23 Aug 2023
 | 23 Aug 2023
Status: this preprint is open for discussion.

Influence of cohesive clay on wave–current ripple dynamics captured in a 3D phase diagram

Xuxu Wu, Jonathan Malarkey, Roberto Fernández, Jaco Baas, Ellen Pollard, and Daniel Parsons

Abstract. Wave–current ripples that develop on seabeds of mixed non-cohesive sand and cohesive clay are commonplace in coastal and estuarine environments. However, laboratory research on ripples forming in these types of mixed-bed environments is relatively limited. New large-scale flume experiments seek to address this by considering two wave-current conditions with initial clay content, C0, ranging from 0 to 18.3 %. The experiments record ripple development and pre- and post-experiment bed clay contents, to quantify clay winnowing. The experiments are combined with previous data to produce a consistent picture of larger and smaller flatter ripples over a range of wave-current conditions and C0. Specifically, the results reveal a sudden decrease in the ripple steepness for C0 > 10.6 %, likely associated with hydraulic conductivity. Accompanying the sudden change in steepness is a gradual linear decrease in wavelength with C0 for C0 > 7.4 %, which may be significant for paleoenvironmental reconstruction. Moreover, for a given flow, the initiation time, when ripples first appear on a flat bed, increases with increasing C0. This, together with the fact that the bed remains flat for the highest values of C0, demonstrates that the threshold of motion increases with C0. The inferred threshold enhancement, and the occurrence of large and small ripples, is used to construct a new three-dimensional phase diagram of bed characteristics involving the wave and current Shields parameters and C0, which has important implications for morphodynamic modelling. Winnowing occurs for both flat and rippled beds, but the rate is two orders of magnitude smaller for flat beds.

Xuxu Wu et al.

Status: open (until 10 Oct 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1375', Sjoukje de Lange, 13 Sep 2023 reply
  • RC2: 'Comment on egusphere-2023-1375', Anonymous Referee #2, 03 Oct 2023 reply

Xuxu Wu et al.

Xuxu Wu et al.


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Short summary
The seabed can change its shape from flat to undulating, known as ripples; the change rate depends on whether the bed is composed of sticky mud. This study, based on experiments, shows a strong reduction in ripple size when the initial mud content over 10.6 %. This could help geologists better interpret the ancient sea conditions. We present a new model to predict ripple size by considering sticky mud. It should help engineers to describe the behavior of the seabed when sticky mud is present.