Flow transformation of subaqueous debris flow: Investigation of turbidity cloud generation based on a two-layered layer-averaged model

Debris flows induced by a subaqueous landslide may experience flow transformation into turbidity currents during its flow path. The initial subaqueous sandy debris flows efficiently change flow properties. Fluidized sands in a flow preferentially settle to form a deposit, whereas clay particles included in a debris flow eject upward and generate a turbidity cloud over the debris flow. However, the proposed mechanism in the literature to create a turbidity cloud has remained unclear due to the lack of quantitative evaluation. This study numerically modeled the generation of turbidity currents by flow transformation of subaqueous debris flows. A two-layered layer-averaged model was used to express the flow transformation. The lower layer represents a debris flow, and the upper layer corresponds to a turbidity cloud. We considered two mechanisms generating turbidity clouds from initial sandy debris flows, which are (1) shear mixing at the surface of a debris flow and (2) release of pore fluid from debris flows. The rate of generation of suspension by shear mixing was modeled to be proportional to the relative velocity between two layers as formulated in the previous study. In modeling the rate of generation of suspension by pore fluid migration, we coupled the release rate of the pore fluid with the deposition rate of sands, assuming that the contractive motion of settling sands squeezes the pore fluid to the upper layer. Numerical experiments for flow transformation of sandy debris flow were conducted, in which debris flow descended a uniform slope. We compared two simulation cases: one incorporated only the process of shear mixing, and the other considered both shear mixing and release of pore fluid. As a result, the case considering the release of pore fluid reproduced the behavior of the flow head reported in the literature better. In contrast, the generation of turbidity clouds only by the shear mixing process was inefficient because the relative velocity between the upper and lower layers became smaller after the development of turbidity clouds. The mechanism by the release of pore fluid was remained effective in the whole duration of flows so far as sands deposited from debris flow. Thus, the contribution of mechanisms generating turbidity clouds varies depending on the development stage of the subaqueous debris flows.