The motion state transformation of submarine mass movement: an experimental study

Submarine mass movement (SMM) is a geohazard that can move along an extremely gentle slope. The motion state of SMM undergoes landslide-debris flow-turbidity current transformation during movement. Because of motion state transformation (MST) process, it is difficult to analyze the mobility and shear stress of the SMM quantitatively. To investigate the MST process, 81 sets of rotational flume-based experiments with different material types, masses, and velocities, were conducted. A densimetric Froude number (Fr) was used to characterize the MST process quantitatively. As velocity increases, the MST initiates at the front and surface of the SMM and progressively propagates toward the rear and bottom. Variations in material type and mass lead to different degrees of MST, which in turn result in distinct patterns of mobility and shear stress of the SMM. The results indicate that three soil–water interactions, i.e., hydroplaning, front erosion, and surface erosion have contributed to the MST process and the mobility of the SMM. During the MST process, Fr = 0.4 and Fr = 0.7 can be used as threshold values to identify the debris flow and turbidity current states, respectively. Furthermore, the MST process induces nonlinear variations in shear stress by altering sediment concentration and shear strain rate. The former dominates throughout the process, while the latter becomes significant only under stabilized motion states. The quantitative analysis of the MST process in this study can provide new insights into the SMM dynamics.