A general model for the helical structure of geophysical flows in channel bends

Abstract

Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record.  First direct measurements of the helical flow structure of turbidity currents as they travel around a bend. Turbidity currents of different thicknesses and velocities exhibit the same helical flow structure. We reconcile current controversy with a new model that explains helical flow structure for a wide range of geophysical flows.© 2017 American Geophysical Union. All rights reserved. AbstractMeandering channels formed by geophysical flows (e.g. rivers and seafloor turbidity currents) include the most extensive sediment transport systems on Earth. Previous measurements from rivers show how helical flow at meander bends plays a key role in sediment transport and deposition. Turbidity currents differ from rivers in both density and velocity profiles. These differences, and the lack of field measurements from turbidity currents, have led to multiple models for their helical flow around bends. Here we present the first measurements of helical flow in submarine turbidity currents. These ten flows lasted for 1-to-10 days, were up to ~80-metres thick, and displayed a consistent helical structure. This structure comprised two vertically-stacked cells, with the bottom cell rotating with the opposite direction to helical flow in rivers. Furthermore, we propose a general model that predicts the range of helical flow structures observed in rivers, estuaries and turbidity currents based on their density stratification.

Rapidly-migrating and internally-generated knickpoints can control submarine channel evolution

“…It has previously been suggested that secondary (across-channel) helical flow causing gradual bend migration, is the main control on submarine channel evolution, as is the case for many rivers. There has been considerable debate over whether the sense of submarine secondary circulation is river-like or reversed 25,31,55,56 . Outer-bend erosion causing lateral migration is common in Bute Inlet and can locally reach rates of over 10 m/year.…”

“…First, it has been proposed that submarine channels evolve in a broadly comparable way to meandering rivers, via gradual outer-bend erosion and inner-bend deposition, and meander bend cut-off 5,23,26 . Bend migration and cut-off is primarily driven by cross-channel (secondary) flow and has long been known to be a dominant control on how rivers evolve [27][28][29][30] , and also occurs in submarine channels 31 . However, submarine channels have been suggested to differ in key regards from rivers 32 .…”

Storm‐induced turbidity currents on a sediment‐starved shelf: Insight from direct monitoring and repeat seabed mapping of upslope migrating bedforms

“…The oblique directions of the flow can either be due to the sharp turn in the canyon axis, which leads to helical flows around meander bends (e.g. Azpiroz‐Zabala et al ., 2017b) or to the shape of the crescentic bedforms that permits flows to travel obliquely on the sides of the canyon. The velocity peaks are coincident with peaks in backscatter in the water column, indicative of peaks in suspended sediment concentration (Fig.…”

Three‐Dimensional Flow Structures and Morphodynamic Evolution of Microtidal Meandering Channels

“…Consistent with these observations, as well as with previous field studies (see Pivato et al, 2018), direct measurements at the study sites highlighted uniform vertical temperature profiles. Hence, we can effectively rule out the chance for the observed secondary flow dynamics to be partially driven by stratification effects (e.g., Azpiroz‐Zabala et al, 2017; Sumner et al, 2014).…”