Numerical study on the heterogeneous bottom bed stress map due to coastal Langmuir circulations*

Langmuir cells generally extend until the mixing layer depth and remain unaffected by bathymetry. However, coastal Langmuir cells are different as they extend to the seafloor and interact with the bottom boundary layer. In this large-eddy simulation study, we further probe into the components of such strong, coherent, full-depth, counter-rotating cells i.e. downwelling and upwelling limbs, and examine their individual roles in shaping the characteristics of the whole flow as well as the heterogeneous bottom bed stress map. As such, analysis of turbulent flow quantities of respective limbs, extracted separately through conditional averages, is performed. We show that the downwelling limbs impinging upon the bottom elevates the bed stress in the zone below and can act as a precursor to sediment erosion and resuspension. Furthermore, apart from the helical motions by Langmuir circulations, elevated bed stress is linked with the passage of inclined, large-scale motions composed of high-momentum fluid consequential to the presence of bottom boundary layer. However, coherent Langmuir cells suppress the structural inclination angle of those large-scale motions.