Three‐dimensional gravity current interactions with oblique slopes: Deflection, reflection and combined‐flow behaviours

ABSTRACT Gravity currents interacting with planar slopes have been thought to always ‘reflect’ a component of flow orthogonal to the slope irrespective of the flow incidence angle. Incoming flows are argued to undergo gravitational collapse, and generate internal waves, that propagate perpendicular to the bounding slope. These processes have been used to explain the widespread observation of palaeocurrents from sole marks at high angles to those in the associated ripple division. This paradigm for gravity current interactions with planar slopes has stood for more than three decades. Herein, these ideas are tested using three‐dimensional low‐density saline currents interacting with (but not overtopping) planar slopes of varying gradients, at a range of incidence angles. Fifteen new experiments show that the dominant flow process transitions from divergence‐, through reflection‐ to deflection‐dominated as the flow incidence angle decreases from 90° to 15° and the slope gradient increases from 20° to 40°. Multidirectional combined flows are documented above topographic slopes, varying as a function of location on a single slope, and the orientation and gradient of the slope. However, discrete internal waves are not observed, likely due to the spatial and temporal variability of flows on the slope. These findings challenge the paradigm of flow deflection and reflection in the existing model; there is not always a component of ‘reflected’ flow orthogonal to the planar slope. A new process model for flow‐planar‐slope interactions is presented. Flows broadly parallel to topographic slopes lead to up‐slope and down‐slope flow oscillations orthogonal to the bounding slope, providing new mechanics for the observation of high‐angle variation between sole marks and ripple derived palaeocurrents in elongate basin‐fills. Results also provide insights into the spatial distribution of distinctive combined‐flow bedforms, which are most applicable to settings where flows interact with high‐relief intrabasinal topography and/or basin margins.