The Structure and Entrainment Characteristics of Partially Confined Gravity Currents

Seafloor channels are the main conduit for turbidity currents transporting sediment to the deep ocean, and they can extend for thousands of kilometers along the ocean floor. Although it is common for channel-traversing turbidity currents to spill onto levees and other out-of-channel areas, the associated flow development and channel-current interaction remain poorly understood; much of our knowledge of turbidity current dynamics comes from studies of fully confined scenarios. Here we investigate the role that partial lateral confinement may play in affecting turbidity current dynamics. We report on laboratory experiments of partially confined, dilute saline flows of variable flux rate traversing fixed, straight channels with cross-sectional profiles representative of morphologies found in the field. Complementary numerical experiments, validated against high-resolution laboratory velocity data, extend the scope of the analysis. The experiments show that partial confinement exerts a first-order control on flow structure. Overbank and downstream discharges rapidly adjust over short length scales, providing a mechanism via which currents of varying sizes can be tuned by a channel and conform to a given channel geometry. Across a wide range of flow magnitudes and states of flow equilibration to the channel, a high-velocity core remains confined within the channel with a constant ratio of velocity maximum height to channel depth. Ongoing overbank flow prevents any flow thickening due to ambient entrainment, allowing stable downstream flow evolution. Despite dynamical differences, the entrainment rates of partially confined and fully confined flows remain comparable for a given Richardson number. Plain Language Summary Turbidity currents are large, underwater flows that can travel for thousands of kilometers across the ocean floor. They carry huge volumes of sediment, causing them to be denser than seawater. It is this density difference that is their main driving force. Not only are they responsible for forming complex seafloor structures, but they can be highly destructive, capable of destroying any seafloor infrastructure in their path. Like rivers, turbidity currents often flow within channels. In this study we show how these channels play a key role in the structure of the currents and how they could be responsible for the vast distances the currents can travel.