Experimental Study on the Movement of Boulders in Debris Flow: Influence of Boulder Size and Initial Orientation

In debris-flow disasters, boulders moving at high velocities cause significant damage to houses and other facilities. Through a flume model test, this study explored the influences of the length/width ratio of the boulders, the angle between the long axis of the boulders and the flow direction, and the density of the mudflow on their starting movement. The experimental results indicate that in the process of a mudflow impacting the boulders, the angle between the long axis and the flow direction influences the magnitude of the component forces of the dragging force in the long- and short-axis directions, thus causing the boulders to deviate. Deflection changes the area of action of the debris-flow drag force on the boulder. Once the boulder gains a certain velocity, it deviates toward a state in which the long axis is parallel to the flow direction to reduce the resistance in the movement process. When the long axis of the boulder is parallel to the flow direction, as the mass of the boulder decreases, the efficiency of the mudflow in transferring the velocity of the boulder increases. When there is an angle between the long axis of the boulder and the flow direction, as the angle increases, the area of the drag force and efficiency of the velocity transfer increase. The movement laws of boulders in mudflows are crucial for engineering construction and sustainable development in mountainous areas.