Floc size asymmetry modulated by tidal dynamics drives net landward sediment transport in a man-made tidal channel

Abstract

In natural aquatic systems, flocculation affects sediment transport by altering particle size and settling velocity. However, the asymmetry in floc size over tidal cycles and its influence on sediment transport, especially in turbid coastal regions affected by human activities, remains understudied. This study uses comprehensive observations from ADV, HR-ADCP, Signature ADCP, LISST, and CTD within a man-made tidal channel located on the west coast of the Korean Peninsula to elucidate the size-dependent sediment transport process. Our findings reveal a tidal asymmetry in bottom shear velocity and particle size distribution (PSD). During ebb tide, the bottom shear velocity was higher, but suspended sediment concentration (SSC), Total Volume Concentration (TVC), and mean size ($M_z$) were lower compared to flood tide. This suggests that advection transport, in addition to resuspension, is a significant source of particles during flood tide. These particles increased SSC, TVC, and landward sediment flux, while low shear velocity facilitated flocculation, resulting in higher $M_z$. Consequently, a higher proportion of microflocs and macroflocs occurred, leading to a higher settling flux from flood tide to high slack water. In contrast, weaker advection transport and low SSC during ebb tide led to lower seaward sediment flux, with stronger shear velocity and lower concentration resulting in weak flocculation and lower $M_z$. During ebb tide, flocculi (10–20 μm diameter) exceeded microflocs and macroflocs, causing a lesser settling flux. This tidal asymmetry in floc size explains the trapping of larger particles during flood tide and high water levels, ultimately leading to net landward sediment flux in the channel over tidal cycles.