Viscous and two-phase debris flows in southern Chins's Yunnan Plateau

Debris flows are often triggered by heavy rainfall and flash floods in mountainous areas. Generally, debris flows are the most disastrous event associated with floods and cause most of the fatalities resulting from flood events. This paper reports a study of debris flows by filed investigations at the Xiaojiang watershed, which is in the upper Yangtze River basin, and flume experiments. Two types of debris flows - viscous debris flows and two-phase debris flows - occur on the Yunnan Plateau in south China. Viscous debris flows consist of clay, silt, sand, and gravel. This kind of flow is non-Newtonian and often develops into roll waves. They sometimes exhibit drag reduction and extremely high super-elevation. The drag reduction is mainly due to the effects of air cushions and the bed smoothening. Two-phase debris flows consist of gravel and boulders and show obvious relative movement between the solid particles and the liquid. Flume experiments show that debris flows may be triggered by torrential ]floods if the channel bed slope is over a critical value. The height of the debris flow head is proportional to the size of the gravel. A striking phenomenon was observed, in which the velocity of particles in the surging head is much lower than those in the following part. The velocity profile of particles in the head is linear and that in the following main flow shows an inverted "s" shape. For the same bed material, the resistance of a two-phase debris flow is higher than the resistance of a water flow because the collisions among the particles create great resistance. The resistance increases with the diameter of solid materials.