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Discontinuity in equilibrium wave-current ripple size and shape and deep cleaning associated with cohesive sand-clay beds

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posted on 2025-03-21, 10:56 authored by X Wu, R Fernandez, JH Baas, J Malarkey, Dan ParsonsDan Parsons

Mixtures of cohesive clay and noncohesive sand are widespread in many aquatic environments. Ripple dynamics in sand-clay mixtures have been studied under current-alone and wave-alone conditions but not combined wave-current conditions, despite their prevalence in estuaries and the coastal zone. The present flume experiments examine the effect of initial clay content, C0, on ripples by considering a single wave-current condition and, for the first time, quantify how changing clay content of substrate impacts ripple dimensions during development. The results show inverse relationships between C0 and ripple growth rates and clay winnowing transport rates out of the bed, which reduce as the ripples develop toward equilibrium. For C0 ≤ 10.6%, higher winnowing rates lead to clay loss, and thus the presence of clean sand, far below the base of equilibrium ripples. This hitherto unquantified “deep-cleaning” of clay does not occur for C0 > 10.6%, where clay-loss rates are much lower. The clay-loss behavior is associated with two distinct types of equilibrium combined flow ripples: (a) Large asymmetric ripples with dimensions and plan geometries comparable to their clean-sand counterparts for C0 ≤ 10.6% and (b) small, flat ripples for C0 > 10.6%. The 10.6% threshold, which may be specific to the experimental conditions, corresponds to a more general 8% threshold found beneath the ripple base, suggesting that clay content here must be <8% for clean-sand-like ripples to develop in sand-clay beds. This ripple-type discontinuity comprises a threefold reduction in ripple height, with notable implications for bed roughness.


Funding

EC, FP7, Ideas, FP7 Ideas: European Research Council. Grant Number: 725955

Leverhulme Trust and Leverhulme Early Career Researcher Fellowship. Grant Number: ECF-2020-679

History

School

  • Social Sciences and Humanities

Published in

Journal of geophysical research. Earth surface

Volume

127

Issue

9

Publisher

John Wiley & Sons

Version

  • VoR (Version of Record)

Rights holder

© The Authors

Publisher statement

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited

Acceptance date

2022-09-07

Publication date

2022-09-23

Copyright date

2022

ISSN

2169-9003

eISSN

2169-9011

Language

  • en

Depositor

Mrs Gretta Cole, impersonating Prof Dan Parsons. Deposit date: 1 October 2024

Article number

e2022JF006771