Ru Wang, Jeff Peakall, David Hodgson, Edward Keavney, Helena Brown, Gareth Keevil

What is the nature of flow reflection, deflection and combined-flow behaviour when gravity flows interact with slopes? In turn, how do these flow dynamics control sedimentation on slopes? Here, these questions are addressed using physical experiments, with low-density unconfined gravity flows interacting with slopes of varying gradients, at a range of flow incidence angles. The present paradigm for gravity current interaction with slopes was based on experiments with high-density flows, conducted in narrow 2D flume tanks, in small (1 m2 planform) 3D tanks, or in large 3D tanks where flows can surmount the topography. Here, larger-scale physical experiments were undertaken in unconfined settings where the flow cannot surmount a planar topographic slope. The experiments show that the dominant flow-process transitions from divergence-dominated, through reflection-dominated to deflection-dominated as the flow incidence angle varies from 90° to 15° and the slope gradient changes from 20° to 40°. Also, patterns of velocity pulsing at the base of, and on, the slope vary as a function of both the flow incidence angle and slope gradient. Furthermore, in all configurations complex multidirectional combined flows are observed on, or at the base of, the slope, and are shown to vary spatially across the slope. The findings challenge the paradigm of flow deflection and reflection in existing flow-topography process models that has stood for three decades. A new process model for flow-slope interactions is presented, that provides new mechanics for the frequent observation of palaeocurrents from sole marks at high angles to those in the associated ripple division. Results provide insights into the formation and spatial distribution of distinctive combined-flow bedforms, sediment dispersal patterns, and process controls on onlap termination styles in deep-sea settings, which can be applied to refine interpretations of exhumed successions.