Initiation and mechanism of two phase debris flow

摘要

Debris flow is the most disastrous chapter of flooding in mountainous areas. This paper reports the main experimental results of initiation of debris flow and the development of debris flow head. Video films of the experiments show that a high rolling head, composed of large gravel and boulders, is a typical character of debris flow. Debris flow may be triggered by torrential flood if the slope of the channel bed is over a critical value. In the initiation stage of a debris flow, particles are removed from the bed and roll in the front of the flow, forming a head consisting of coarse particles. High concentration of particles, up to 1100-1600 kg/m~3, was carried down the flume with the head in the experiments. The height of the debris flow head is proportional to the size of gravel. It is striking that the velocity profiles of the particles in the head are quite different from those in the main flow. The particle's velocity in the main flow is about 2 times of those in the head. The velocity profiles in the head are nearly linear and those of the main flow are more curved. The mechanism of the different velocity profiles is studied. The instantaneous velocity of small particles is higher than that of large particles but the average velocity of small particles is lower than large particles. A layer of small particles is often seen between the stagnant bed and the moving particles in the debris flows. For the same bed material, the resistance of debris flow is about 10 times higher than the normal sediment-laden flow because the collisions among the particles and between particles and the bed create a great resistance. The resistance increases with increasing diameter of solid material.