Effects of bed permeability and roughness on the mobility of submarine debris flows: experimental insights

Submarine debris flows pose a threat to offshore development. Field evidence shows that these flows travel on sandy beds that are rough and permeable. However, existing experimental studies idealize bed conditions as smooth and impermeable. A series of experiments were carried out to investigate the effects of bed roughness and permeability on the prevailing flow dynamics. Experimental results show that excess water pressures at the flow-bed interface of hydroplaning flows are mainly caused by dynamic pressure as debris moves against the ambient fluid. In contrast, excess water pressures at the flow-bed interface for non-hydroplaning debris flows are caused by rapid loading on a sandy bed. Excess pressures from rapid loading provide another explanation for the long travel distances of submarine debris flows. Permeable beds provide an additional path for the diffusion of fluid pressures at the flow-bed interface, which increase bed normal stresses and decelerate a flow. Bed permeability and roughness are observed to regulate the bipartite flow dynamics (i.e., turbidity current overlying dense laminar layers). Findings suggest that existing thresholds used to describe the onset of hydroplaning may only be relevant for smooth and impermeable beds and hydroplaning alone may be insufficient to explain the high mobility of submarine debris flows if more realistic boundary conditions are considered.