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Relating the Flow Processes and Bedforms of Steady-State and Waning Density Currents

de Cala, Isabel; Ohata, Koji; Dorrell, Robert; Naruse, Hajime; Patacci, Marco; Amy, Lawrence A.; Simmons, Steve; McLelland, Stuart J.; McCaffrey, William D.

Authors

Isabel de Cala

Koji Ohata

Hajime Naruse

Marco Patacci

Lawrence A. Amy

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Dr Stuart McLelland S.J.McLelland@hull.ac.uk
Deputy Director of the Energy and Environment Institute

William D. McCaffrey



Abstract

© Copyright© 2020 de Cala, Ohata, Dorrell, Naruse, Patacci, Amy, Simmons, McLelland and McCaffrey. The interaction between turbidity currents and mobile substrates can lead to the development of different types of bedforms. Although much research has been conducted on bedform development beneath open channel flows, research into bedform development beneath waning gravity currents is relatively rare. Analysis of density current-related bedform development has therefore relied upon open channel flow phase diagrams. We report on an experimental study designed to assess the development of bedforms under steady and waning saline density currents. The experimental density currents developed stepped density profiles in which a higher-density basal zone was separated from the ambient fluid by a zone of intermediate density; any bedforms that developed were contained within the bottom layer of the current. Under different conditions ripples, dunes, downstream migrating antidunes and long wavelength antidunes were observed to form and could be distinguished based on their interactions and phase relationships with the upper surface of the lower denser layer of the current. Due to limited mixing between the upper and lower layer of the current and maintenance of current momentum, currents set with slowing discharge flow rates maintained a steady flow velocity in the lower layer of the flow. As a result, sustained bedform formative conditions were achieved within this lower layer, while waning current conditions effected the rest of the flow. Under waning currents, it was seen how pre-existing bed states can determine the subsequent evolution of bedforms. This illustrates the limitations of existing phase diagrams as they do not account for trajectory or rate of passage of flows through different bedform phase spaces. In order to establish a reliable quantitative association between the flow regime and the type of bedform development, it is critical to adopt an appropriate Froude number calculation method for stratified flow. The updated density current phase diagram indicates supercritical flow can be achieved at lower flow velocities than for open channel flows due to the effects of reduced gravity. Bedform depositional structures found in outcrop and on the modern sea floor provide data that helps to interpret the hydrodynamic and sedimentological character of the current that formed them. Therefore, understanding the processes involved in bedform development beneath density currents will enable more accurate estimation of the properties of flows.

Citation

de Cala, I., Ohata, K., Dorrell, R., Naruse, H., Patacci, M., Amy, L. A., …McCaffrey, W. D. (2020). Relating the Flow Processes and Bedforms of Steady-State and Waning Density Currents. Frontiers in Earth Science, 8, https://doi.org/10.3389/feart.2020.535743

Journal Article Type Article
Acceptance Date Aug 28, 2020
Online Publication Date Oct 22, 2020
Publication Date Oct 22, 2020
Deposit Date Nov 6, 2020
Publicly Available Date Nov 9, 2020
Journal Frontiers in Earth Science
Print ISSN 2296-6463
Electronic ISSN 2296-6463
Publisher Frontiers Media
Peer Reviewed Peer Reviewed
Volume 8
Article Number 535743
DOI https://doi.org/10.3389/feart.2020.535743
Keywords Bedforms; Saline density current; Turbidity current; Dunes; Ripples; Antidunes; Experimental
Public URL https://hull-repository.worktribe.com/output/3654902
Publisher URL https://www.frontiersin.org/articles/10.3389/feart.2020.535743/full

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Copyright Statement
Copyright© 2020 de Cala, Ohata, Dorrell, Naruse, Patacci, Amy, Simmons, McLelland and McCaffrey. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.



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