Depositional mechanisms and morphology of debris flow: physical modelling

A comprehensive understanding of the deposition mechanisms and morphology of debris flows is necessary to delineate the extent of a debris flow hazard. However, due to the wide range of debris flow compositions and the complex topography in the field, there remains a deficiency of fundamental understanding on how the effects of grain-size distribution, water content, and channel slope influence the deposition mechanisms and morphology of debris flow. In this study, a series of experimental tests were carried out using a flume with a horizontal outflow plane to discern the effects of particle size, water content, and slope on the deposition morphology and grain size segregation on the deposition fan. Results reveal that the experimental debris flows are under either viscous or collisional flow regimes. Most experimental debris flow fronts lack high pore fluid pressures, emphasizing the formation of deposits via grain-grain and grain-bed friction and collisions; also high excess pore fluid pressure (positive) behind the front head is measured and it is beneficial for the mobility of debris flows. Both the deposit area and runout-width ratio are positively correlated to the Bagnold and Savage numbers and the initial water contents. Furthermore, an increase of fines content reduces the runout distance. However, this feature is not as obvious for high water content flows (w = 28.5% in this study). Moreover, smoother transition topography between the transportation and deposition zone leads to longer runout distances. For debris flows with a high solid fraction (Cs > 0.52 in this study), particle sorting is quite inhibited in the deposit fan.