Modified Stokes drift due to surface waves and corrugated sea-floor interactions with and without a mean current

TL;DR

This study investigates how surface waves interacting with corrugated sea-floors via Bragg resonance can significantly alter Stokes drift, affecting particle trajectories, with implications for pollution mitigation and oceanic tracer transport.

Contribution

The paper provides a theoretical and numerical analysis of how bottom ripples influence Stokes drift through Bragg resonance, including effects of mean currents and potential applications.

Findings

Reflected waves can reverse particle drift direction.

Ripples act as particle traps or reflectors.

Increased ripple size enhances reflection and drift alteration.

Abstract

In this paper, we show that Stokes drift may be significantly affected when an incident intermediate or shallow water surface wave travels over a corrugated sea-floor. The underlying mechanism is Bragg resonance -- reflected waves generated via nonlinear resonant interactions between an incident wave and a rippled bottom. We theoretically explain the fundamental effect of two counter-propagating Stokes waves on Stokes drift and then perform numerical simulations of Bragg resonance using High-order Spectral method. A monochromatic incident wave on interaction with a patch of bottom ripple yields a complex interference between the incident and reflected waves. When the velocity induced by the reflected waves exceeds that of the incident, particle trajectories reverse, leading to a backward drift. Lagrangian and Lagrangian-mean trajectories reveal that surface particles near the up-wave side of the patch are either trapped or reflected, implying that the rippled patch acts as a non-surface-invasive particle trap or reflector. On increasing the length and amplitude of the rippled patch; reflection, and thus the effectiveness of the patch, increases. The inclusion of realistic constant current shows noticeable differences between Lagrangian-mean trajectories with and without the rippled patch. Theoretical analysis reveals additional terms in the Stokes drift arising from the particular solution due to mean-current--bottom-ripple interactions, irrespective of whether Bragg resonance condition is met. Our analyses may be useful for designing artificial, corrugated sea-floor patches for mitigating microplastics and other forms of ocean pollution. We also expect that sea-floor corrugations, especially in the nearshore region, may significantly affect oceanic tracer transport.