The Structure and Entrainment Characteristics of Partially Confined Gravity Currents

Abstract

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.

Controls on the architectural evolution of deep-water channel overbank sediment wave fields: insights from the Hikurangi Channel, offshore New Zealand

“…A systematic down-channel decrease in the magnitude and velocity of overspill from the Hikurangi Channel is attributed to the process of flow 'tuning'. This arises from the loss of material from the dilute, upper parts of flows in up-channel locations, causing the range of flow heights to decrease down-channel as flows progressively lose material, with thicker flows losing more than thinner-ones (Figure 21C; Mohrig and Buttles 2007;Kelly et al 2019). Flow tuning has generated a down-channel decrease in the size of the outer-bend wave fields (Figure 22).…”

“…Previous studies have led to the development of models of overbank flow and architecture evolution in which progressive trends of diminishing grain size (typically from fine sand to mud) and deposit thickness are seen in transects away from the levee crest (Kane et al 2007;Morris et al 2014). However, these simple trends may be complicated by the influence of factors such as: variations in the size of turbidity currents relative to their host conduit (Dennielou et al 2006), flow 'tuning' (Mohrig and Buttles 2007;Kelly et al 2019), variations in overbank slope gradient (Kane et al 2010;Nakajima and Kneller 2013), sinuosity (Timbrell 1993;Kane et al 2008), structural confinement (Clark and Cartwright 2011), the Coriolis force (Klaucke et al 1998;Cossu et al 2015), and contour currents (Fuhrmann et al 2020;Miramontes et al 2020).…”

Graphics processing unit accelerated lattice Boltzmann method simulations of dilute gravity currents

“…Reynolds-averaged Navier-Stokes (RANS) models, derived by time-averaging of the governing equations, have been applied to the simulation of dilute gravity currents with both fixed and deformable boundaries. [27][28][29][30][31][32] Although RANS models are far less computationally expensive than DNS models, which has enabled the simulation of environmental scale flows, the inherent averaging of the governing equations reduces the accuracy with which they can capture the complex time-dependent turbulent flow features of gravity currents.…”