flow depths > 1–2 m) tend to lower the elevation of the channel bed, while smaller debris flows and debris floods either deposit sediment or produce no net change. The erosion sensor data, which allow determination of the timing and magnitude of channel-bed erosion within an individual debris flow surge, indicate that sediment entrainment from the bed typically takes place at the debris flow front region. We analyzed the data from the vertically-oriented array of force plates to investigate the mechanism of erosion. The force plate data were decomposed into normal and fluctuating components, and they reveal that the fluctuating component is largest at the head of the debris flows and rapidly decreases with time after the passage of the flow front. This indicates that the inter-granular particle collisions (or inertial solid stresses) are strongest at the head debris flows. The results suggest that sediment entrainment at the flow-bed interface may be controlled by the inter-granular particle collisions within the flow.