Influence Of Typhoon On Sedimentary Dynamic Process Of Intertidal Wetland In Yangtze Estuary

Tropical cyclones are one of the most serious natural disasters to human beings.The typhoon is a high-intensity tropical cyclone that has a huge impact on many coastal areas.Nowadays,at the background of global warming and sea level rising,the frequency and intensity of typhoons?hurricanes?and the damage caused by them have gradually increased,which has become an indisputable fact.The impact of the typhoon on the social economy has been widely known.However,there are few studies on the impact of typhoons on coastal ecological environment.Intertidal wetlands are fragile ecosystems affected by typhoons.It is important to carry out in-situ monitoring of tidal-sedimentary wetland sedimentary dynamic processes.This paper selected the Nanhui tidal flat?no tidal creek development?and a tidal creek of Eastern Chongming in Yangtze River estuary as research objects,using advanced high-resolution tidal currents,waves,suspended sediment concentration?SSC?,elevation and other measuring instruments for in situ observation and obtained a large amount of data before,during and after typhoons?involving 15 typhoons,which are 60 to 4000 km away from the study area?,and carried out an original study of the typhoon on sedimentary dynamic process of intertidal wetlands.The main conclusions are as follows:1)Impacts of remote typhoons on coastal hydrodynamics.The tidal flat hydrodynamics were significantly enhanced due to the impact of remote typhoons.The correlation analysis between the significant wave height at the nearshore station and the impact factor of the remote typhoons shows that the nearshore wave height is not completely synchronized with the typhoon intensity,there is a time lag of 1⁴tidal cycles between them.Because there is a long distance between the typhoon eyes and the study area,and the swells generated by remote typhoons requires some time to spread to the Yangtze Estuary.In order to eliminate the influence of tidal range on tidal flow velocity and wave height,a comparative study on hydrodynamics was conducted between two similar tidal range tidal cycles,which were during calm and typhoon weather conditions,respectively.The results show that during the typhoon period,the tide-averaged significant wave height is 3.1 times larger than the value under calm weather conditions,the tide-averaged current velocity is 1.5 times larger than the value under calm weather conditions,tide-averaged?_cw is 1.8 higher than the value under calm weather conditions.2)Impacts of remote typhoons on SSC.There is a significant positive correlation between the tide-average SSC and?_cw,and SSC increases with?_cww following a power trend.This shows that the remote typhoons affects the SSC in tidal wetland by affecting the hydrodynamics of the tidal flat.In the typical comparison between two tides that are equal in tidal range and high tidal level,the SSC in the typhoon-influenced tides were 1.7².9 times higher on average than the SSC in the calm weather conditions.At the moment when the typhoon affected significantly,the SSC near the bottom of the tidal flat exceeded the upper limit of instrument detection,and the maximum thickness of the fluid mud layer was close to 1 m.The influence of typhoon on SSC lasts longer than the impact on hydrodynamics,because the hydrodynamics gradually weaken in the later stage of typhoon impact,and the deposition of suspended sediments in water column takes some time,that is,the sedimentation hysteresis effect of suspended sediments.3)Impacts of remote typhoon on seabed stability.The high-frequency relative elevation measurement of tidal flat shows that the tidal flat elevation is obviously affected by the remote typhoons.During the typhoon Lionrock,Twin Typhoons Meranti and Malakas and Megi,there was obvious erosion on the surface of the tidal flat.After Twin Typhoons Meranti and Malakas,the tidal flats showed a state of rapid accretion.Near the observation tripod,the change trend of the relative elevation of the tidal flat is negatively correlated with?_cw,and the correlation coefficient between these variables is 0.6.This indicates that the mudflat tends to erode as the?_cww increases,and vice versa.On the cross-shore elevation profile,the tidal flat elevation observation shows that during the influence of the Twin Typhoons,the inner salt marsh area and the middle part of the intertidal zone caused rapid accretion,with a mean value of 0.21 m,a maximum value of 0.45 m;At the transition zone of salt marsh and mudflat,and the lower intertidal zone are in an erosive state,and the erosion at the junction of the mudflat and salt marsh is the strongest,with a maximum erosion thickness of 0.28 m.In addition,a sand bar at the junction of salt marsh and mudflat,the sand bar retreated landward about 30 m after Twin Typhoons.The top of the sand bar slope narrowed and the slope became steeper,the retreated sand body buried the original plants.4)Impacts of remote typhoons on wetland surficial sediment.The grain size and composition of surficial sediment in tidal flats are very sensitive to the hydrodynamic changes of tidal flats.During the typhoon period,the tidal flat hydrodynamics is greatly enhanced,and the fine-grained suspended sediments are difficult to settle.The original fine particles in the tidal flat surface may be resuspended into the water column,then transported and redistributed along with the tidal currents.The grain size appears coarser,and the sand content in the sediment component increases.After the typhoon influences,the tidal flat hydrodynamics gradually weakens,the tidal flat surface begins to accept the sedimentation of suspended particles in the water column,and the grain size of surficial sediment appears rapidly refine.The content of clay and silt in the sediment component is increased.The variation of the surface sediment size and composition of the tidal flat is not completely synchronized with the change of the remote typhoon intensity,with a time lag of 1⁴ tide cycles.Because the remote typhoon is far away from the study area,it takes a period of time for the typhoon-generated swell to spread to the near shore.In addition,the sediments in different sections of the intertidal wetland have different responses to typhoons.The surface sediments of the bare mudflat are most affected by typhoons,and the sediment coarsening can reach more than twice the particle size of the original sediments.Second in the tidal creek,middle and lower part of the salt marsh flat,the middle and upper parts of the salt marsh tidal flat have the least impacts.After the typhoon,the surficial sediments were better sorted than before the typhoon,and the kurtosis became narrower.5)Influence of typhoon on transport of water and sediment in tidal creek.Compared with the calm weather,the nearshore wind speed increased by 3⁴ times,the siginificant wave height increased by 7¹5 times,and the tidal creek current velocity increased by about 1² times during the influence of typhoon Yagi and Rumbia.The increase of storm waves,heavy rainfall and current velocity has increased the disturbance to the water column.The bed sediments are suspended into the water column in a large amount,so that the concentration of suspended sediment in the water column is 3¹1 times larger than that in the calm weather?similar tidal conditions?.Sediments entering the water column have moved away and redistributed with the tidal current,causing erosion of the tidal flats.The water column with increased sediment content enters and outputs the inner of the salt marsh and tidal creek.The sediment transport per unit wide during storms was 4³3 times larger than the value in calm weather,and the net sediment transport per unit wide increased by8¹7 times.However,due to the weak hydrodynamics inside the salt marsh,the dense salt marsh vegetation has the function of catching sand and preventing beach erosion,so that the net sediment transport per unit wide was transported to inner salt marsh tidal flat,which promoted the deposition of sediment in the salt marsh tidal flat.In summary,typhoons?even remote typhoons that passing by hundreds or thousands of kilometers away?have an important impact on the sedimentary dynamics of the intertidal wetlands in the Yangtze River estuary,and should be given high priority in the protection and management of coastal wetlands.