On the effect of surface roughness on velocity profiles and runout lengths of dry granular flows

Gravitational mass flows like debris flows are often physically modelled as an assembly of particles flowing in simplified flume configurations. There is indication that natural flows exhibit a combined movement of sliding and internal deformation, which is not well understood and underrepresented in scaled laboratory experiments. In this study we investigate the effect of the surface roughness on the velocity profile and the runout of small-scale, dry granular avalanches. The experimental set-up is a 0.17 m wide flume with an inclination of 34° for the first 1.5 m, following an 0.8 m curved transition zone with a radius of 1.7 m, and ending in a runout zone with an angle of 4°. The tested material consisted of non-perfect spherical ceramic beads with a diameter of 2.8 to 4.3 mm. We tested four different types of surface roughness ranging from 0 to 6 mm height and additionally one macro roughness, which was higher than the maximum flow height. To also get information about the influence of the relative roughness experiments with three different starting volumes were undertaken. All fourteen experimental variations were repeated three times. Flow heights were measured with laser sensors at four different positions along the channel. Three of them were used to calculate the mean front velocity of the flowing mass in two cross sections. Furthermore, the experiments were recorded with a high-speed camera through one sidewall out of acrylic glass. The recordings were analysed using a PIV (Particle Image Velocimetry) software to derive velocity profiles in 1/1500 second time steps. Results show that the velocity profiles changed from the head to the tail of the flow and that the profiles of the two roughest surfaces are more alike than the smooth roughness configurations. The fraction of sliding on the total movement varied between 0 and close to unity. The runout length decreased the higher the roughness was and increased with higher starting volume. The shape of the velocity profiles at the deepest sections of the flows changed with surface roughness and with starting volumes. Only the velocity profiles for the two roughest surfaces show an inflection point. Our experiments highlight the importance of surface roughness as well as relative roughness for granular mass flows and provide data for model testing.