A Study Of The Suspended Sediment Concentration In Response To The Typhoon In The Yangtze Estuary And Its Dynamic Mechanism

The windstorm is one of the most frequent natural disaster-making factors in the coastal zone,and China is one of the most frequent and severe windstorm-ravaged countries.Suspended sediment concentration(SSC)is an important index to measure water quality,and its variations have major influences on seabed erosion/accretion,biological primary productivity and restoration/ loss of land resources.Therefore,it is of great theoretical and practical significance to study the effects of windstorms on the suspended sediment concentration in estuaries and coasts.In this paper,we use the data of synchronized meteorological data(wind speed,wave heights,tides,etc.)and surficial SSC(twice a day)at six stations in the Yangtze estuary in continuous in recent years,combined with high-resolution observational data from the observation platform in typical tidal flat and 20 m-depth seabed.8 typical windstorm events are screened to study(Typhoon " Kompasu"," Meari"," Haikui","Sanba","Fung-wong"," Vongfong"," Moranti ","Malakas"),of which "Moranti " and "Malakas" are the Double Typhoon event(It has the synchronized data of surface survey data,20 m-depth seabed survey data and tidal flat near-bottom measurement data in typhoon events).Through the comparison of the change of water dynamic and SSC during the windstorm period,the intensity and spatial difference of the windstorm to the SSC can be revealed,and we can discusse the regularity and dynamical mechanism of the influence of windstorm on the concentration of estuarine sediment.The main findings and conclusions are:(1)Impact of windstorms on dynamic conditionsThe wind speed of surface survey station under 6 typical Typhoons("Kompasu","Meari","Haikui","Sanba","Fung-wong","Vongfong")was 1.7 times higher than that of before typhoon on averagethe wind speed under "Moranti " and "Malakas" affected on average were 1.7 times(surface),2.2 times(seabed)and 1.6 times(tidal flat)higher than that before the typhoon,the maximum value reached 3 to 5 times than that beforethe typhoon.During the 6 typhoon events,the average significant wave height of the surface survey station was 2.2 times higher than normal weather(0.8 m)before typhoon,and the maximum value of Typhoon " Kompasu " can reach 3.1 times;The ratio of significant wave height during the Double Typhoon events to the significant wave height before Typhoon in surface,20 m-depth seabed and tidal flat near-bottom were 2.4,2.3 and 1.7 respectively,the maximum ratio reaches 6.2 times,9.8 times and 3 times respectively.Under the influence of Double Typhoons,the flow velocity of 20 m-depth seabed and tidal flat near-bottom were 1.4 times and 1.2 times higher than that in the same tidal range before typhoon.During the period of "Moranti"-"Malakas" Double Typhoons,the mean wave-flow combined shear stress were 1.9 to 2.5 times higher than that in the near tidal range of typhoon,which were much larger than the maximum of critical erosion shear stress of sediments.(2)The effect of windstorm on the SSC The average suspended sediment concentration in the designated waters during the 6 typhoons were twice than that in the near tidal range before the typhoon(0.32 kg/m3),and SSC during the Typhoon " Kompasu " was 5.1 times higher than that before the typhoon.The SSC of surface,20 m-depth seabed and tidal flat near-bottom under Double Typhoons "Moranti"-"Malakas" condition were 6.1 times,5.8 times and 3 times higher than before the typhoon respectively,the maximum reached 8.4 times,78 times and 11.5 times than before the typhoon respectively.The thickness of the sediment concentration layer(>10 kg/m3)in the near-bottom of tidal flat was 0.5 m in normal weather,but it reached 0.8-1 m during typhoon.(3)Spatial difference of the influence of windstorm on the SSC The Sheshan station and Luchaogang station outside the Yangtze estuary are located in open waters at six stations,its response of suspended sediment concentration to windstorm events(rate of change are 167.1 % and 143.7 % respectively)compared with the more concealed 4 stations in the mouth(Hengsha,Gaoqiao,Xuliujing,Qinglonggang,the rates of change are at 42.3 % to 116.5%)were more sensitive.Double Typhoon "Moranti"-"Maleka" caused the change of SSC were multiplied whether in the surface,seabed or tidal flat area,the maximum change wasthe 20 m-depth near the bottom,due to the maximum changes of wind speed and wave height,the maximum change reached 78 times than before the typhoon.(4)Dynamical mechanism of the effect of windstorm on the SSC Hydrodynamic change is the key to the effect of windstorm on the SSC.When the windstorm transmits the huge energy to the water body,the wave-flow power is enhanced(as compared with the similar tidal conditions),it make the shear stress increase,then the difference between the shear stress and the sediment shear stress is enlarged,which enhance the re-suspension.The re-suspension accelerates the exchange of sediment and suspended sediment frequencyly,it causes the sediment to be active in the water,which plays an important role in increasing the SSC.On the contrary,after the windstorm the wave-flow force weakens.Then the suspension capacity decreases,and finally it leads the suspended sediment concentration decrease.