| Intertidal mudflats are located in typical areas of sea-land interaction and are widely distributed in the global coastal system,occupying a unique position in carbon absorption and conversion,biological habitat,wave reduction,shore protection,and land reserve,etc.They have valuable ecological and environmental service functions and high economic and social values.Therefore,exploring the evolution process and mechanism of intertidal mudflats has been the focus of estuarine and coastal research.In recent years,against the background of global warming,frequent extreme weather and increasing human activities,mudflats in estuarine coastal areas have been facing threats such as storms,erosion retreat and ecosystem degradation.Short-term storms cause a significant increase in the hydrodynamics of the intertidal mudflats,resulting in dramatic changes in the morphologic of the mudflats.However,the morphodynamic processes and mechanisms over the mudflats under the influence of storms are still unclear,and field observations and numerical simulations are needed to systematically explore and predict the evolution processes and mechanisms of mudflats under extreme weather conditions,so as to provide the scientific basis for the integrated management and sustainable development of estuarine coastal areas.In this study,field observations and numerical simulations were carried out at the intertidal mudflats of East Chongming and Nanhui in the Yangtze River Estuary during the storm season（July 20,2021,to August 21,2021,and September 22,2014,to September 24,2014）,respectively.High-frequency hydrodynamic,sediment and geomorphology data such as waves,currents,sediment,erosion and accretion during the tidal cycle were obtained before,during,and after the storm using high spatial and temporal resolution tidal shallow water instruments.Based on the Delft3D numerical model and field observations data,a hydrodynamic geomorphological model of the intertidal mudflats is built to investigate the spatial response characteristics of the mudflats morphodynamic under the influence of storms,focusing on the effects of wind speed changes on the spatial patterns of intertidal mudflats morphodynamic during storms.The main conclusions are as follows.（1）Under the storm,the morphodynamical activity of intertidal mudflats intensifies.The maximum water depth during the storm period in similar tidal cycles is1.33 times that of normal weather,the maximum significant wave height is 1.58～4.35times of normal weather,the wave energy reaches 3.20 times of normal weather,the current velocity is 1～1.43 times higher than that of normal weather,the suspended sediment concentration is 1.16～10.56 times of normal weather,the bed shear stress reaches 1.71～2.63 times of normal weather,the suspended sediment transport rate is increased by one order of magnitude compared with the similar tidal cycles in normal weather,and the maximum bed-level change can reach 2.70 times of the same period in normal weather.The storm period changed the current direction of the normal tidal condition over the mudflats,affecting the erosion and accretion patterns during spring-neap tides of the mudflats so that the accretion dominated from neap to spring tides changed to erosion dominated,and the overall erosion magnitude at the tidal flats increased.（2）Spatial differences occur in the morphodynamic response of the intertidal mudflats to the storm.The values of current velocity,significant wave height,suspended sediment concentration,suspended sediment transport rate and bed shear stress during the typhoon Fung-wong vary significantly throughout the mudflats.In the same cross-section,the maximum velocities of the three points distributed from shore to sea during the storm are 0.28,0.33,and 0.56 m/s,the maximum significant wave heights are 0.95,1.25,and 1.49 m,the maximum suspended sediment concentration are6,5.59,and 4.20 kg/m3,the maximum suspended sediment transport rate are 3.77×10-4,8.54×10-4 and 2.20×10-3 m3/s/m,the maximum bed shear stresses are 1.74,0.87,5.02N/m2,and the bed-level changes during the typhoon Fung-wong are-0.13 m,-0.15 m and-0.17 m（negative sign indicates erosion）,respectively.The whole mudflats show a spatial trend of increasing hydrodynamic and sediment transport rate from high to low mudflat during the storm,and the erosion magnitude gradually increased.The main reason for this spatial pattern is the difference in submersion duration and hydrodynamic strength of the low mudflat compared with the high mudflat.The low mudflat is submerged for a long time;the hydrodynamic process is strong,but the suspended sediment concentration is low.The energy tends to increase from high to low mudflat,leading to the intertidal mudflats becoming more sensitive to the morphodynamic response of the storm from high to low mudflat.（3）Wind speed during the storm has a significant effect on the intertidal mudflats morphodynamic.Increasing the wind speed of the storm enhances the hydrodynamics and erosion-accretion magnitude on the mudflats but does not change the trend of erosion and accretion from high to low mudflats during the storm.The maximum wind speed in the region gradually increases from 15 m/s to 20 and 25 m/s;for every five m/s increase in wind speed,the maximum flood velocity in the study area increase by0.20 and 0.30 m/s,the maximum ebb velocity increase by 0.15 and 0.25 m/s,the maximum wave height in the region increase by 0.17 and 0.16 m,the maximum suspended sediment concentration increase by 1.23 and 0.57 kg/m3,and the average maximum suspended sediment transport rate increased by 0.82×10-3,1.30×10-3 m3/s/m.The average maximum bed shear stress increased by 0.10 and 0.30 N/m2,and the erosion thickness increase by 0.07 and 0.09 m during storm.With the increase in wind speed,the spatial difference between the flood and ebb velocity,significant wave height and sediment transport rate on the intertidal mudflats becomes larger,and the slope of mudflats topography becomes steeper.However,the wind speed does not change the submersion time over each part of the mudflats,and the spatial trend of increasing bed-level change from the high to the low mudflats during the storm remains consistent.When the wind speed increase to 20 m/s and above,the current direction difference between the flood and ebb disappear,and the current direction is controlled by the wind and remains consistent with the wind direction.A NE–SW-oriented erosion zone appears in the study area for all storm conditions,which is related to the prolonged strong ENE winds that occurred during the storm. |
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