The Multiscale Characters And Mechanism Of Deposition-erosion In The East Chongming Tidal Flat

The estuary delta is one of the most typical regions of where land and ocean are convergence,with richness and diversity of resources and environment.With the growth of people's demand for exploitation and utilization of resources and the development of social economy,most of the estuarine deltas in the world have become the most densely populated,socially and economically developed regions.Therefore,the discussion on the formation,development and evolution of delta has always been the focus of people's attention,as well as one of the hot issues concerned by physical geography,social economy and other disciplines.Estuarine tidal flat especially estuarine sand island tidal flat is the core area of delta formation,development and evolution.The erosion and deposition of tidal flat at different time scales is the basic representational process of the formation,development and evolution of tidal flat,and the erosion and deposition process of the tidal period characterized by hours and the tidal period characterized by days is the key to the above representational process.However,provious studies have little attention to above scientific processes due to lack of multi-dynamic observation technology and difficulty of field continuous observation for long time.We laied out six observation sites in the East Chongming Island tide flat in Augest and Septemper,2018.The study utilized advanced instruments such as precision ADV(Acoustic Doppler Velocimeter),HR(High Resolution ADCP),OBS-3A(Optical Back Scatter Turbidity Meter),SBE(Sea Bird wave meter)to measure current,suspended sediment concentration,high-resolution depth and wave.Based on in-sites measure data,we try to discuss the sediment dynamic process of deposition and erosion in the estuarine tidal flat.The important found points are as follows.(1)The characteristics of different periodic deposition-erosion of the East Chongming tidal flat.1)Four different sediment stages such as flood erosion,flood deposition,ebb erosion and ebb depsotion periods were observed in every tide cycle;furthermore,the sediment process of every spring stage generally characterized by net depositon,and net erosion for every ebb stage.2)From neap to spring tide,the sediment process of tidal flat generally showed erosion trend,but from spring to neap tide,the sediment process of tidal flat generally showed deposion trend.3)In north and middle tidal flat,the sediment process of neap-spring cycle showed general net deposion such as 25 in intertidal zone and 7 mm in subtidal zone;but,in south tidal flat,the process neap-spring cycle showed general net erosion such as 20 mm in intertidal zone and 100 mm in subtidal zone(2)Dynamic mechanism of different periodic tidal flat deposition-erosion process.1)Four tidal current stages drive four sediment statge,i.e.flood velocity increase,flood velocity increas,ebb velocity devease drive flood erosion,flood deposition,ebb erosion and ebb deposition process,respectively.2)Suspended sand setting in flood decrease period result in net deposition in whole flood tide cycle,and long-term flow in ebb increase period result in net erosion in whole ebb tide cycle.3)increase of mean tidal velocity and low suspended sand concentration cause to net erosion from neap to spring tide,and the decrease of mean tidal velocity and high suspended sand concentration resulte in net deposition from spring to neap.4)The changes of salinity and flow direction indicate that the net increase of suspended sediment concentration in north and central is mainly caused by the sharp increase of advection sediment transport in later period.The net decrease of suspended sediment concentration in the south is a sharp decrease in the amount of suspended sediment input from the outer tidal flat.(3)Typhoon has a significant impact on deposition-erosion dynamic environment in flat.Duing typhoon period,the water level in north,central and south zone increased 30,40 and 44 cm,respectively;and the wave height and energy density increased 1.8~3.4 times and 3.7~4.3 times more than that of normal wearth in intertidal zone.The amplitude of deposition-erosion increased 1.6 times more than that of the normal weather.The above results has deepened understandant of tidal flat erosion-deoposition process in estuarine tidal flat,and provide references for risk evaluation of erosiondeposition in esturine tidal flat,especially under the background of sediment flux into sea sharply decline and sea level rapid rise.