Measurement of Velocity Profiles and Fluid Pressure in Natural Debris Flows

Debris flows are mixtures of sediment and water that can have a wide range of grain size distributions and water contents. The connection between pore fluid pressure and the internal deformation behavior is of fundamental interest for understanding the mechanics of these geophysical processes. We measure grain velocities at a certain height of the flow by cross-correlating signals from paired conductivity sensors. Fluid pressure is measured using a pressure transducer shielded by a 2 mm steel mesh to avoid direct contact with coarse grains. In this contribution we review results of systematic measurements of velocity profiles and fluid pressure in experimental sediment-water flows and present respective measurements of a natural debris flow recorded at a new monitoring station in an Alpine creek. Velocity profiles in experimental mixtures strongly vary with grain size distribution and associated excess pore fluid pressure. The recorded natural flow displayed variations of velocity profiles from front to tail, with pore fluid pressures close to normal stress in some sections of the flow.