Sediment Dynamical Processes In The South Passage Of The Yangtze Estuary During The Typhoon Lan

The estuary is an active part of land-sea interaction.The river carries large amounts of water,sediment,and nutrients into the ocean through the estuary.The Yangtze River estuary suffers from typhoons on average 3.5 times per year.Typhoon has an important impact on the sediment dynamical processes of the estuary.The erosion of wind and waves on sediment is strong,which causes channel deposition.However,previous observations on the sediment dynamical processes were mostly carried out in the calm weather.At the same time,it is difficult to obtain high-quality data near the bottom due to the limitations of the traditional observation methods.This thesis uses a variety of high-resolution instruments to observe the first-hand continuous data of sediment dynamical processes during a typhoon event and compares with those in the calm weather.This study helps to understand the impact of the typhoon on the sediment dynamical processes of the estuary and related physical processes.This thesis focus on the South Passage of the Yangtze River estuary.Field observations were carried out in October 2017 during the occurrence of the Super Typhoon Lan by a shipborne system and a bottom-mounted system with various instruments.The data of temperature,waves,current velocity,salinity,and suspended sediment concentration(SSC)for 15 consecutive tidal cycles were collected.Based on laboratory experiments and formula calculations,the time series of wind speed,wave,current velocity,salinity,SSC,bed shear stress,bed erosion,and sediment transport during the typhoon were shown.The influences of different factors on SSC were analyzed.The applicability of the parabolic method to fit the current velocity of the bottom blind zone of ADCP was discussed.The mechanism of water and sediment transport was summarized.It was found that the waves had an improtant contribution to the bed shear stress,the wave height and wave shear stress had flood-ebb asymmetry,and the SSC increased significantly.The main phenomena and conclusions obtained are as follows:During the period of Typhoon Lan,the average wind speed near the South Passage was 2.2 times that of calm weather.The maximum wind speed measured was 16.5 m/s.On the one hand,the northwest wind not only suppressed the flood current velocity and duration but also increased the ebb current velocity and duration.On the other hand,the northwest wind inhibited the intrusion of saltwater,and enhanced the stratification,reduced the salinity of the water column significantly.While the processes were reversed with regard to the northeast wind.The maximum value of the maximum wave height in the South Passage was 2.14 m,the maximum value of the significant wave height was 0.76 m.The significant wave height during flood tide was about 1.5 to 2.7 times that of the ebb tide,which contributed to suspend more sediment.The bed shear stress was significantly increased by the combined action of wave and current during the typhoon,which was about 5times that of calm weather.During periods of strong winds and waves,slack current of spring tide,and neap tide,waves made a significant contribution to the total bed shear stress.During the observation period,the SSC was high.The depth-average SSC of a tidal cycle was 0.12~1.29 kg/m~3 while the near-bottom SSC was 0.55~5.79 kg/m~3.During the tidal cycle when the wind speed gradually weakened,the fluid mud appeared near the bottom with the maximum thickness exceeded 1.15 m.The source of sediment was probably related to the strong erosion of the surrounding shallows by the strong winds and waves during the typhoon.In a tidal cycle with fluid mud,when the hydrodynamic condition was strong and the salinity stratification was weak during the peak current,the thickness of the fluid mud increased,and the situation was opposite during the slack current.Nearly 90%of the velocity profiles near the bottom did not conform to the logarithmic distribution,the applicability of the parabolic method to fit the current velocity of the bottom blind zone of ADCP was discussed.The results showed that this method had good applicability.In one tide cycle,the relative errors were lowst in peak current,and the relative errors in the flood tide was significantly lower than that in the ebb tide.When fluid mud appeared,the actual current velocity near the bottom was obviously greater than the fitted current velocity.The net transport direction of water and suspended sediment in the South Passage were seaward during the observation period.The Euler residual current effect was the main mechanism of suspended sediment transport to the sea,and the Stokes drift and vertical gravitational circulation were the main mechanisms of suspended sediment transport to the land.The tidal pumping transport to the sea significantly in the tidal cycle with fluid mud.In the boundary layer near the bottom,although the net suspended sediment was transported to the land in some of the tital cycle during spring tide and the neap tide with gravity circulation,the net suspended sediment was transported to the sea significantly in some tidal cycles with fluid mud,which caused the suspended sediment net transport to the sea near the bottom during the observation period.