Image analysis for debris flow properties estimation

On August 1998 some debris flows were recorded by an automated monitoring system at Acquabona Creek (Eastern Italian Alps). In this paper, motion picture of the flow, timing of single coarse particles on the flow surface, geophone recordings and ultrasonic logs were used to obtain velocity, depth and particle size data for the August 17 debris flow. Analogical images, recorded from zenithal camera located along the flow channel axis, have been converted in digital format and then processed in order to obtain a suitable identification of single flowing particles. Through image analysis techniques, grain-size and surface velocity distributions of debris flows are estimated. These properties are of the outmost importance in developing a proper rheological model for debris flows. Front velocities ranged from 1.8 to 4 m/s and corresponding flow surface velocities ranged from 2.26 to 4.18 m/s. A range of slurry shear strengths and viscosities occur in different surges of the same event. These differences in rheological flow behaviour appear to be influenced by concentration of particles and by flow particle-size distributions too. Coarser slurry are related a Bingham flow and finer slurry is associated to Newtonian flow. The development of rigid plugs is an index of rheological behaviour. In the observed surge Bingham flow is characterised by plug widths between 62 and 94% of flow width for flow depth higher than 45 cm. When depth dropped below 45 cm, slurry is finer and more liquid and flow transformed to hyperconcentrated flow. Turbulent small less concentrated surges are caused by a sort of pushing effect of the following coarser debris surge.