| The seawater with low suspended sediment concentration(SSC)dominates the sea area off Qinhuangdao all year round,especially in winter,which is an exceptional case since most of coastal waters are turbid in the Bohai Sea.Being one of the most famous coastal tourist resorts,it is crucial to keep the seawater around Qinhuangdao clear and unpolluted.In this paper,the sea-base-observation system was used to conduct synchronous observation of water and sand elements in Qinhuangdao sea area with a long time sequence and full depth,including two observation batches in winter of 2017 and winter of 2018.Three bottom-mooring systems were deployed in the low SSC region from February to March 2017 up to 22 days to reveal the process and mechanism of local sediment resuspension.The observation results showed that during the strongest wind of the whole deployment period,the sediment resuspension process included a turbulent resuspension event occurred in the high turbulent kinetic energy period,and a continuous resuspension event when the bed shear stress(BSS)larger than the critical.The turbulent resuspension event was characterised by high-turbidity resuspended sediments at the bottom in the form of pulsation with an influence depth not more than the thickness of the bottom boundary layer.It was found to be triggered by the storm-wind-driven strong turbulence.For the continuous resuspension,however,it could be attributed to the wave-dominated large BSS.During storm winds,the high concentration of suspended sediment in the upper and middle layers appeared to lag behind the high wind speed by about 8-10 h.In contrast,the turbulent resuspension event occurred in synchronization with the strong winds.The in-situ measurements from all the three moorings indicate that it is the local weak sediment resuspension,even under the influence of storm winds in winter,that causes the formation of the low SSC area off Qinhuangdao.The influence of sea ice on the hydrodynamics,sediment resuspension,and suspended sediment concentration(SSC)in the coastal area of Qinhuangdao was systematically investigated using 45-day in-situ measurements at two stations(with ice at M1 and without ice at M2)in the Bohai Sea in the winter of 2018.It was found that the daily fluctuations of temperature and salinity at M1 are more significant than those at M2.During a typical seawater icing event on January 28,the temperature and salinity of the bottom water at M1 were decreased by 1.77? and increased by 0.4 psu,respectively.Moreover,due to the shielding effect of the sea ice,the residual current was much less affected by the wind at M1 than at M2.For the vertical distribution of current velocity,it changed from traditional logarithmic type under ice-free condition to parabolic type under ice-covered condition due to larger drag coefficient of water body by the solid ice surface.For the SSC and turbidity at the bottom layer,the average values were 4.9 ?L/L and 8.6 NTU at M1,respectively,approximately half of those at M2.The smaller SSC and turbidity at M1 can be attributed to the lower near-bottom turbulent kinetic energy(TKE).At M2,however,the larger SSC is closely related to the strong wind forcing which could induce higher TKE without sea ice cover and hence stronger turbulent resuspension.The seabed sediment analysis results showed that in the study area,fine sand is most likely to resuspend and cohesive particles would resuspend only under strong hydrodynamic conditions. |
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