MEASURING VELOCITY PROFILES IN LARGE SCALE DEBRIS FLOW EXPERIMENTS

The shear rate represents an important parameter for understanding the flow mechanics in geophysical flows, like debris flows. Often the analysis of debris flow experiments is based on an assumed velocity field within the flow or on observations of the velocity distribution at the side walls. To estimate the effect of side wall friction and to derive a representative shear rate, a probe to measure the mean particle velocity at different depths and different locations within the flow has been developed. The system is based on the measurement of small variations of conductivity of the passing material at the tip of a steel tube which is lowered into the flow. Calculating the time lag between signals from two independent conductivity measurements at a small, known distance apart yields the velocity of the material passing by the tip of the sensor.

Experiments were conducted in a 4 m diameter, 0.8 wide rotating drum at the University of California at Berkeley. The large scale enables us to use grain sizes similar as that found in the field, and to create longer, deeper flows compared to traditional flumes. A series of runs with grain - fluid mixtures of varying coarse and fine sediment concentration have been carried out. Additional measured parameters include the flow depth, normal stress and pore fluid pressure along the base. Results show that the velocity field is qualitatively similar in the centre and on the side for most mixtures, but can vary along the flow. The magnitude of the effect of wall friction is estimated for different mixture compositions and solid concentrations. These new measurements enhance the understanding of the flow mechanics of our laboratory debris flows and may help to improve constitutive models that describe natural events.