Surface slopes, velocity profiles and fluid pressure in coarse-grained debris flows saturated with water and mud

Data on the internal velocity distribution of flowing sediment–fluid mixtures such as debris flows are rare, but necessary for model development and testing. A probe to measure the mean particle velocity at different depths and different locations within experimental debris flows in a 4 m diameter rotating drum was developed. In addition, the flow depth, basal normal stress and basal pore fluid pressure were also measured. Results show that for a given sediment–fluid mixture the velocity profiles collapse to distinct non-dimensional profiles. Macroscopic flow behaviour shows great similarity, with mean surface slopes weakly dependent on the shear rate for water-saturated gravel, but strongly shear-rate-dependent when pores are filled with mud. Poorly sorted material with a high content of fines produced fluid pressures close to normal stress and sidewall friction had a strong effect on the flow pattern. Our results reveal variability in profile characteristics for flows displaying similar macro-dynamics and provide data for model testing.