Turbidity Current Dynamics: 2. Simulating Flow Evolution Toward Equilibrium in Idealized Channels

Abstract

This study simulates turbidity currents through a number of idealized channels using a steady, one‐dimensional, depth‐averaged model to determine if modeled flows starting from a wide range of initial conditions might reach an equilibrium state where clear‐water entrainment balances fluid mass lost to flow stripping or overspill processes. To accomplish this, we calculated flow dynamics based on 1,000 sets of randomized initial conditions, identified flows that successfully traversed the system, and then extracted the flow height, velocity, and sediment flux at the channel terminus. We then systematically changed channel and flow properties using a wide range of values and calculated the length required for the simulated currents to reach the new equilibrium conditions. We found that modeled turbidity currents may evolve to a single equilibrium state consistent with the physical channel and flow properties regardless of the initial flow conditions. Additionally, we found that simulated turbidity currents generally required 1–40 km of downflow distance to adjust to new equilibrium conditions resulting from changes in the channel slope, channel height, channel width, coefficient of bed friction, and meander radius of curvature. Changes in sediment grain size and suspension cloud concentration resulted in adjustment lengths exceeding 100 km. Shorter adjustment lengths resulting from flow stripping suggest that this process might play an important role in flow filtering. The fact that modeled flows adjust to new equilibria over tens of kilometers suggests that they are less sensitive to upstream flow conditions in longer channels.

Controls on submarine channel-modifying processes identified through morphometric scaling relationships

“…7), indicating that the stable widths and aspect ratios observed in the upper Bengal 1 channel and the GoM 12 channel may be typical of medial to distal portions of submarine channels. This is consistent with the modeling results of Traer et al (2018aTraer et al ( , 2018b) that suggest turbidity currents can achieve flow equilibrium within the upstream reaches of a channel and maintain it over long distances, producing a flow filtering effect and consistency of flows that traverse the full length of the channel. The dramatic depth decrease at the end of the Bengal 1 channel can be attributed to the channel-lobe transition (see below), which is not mapped in the Amazon 1 and GoM 12 channels.…”

“…11; Conway et al, 2012). Turbidity current modeling efforts by Traer et al (2018aTraer et al ( , 2018b show that currents may take tens to almost 100 km to achieve flow equilibrium (between entrainment and overspill or flow stripping) following a perturbation to channel slope, depth, or other parameters. These findings are consistent with our observations from the Niger 9 channel and suggest that the long adjustment period of flows relative to topographic irregularities and channel length could be reflected in the morphologies documented here.…”

“…We hypothesize that fluvial scaling relationships that arise from terrestrial processes, such as downstream increases in discharge driven by precipitation and overland flow, will be different in submarine systems where such processes do not occur. This study presents a rich data set of submarine channel morphometrics that can be further employed in numerical modeling of turbidity currents (e.g., Sequeiros, 2012;Traer et al, 2015Traer et al, , 2018aTraer et al, , 2018b, offshore energy resource exploration, seafloor infrastructure hazards assessment (e.g., Piper et al, 1999), and marine habitat and ecosystem research (e.g., Vetter and Dayton, 1998). By directly testing whether certain fluvial scaling relationships are also present in submarine channels, we address whether morphological similarities reflect analogous processes, and assess the relevance of fluvial analogs in deep-water systems (Mitchell, 2004;Keevil et al, 2006;Hughes Clarke, 2016).…”

New flow relaxation mechanism explains scour fields at the end of submarine channels

“…If turbidity currents are too small to reach the end of the channel, flow relaxation cannot occur and the channel will not propagate, as seen for the Southern Channel in the Squamish Delta between 2004 and 2006 43,44 . On the other hand, if a turbidity current is too large for the channel, then the flow will overspill the channel, thereby reducing its size until the flow matches the channel dimensions 45 , in which case flow relaxation will occur.…”

Self-stratifying turbidity currents

“…Traer et al (2018a) proposed a four equation model capturing both mass lost due to lateral overspill by pressure and that lost due to the sinuosity of the current. This model is then extensively investigated in Traer et al (2018b), demonstrating a range of interesting behaviours comparable to those seen in real physical flows. However, the levee overspill is derived from a consideration of viscous flow dynamics, not inviscid, which results in the wrong dependence on the excess depth of the current (compare eq.…”