| The intertidal flat is one of the important ecosystems on the earth. It provides habitats for migratory birds and benthos, feeding areas for fish, dissipating waves for protection of seashore, and filtering contaminants. It plays an important role in resource and environment of coastal zone. In the tendency of the growing speed of the global sea level rise, the intertidal flat can also resist erosion of bed, dissipating water energy and accelerating sedimentation process, and reducing cost of coast engineering. The intertidal flat is becoming hot topic of multi-disciplinary and multi-field research, and is an important component in the study of coastal and estuarine.In this study, the exposed mudflat-salt marsh transition was selected as the study regions. The content of measurement is included:(1)Water depth, Maximum wave height, Significant wave height and Wave energy using SBE26plus Wave and Tide Recorder, in Eastern Chongming, Yangtze Delta;(2) measuring current profile using PC-ADP和P ADP-XR;(3) measuring near bottom suspended sediment concertration using OBS-3A;(4) surveying bed level change in situ using RTK-GPS, Double-rods and ADV;(5) sampling sediment for analyzing water content and grain size; The results of calculation are:(1) Bed shear stress due to current, Bed shear stress due to waves, and Bed shear stress due to current and waves;(2) Critical shear stress for the erosion of bottom sediment;(3) Phases of erosion and deposition, and net bed level changes. The main results are as following:1) wave characteristics and wave attenuation over transitional zone of mudflat and salt marsh. The change of wave parameters is different in this study area in space and time, Average wave height, Significant wave height, Maximum wave height, Average wave energy and Significant wave period ranged from0.2to0.5m, from0.3to0.7m, from0.6to1.3m, from～0to155J/m2, and from2to8s, respectively. The variation characteristics are:(1) the significant wave height at the southern section is greater than that at the northern section; there are close relationship between Maximum wave height and significant wave height (P<0.01);(2) Take the northern section for an example, when the waves passed through a cross-shore distance of610m over the bare flat; the significant wave height, mean wave height and mean energy decreased by55.3%,56.8%and80.3%, respectively, The significant wave height, mean wave height and mean energy decreased by24.1%,21.8%and33.3%, respectively, when the waves passed through a cross-shore distance of9m over the Spartina alterniflora marsh, The decreasing rates (relative decrease per meter of cross-shore distance) of the tidal cycle maximum of burst-based mean wave height, the tidal cycle average of burst-based mean wave height, the tidal cycle maximum of burst-based significant wave height, the tidal cycle average of burst-based significant wave height, the tidal cycle maximum of burst-based mean energy and the tidal cycle average of burst-based mean energy, are14.9,25.6,24,29.7,47.5and58.1times of higher over the Spartina alterniflora marsh than over the mudflat, respectively;(3) in time, wave height and wave period during storm day are greater than that during calm day, and that during high water are greater than that during other phases.2) Current and direction characteristics over transitional zone of mudflat and salt marsh.(1) The current direction was clockwise rotary. It tended to be southwestward (onshore) at early flood, to be alongshore at the high tide, and to be eastward or southeastward (offshore) at late ebb.(2) the current ranged from～0to～0.8m/s, the flood period was shorter than the ebb period, and the flood current velocity was higher than the ebb current velocity, showing tidal asymmetry. On average, the ratio of flood to ebb period was0.75, and the ratio of flood current velocity to ebb current velocity was1.8;(3) the major portion of the current profiles (78%) accorded with the lognormal distribution.3) Characteristics of suspended sediment concertration (SSC) over transitional zone of mudflat and salt marsh. In this study area, suspended sediment concertration (SSC) ranged from～0.2to6.8g/1. Average SSC (35cm above the bed) at the mudflat, the border between bare flat and marsh, the marsh was1.2,1.8and1.9g/l, respectively, and the maximum of that was5.2,4.9and5.1g/1, respectively. On average, SSC during the flood period was three times than that during the ebb period4) Characteristics of Bed shear stress of combined current-wave action over transitional zone of mudflat and salt marsh.(1) In the middle section, Bed shear stress due to current (τc) ranged from～0to0.87N/m2(0.21N/m2on average), waves (τw) ranged from0.03to0.49N/m2(0.20N/m2on average). On average, τc is equivalent to τw;(2) Bed shear stress of combined current-wave action (τcw) ranged from0.01to 1.11N/m2(0.27N/m2on average) using van Rijn model and Soulsby model. On average, the results from van Rijn model and Soulsby model were close (the difference is2%), but τcw(van Rijn model)> τcw (Soulsby model), when τc<τw, and τcw(van Rijn model)<τcw (Soulsby model), when τc> τw.(3) Comparing coast with reversing current, Bed shear stress of combined current-wave action in Eastern Chongming Island was complicated.(4) Bed shear stress of combined current-wave action in the mudflat was one order bigger than that at the nearby marsh.5) Erosion and deposition under combine current-wave action.(1) Base on sampling and analyzing bottom sediment, Critical shear stress for sediment erosion (τce) at the mudflat and the marsh was0.103N/m2and0.116N/m2, respectively.(2) Intertidal phases of erosion and deposition determined by comparison between τcw> τce and τcd (Critical shear stress for the deposition of suspend sediment, τcd=0.05N/m2). The results showed that on the mudflat, τcw was larger than the critical erosion shear stress (τce=0.103N/m2) for70%of the period of submergence, and the mudflats were dominated by erosion, whereas at the salt marsh, they were governed by deposition;(3) the results mentioned was in agreement with the observed bed-level change during the study period.Overall, there existed significant differences in sediment dynamics in space and time over transitional zone of mudflat and salt marsh in Eastern Chongming Island. Given normal weather, the value of τc is approximately equal to that of τw, we predicate that the value of τc is more than that of τw under storm weather. Therefore, we should take over combined current-wave action when we understand sediment dynamic in the mudflat. In view of this fact that tidal salt marsh vegetation affects the hydrodynamics and sedimentation process greatly, it is very important for protection of salt marsh in the context of growing speed of the global sea level rise and increasing frequency to storm weather. This result from present study may be incorporated into other places. However, there has a long way to study sediment dynamic process in the intertidal flat with the aid of advanced oceanographic instruments and accumulating first-hand data.
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