Contribution of local erosion enhanced by winds to sediment transport in intertidal flat

Abstract

Field work combining in-situ mooring and erosion experiments was conducted in the intertidal flat of Asan Bay to reveal the relationship between sediment erodibility and external forcing and the contribution of local erosion to sediment transport. A total of 25 tidal cycles occurred during the mooring period with 6-h submergence and 5.7-h air exposure. The erosion rate (E) for sediment bed during air exposure was 0.007 g m−2 s−1, which was 36% of E (0.019 g m−2 s−1) during submergence. Although the sediment bed was influenced by consolidation during air exposure, the erodibility of the submerged bed was closely associated with the unconsolidated deposits of sediments newly supplied from tidal channel and bed shear stress (τcw) induced by current-wave interactions. In particular, τcw could be enhanced by strong winds, which depended on the ratio (Hs/h) of significant wave height and water level, directly disturbing the sediment bed. Under weak winds (Hs/h < 0.06), the newly deposited sediments with high erodibility increased the bed elevation. A small amount of the sediment bed was eroded (E: 0.006 g m−2 s−1), as a floc erosion, with a slight decrease in suspended sediment concentration (SSCT). This increased the contribution (57.2%) of eroded sediments to SSCT. Under strong winds (Hs/h > 0.06) with enhanced τcw, meanwhile, the erodible sediment in the bed was depleted accompanied by a decrease in bed elevation. A large amount of sediment bed was eroded (E: 0.025 g m−2 s−1), as a surface erosion, resulting in the rapid increase in SSCT and the enhanced seaward sediment flux. The sediments eroded from the bed were supplied to water column accounting for 86.8% of SSCT. This suggests that seaward sediment transport under strong winds could be caused by surface erosion shifting from floc erosion in the intertidal flat.