Model calculation of the velocity of debris flows containing finer sediment

When applying a model to debris flows containing finer sediment, an increase of pore fluid density and representative diameter should be considered because the finer sediment behaves as a liquid phase. The liquid phase sediment is thought to contribute to the stress of the debris flow as pore fluid rather than as particle-particle interactions. This study was performed to reproduce the velocity of such debris flows, obtaining the pore fluid density from the sediment concentration and using the volume median diameter or the volume mean diameter as the representative diameter to test for which is more appropriate for reproducing the velocity. To compare measured and calculated velocities we conducted debris flow flume experiments over an erodible bed. In the experiments, four particle size distributions were used. We set the sediment concentration as the experimental condition and measured flow depths, deposition slopes, and velocity profiles. The measured mean velocities were evaluated by dividing the flow rate by the flow depth. In contrast, the calculated mean velocities and the velocity profiles were computed using the model. In the model calculation, the increase of pore fluid density was evaluated such that the ratio of liquid phase sediment in the particle size distributions reproduced the concentration, and the representative diameters were determined, except for the liquid phase sediment. As a result of the comparison between measured and calculated velocities, the mean velocities, calculated using the mean diameters, were more reproducible than those calculated using the median diameters. Moreover, the velocity profiles were well reproduced when using mean diameters, especially when the particle size distributions were wider. These results led us to conclude that the velocity of debris flows containing finer sediment can be described using the pore fluid density, evaluated by the concentration and by using the mean diameter as the representative diameter.