Physical modelling of submarine debris flows : scaling and bed conditions

Abstract

Submarine debris flows are among the largest types of landslides on planet earth. They occur under the cloak of the sea so little is known about how they can reach such large volumes and travel such long distances. Over recent decades, scientists have modelled submarine debris flows in laboratory settings, which have provided a preliminary understanding of submarine flow dynamics. However, when conducting physical experiments, a fundamental challenge is linking the dynamics of miniaturized flows to field ones. Surprisingly, little attention has been paid to scaling considerations to guide physical modelers in the literature. Furthermore, field evidence suggests that the entrainment of the sediment-rich sea floor enables the growth of submarine debris flows to giant scales. However, there is a dearth of understanding on the dynamic process of entrainment owing to a lack of physical evidence on submarine debris flows travelling on more realistic rough and erodible bed conditions. A systematic dimensional analysis is first carried out in this study to describe the dynamics of submarine debris flows from both macroscopic and mesoscopic perspectives. Based on a systematic review of laboratory and field data reported in the literature using the derived dimensionless numbers, some scale invariant criteria are proposed to bridge the gap between miniaturized and field flows. Furthermore, the shed light on the mechanisms of submarine debris flow entrainment, a new analytical entrainment model is proposed. The model considers the relative contributions from basal grain friction and fluid viscous stresses that drive the entrainment process, and the effects of hydroplaning. To evaluate the new analytical model, a new experimental setup is developed to simulate submarine debris flows, with clay contents from 4% to 12%, overriding an entrainable loose sand bed. The clay content has profound effects on the entrainment dynamics. Flows with high clay contents (i.e., ≥10 vol.%) hydroplane, which enhances the mobility of the flow but reduces its entrainment potential. The effects of bed conditions on flow mobility are also investigated experimentally by modifying the new physical model with smooth and impermeable beds. A series of experiments are 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 generated as a debris moves against the ambient water. In contrast, excess water pressures at the-flow-bed interface for non-hydroplaning debris flows are caused by rapid loading on a sandy bed. Bed permeability and roughness are observed to regulate the bipartite flow dynamics (i.e., debris layer and turbidities above). Findings also suggest that existing criteria used to govern 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. The proposed theoretical model and unique experimental evidence can be used as guiding tools to progress towards the inclusion of entrainment for vulnerability assessments of offshore infrastructure.