Study On The Hydrodynamics And Sediment Transport Characteristics On Tidal Flat With Particular Sediment In Yangtze Estuary

The surface sediment grain size distribution (GSD) and hydrodynamic processes are distinct in different parts of the estuarine tidal flat under the combined action of tide and runoff. Field investigation was carried out in northern and southern parts of Eastern Chongming Island in Nov.2010. With the help of tripod observation system, high-accuracy data was obtained including the current, wave and surface sediment. Begin with the sediment components analysis, the characteristics of the sediment and hydrodynamics are classified and summarized, the suspended sediment-hydrodynamic response machanism and sediment transport process are discussed. The main conclusions are as follows:1. The surface sediment on the northern and southern parts of the Eastern Chongming Island are totally different. The feature of muddy north and silty south is discovered.The medium grain size of the surface and suspended sediment are around8μm and10.4u m respectively in the north. And the sediment is mainly composed of clay and silt, the proportion of sand is zero. So the northern district could be named as muddy flat. The medium grain size of surface and suspended sediment are around64μ m and40.5u m respectively in the south. And the sediment is mainly conposed of silt and sand, the proportion of clay is nearly zero. So the southern tidal flat can be named as silty flat.By comparision of the surface and suspended sediment GSD, higher similarity is found in muddy flat which showing greater exchange ratio between surface and suspended sediment. Similar phenomenon turn up between the flat and channel nearby.2. The current velocity (30-50cm/s) on silty flat is stronger than that(<20cm/s) on the silty flat. It is worth mentioning that the current has the deviation to the saltmarsh in the north, while it is parallel to the saltmarsh edge in the south.The near bed velocity profile fits the logarithm law well except several points closest to the bed surface. And tidal flat bottom boundary layer parameters are caculated. Friction velocity(U*) and shear stress caused by tidal current is around3.5cm/s and1.3N/m2, roughness length Zo=4.35cm in the north. Corresponding value are3.2cm/s,1.ON/m2and1cm in the south.3. The wave attenuation on the muddy flat is greater than that on the silty flat. Along with the increasing distance and elevation, the wave attenuation ratio on the mud flat is1.4and2.9times than that on the silty flat respectively. The relative wave height(average wave height/water depth) is0.04-0.16, having positive linear relationship with wave attenuation. Attenuation of wave with high frequency dominates when the wave is propagating landwards, and there is a clear variation of wave spectral shape with the peak moving to the long period. The shear stess caused by the wave reaches the peak at the beginning and end of the tidal cycle.4. The average SSC has a range of1-1.5kg/m3in the north and it is higher during the flood phase. In the south, the average SSC is about1.2-2kg/m3, no obvious flood time advantage, sometime higher in the ebb time. The SSC tidal asymmetry has a very important effect on the depositon of tidal flat. Generally, the shear stress caused by the current is larger than that caused by the wave. Tidal current is the key factor controlling the SSC. And the relation between the current and SSC is discussed carefully.5. The sediment flux is greater during the flood phase on both sides of Eastern Chongming Island. Total sediement flux is larger in the south than that in the northern part. However, the shoreward net sediment flux is less in the south. The path of the net sediment flux in the north is directing the upstream of the north branch and the saltmarsh. While the net sediment flux is reciprocating between the north channel and subtidal zone in the south. So the sediment transport pattern is fit for flat deposition in the north but erosion in the south.