In shallow marine environments gravity-driven currents (e.g., hyperpycnal flows) often traverse surface wave fields, and the resulting complex flows are key mechanisms for offshore sediment transport. Our laboratory experiments illustrate how surface waves alter sediment transport in gravity-driven density currents. The addition of a wave field to a gravity-driven current resulted in a 7-8.5 increase in the downslope transport of the deposit volume. Additionally, oscillatory velocities recorded at downslope locations in surface-wave-altered turbidity currents were larger than wave-field velocities measured at the same location without a turbidity current. These observations indicate that surface waves alter turbidity currents in a longitudinally complex manner whereby the influence of oscillatory currents is transported downslope within the body of the turbidity current. These effects were observed in a conservative case: the maximum orbital velocities of the wave field were an order of magnitude less than the maximum unidirectional velocities of the current. We predict that if the velocities of the wave field and the current were sub-equal, a plausible scenario for hyperpycnal flows in near-shore, deltaic, and proximal shelf environments, these effects would be substantially more effective. This work has significant implications for modeling offshore sediment transport in shallow marine environments and for interpreting the deposits of such flows, most notably that the presence or absence of combined-flow ripples might not indicate whether the current was deposited above or beneath wave base.
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