Discontinuity in Equilibrium Wave-Current Ripple Size and Shape and Deep Cleaning Associated With Cohesive Sand-Clay...

Area of Science:

• Earth and Environmental Sciences
• Fluid Dynamics
• Sedimentology

Background:

• Sand-clay mixtures are common in aquatic environments like estuaries.
• Previous studies on ripple dynamics focused on current-alone or wave-alone conditions.
• Combined wave-current conditions are prevalent but understudied for sand-clay ripple formation.

Purpose of the Study:

• To investigate the effect of initial clay content (C0) on ripple development under combined wave-current flow.
• To quantify how varying clay content influences ripple dimensions and bed characteristics.
• To understand the mechanisms of clay winnowing and its impact on ripple morphology.

Main Methods:

• Conducted flume experiments using sand-clay mixtures with varying initial clay content (C0).
• Applied a single, consistent combined wave-current condition.
• Quantified ripple dimensions, growth rates, and clay transport rates during ripple development.

Main Results:

• Inverse relationship observed between initial clay content (C0) and ripple growth rates.
• Clay winnowing transport rates decreased with increasing C0 and as ripples approached equilibrium.
• A critical threshold (around 10.6% initial clay, corresponding to <8% beneath the ripple base) was identified, distinguishing between large, clean-sand-like ripples and small, flat ripples.

Conclusions:

• Initial clay content critically controls sand-clay ripple morphology and development under combined wave-currents.
• Higher clay content inhibits 'deep-cleaning' and leads to smaller, flatter equilibrium ripples, significantly reducing bed roughness.
• The findings highlight a discontinuity in ripple types based on clay content, with implications for sediment transport and coastal morphology.