Morphological Response And Dynamic Mechanism Of Mudflat To The Storm Events

Storm is one of the most dangerous natural disasters in the world,and it is also the most important dynamical force for the occurrence of short-term natural disasters in coastal areas of China.Mudflat or muddy flat is widely distributed all over the world and one of the most important features of coastal landforms in China.Muddy flat has ecological environment service functions,shore embankment protection functions,tourism functions,and it also was potential land resources.Therefore,the morphological evolution caused by the strom has great significance on the coastal geomorphology,sedimentary engineering and ecology.In this paper,the study area is the open tidal flat(Nanhui tidal flat)in the front of the Yangtze River Delta.In the typhoon season,RTK-GPS,triple-rods method,buried plate method and ADV method are used to observe morphological evolution;and other high-resolution advanced instruments like HR-Profilereter,AEM-HR/USB,ASM-IV,OBS-3A and SBE-26 were used to observe the hydrodynamic process and suspended sediment.The surface sediment samples collected from the surface were analyzed by laboratory analysis of sediment particle size and water content.At the same time,collected data about the typhoon tracks and the adjacent station wind speed.Based on this,the changes of hydrodynamic process,suspended sediment concentration,bed particle size and the morphological evolution of the tidal flat during the storm events were studied,and dynamic mechanism was analyzed.The main results and conclusions are as follows:1)Impact of storm on dynamic variations in muddy flatThe measured wave height under storm weather in the muddy flat is significantly higher.The maximum values of the significant wave height(2.18 m,1.53 m,1.93 m,2.92 m,2.50 m and 2.09 m)in the strongest stage of the Fung-wong,Vongfong,Phanfone,Chan-hom,Molandi and Mahalka are 2 to 2.9 times higher than that of the calm weather.Storms on the muddy flat flow velocity increased significantly,the maximum flow velocity under typhoon condition is 1.4 to 1.5 times faster than that of the calm weather.The maximum bed shear stresses(?cw)under typhoon condition is 1.4 to 6.2 times higher than that of the calm weather.2)Impact of storm on suspended sediment concentration and sediment particle sizeSuspended sediment concentration on the muddy flat increased dramatically under storm events.The vertical average suspended sediment concentration of near-bottom 0.9 m water layer in the strongest tidal period is 2.0 to 2.6 times higher than that of the calm weather.High suspended sediment concentration layer near bottom(suspended sediment concentration is more than 10 kg/m3)under storm events has a duration of 7 to 8 h in one tidal cycle(2 to 4 h in calm weather).The thickness of high suspended sediment concentration layer near bottom exceeds 0.9 m in one tidal cycle(less than 0.6 m in calm weather).The median particle diameter(37.5?m)on the muddy flat under storm events is 1.2 times bigger than that of calm weather.Surface sediments particle size under storm events are usually thicker than calm weather.3)Impact of storm on muddy flat morphological evolution and its mechanismDuring the storm events,the muddy flat often experienced large-scale accretion and erosion,intensity of morphological change is several times higher than the same tidal condition under the calm weather.The maximum net erosion depth observed in a typhoon event reached 0.7 m and the maximum net accretion thickness reached 0.4 m.Because of the different geomorphological condition before the storms,the different tidal condition and tropical cyclone tracks,the storms are different,the different winds condition between tidal flat and the adjacent tidal waves,the details of accretion and erosion on muddy flat are complex.The change of different transection under the storm event can be net erosion or net accretion.The accretion and erosion of different parts of the transection in the storm may also be different.During the storm,the dynamic mechanism of muddy flat subjected to morphological change is that the storm transmits the huge energy to the water body,which leads to the obvious enhancement of the wave and the dynamic of flow,and the shear stress of the wave and the base bed is obviously increased and the capacity of carrying sediment is obviously enhanced.During the storm,the sediment mechanism of muddy flat subjected to morphological change is that the water content of loose sediments is higher,and the critical shear stress(?e)is lower than that of clam weather.Therefore,the bed shear stress(?cw)is much higher than the critical shear stress(?e)during the storm,which leads to a large number of sediments resuspension,the suspended sediment concentration increases obviously,and the sediment is transported to the adjacent areas,may be in some weakening stage(such as the stage of slackwater)will produce strong accretion in different areas.From the point of sediment balance,the intense sediment exchange between the bed and the water takes place on muddy flat.In addition,the large-scale accretion and erosion are also a kind of sediment exchange,that is,the sediment spatial exchange.