Process of Occurrence, Flow and Deposition of Viscous Debris Flow

Abstract

Debris flow generally contains particles whose size distributes in a very wide range, and the fine fraction of them is usually incorporated in the interstitial water to behave as a kind of fluid. When the fine fraction is composed of cohesive solids and it accounts for a significant ratio in the entire particle fractions, the interstitial fluid becomes highly viscous slurry. In addition to the large viscosity of the slurry, the dense content of coarse particles enhances the apparent viscosity of the whole material. Therefore, in such a debris flow, the viscous stress is more dominant than the stresses due to the inter-particle collisions and/or the macro turbulent mixing. This kind of debris flow is called as the viscous type which typically occurs at Jiangjia Gully, Yunnan, China. Field observations of running viscous debris flows and some field inspections of the debris flow deposit suggest that viscous debris flow arises from landslide. A qualitative model for the process of transformation of a soil mass into debris flow is introduced. Once debris flow is generated it surges down the slope or the channel in fantastic speed. Such a small resistance to flow can only be fulfilled by nearly homogeneous dispersion of coarse particles within flow. Here, the particle dispersion mechanism in laminar flow is attributed to both the drag of the squeezing flow in the viscous interstitial fluid and the higher resistance to the particle motion toward the more densely particle concentrated area. When a viscous debris flow comes down to a mildly sloping area its competence for dispersing the coarse particles diminishes due to decrease in the velocity of flow, and it begins to deposit. The sediment concentration in viscous debris flow is so dense that the volume change before and after deposition is little, and the fresh deposit is very soft, virtually not different from the material in flow. A newly arriving surge easily entrains the deposit and goes together downstream. Soon, as the depth of flow and the velocity decrease in the rear part, deposition takes place again. Repetition of such the phenomena continues long at Jiangjia Gully. A Newtonian fluid model introduced herein is examined by the flume experiments as well as by the field observations.