Chongming Dongtan Salt Marsh Near The Bottom Of The Water And Suspended Sediment Change Process

With the rapid development of cities in Yangtze River Delta, the potential of available resources and environmental benefits of salt marshes in Yangtze River Estuary are getting more and more concern by experts and scholars. Tidal Flat salt marshes were influenced by the interaction between the land and sea. Tide hydrodynamics played an important role during the morph dynamic process of the tidal flat. The sediment transport and material exchange occurred frequently during the process. Salt marsh not only plays a role of barrier to protect the coastal zone, but also sensitively responds to the environment changes. The bottom of salt marshes is the main place where sediment suspension, settlement and exchange happen. The flow and sediment transport mechanism is the hot issue on tidal beach erosion and deposition and material circulating.In this paper, a salt marshes area of Chongming Dongtan in the Yangtze River Estuary was the study area and the observation periods were spring (in May) and summer (in July and August). There were several main indicators, such as tide water level, near-bottom current velocity, near-bottom sediment concentration and sediment grain size. The process of the near-bottom water and sediment changes in salt marshes were studied. Main conclusions were drawn as follows.(1) According to the measured data in spring and summer, near-bottom average current velocity in the salt marshes was generally low, which was between 3.24cm/s and 7.29cm/s. The near-bottom average current velocity value was less than that of the average upper velocity which was 11.75cm/s. The average velocity of spring tide was 1.5 to 2.1 times than that of neap tide. The average velocity of flood tide about 5.73cm/s was faster than that of ebb tide about 4.03cm/s. There is a shoreward flow and an offshore flow at the peak phases in the flood tide and ebb tide of spring tide. The peak phase of the velocity to shore stage was more obvious than that of offshore flow. The peak phase in neap tide was not obvious. The flow velocity was fast in medium-term tide.The three-dimensional velocity data were measured by the Vectrino Velocimeter. The near-bottom turbulence energy and shear stress were measured by TKE method. The average near-bottom shear stress of spring tide about 1.2N/m~2 was stronger than that of neap tidal about 0.7N/m~2. There was a "bimodal" feature in spring tide and a "single-peak" feature in neap tide. In spring tide, the near-bottom velocity and the near-bottom shear stress were stronger whereas the near-bottom shear stress of flood tide about 1.33N/m~2 was slightly stronger than that of ebb tide about 1.08N/m~2. In neap tide, near-bottom shear stress would increase with the rise of tidewater. The maximum value would occur in medium-term tide. (2) Near-bottom turbidity values were measured by the Solitax sc100 Turbiditor in salt marshes, and suspended sediment concentration data were obtained through the scene of sediment calibration method. During tide periods in the spring, the average concentration of suspended sediment was 2.45g/L, and the average suspended sediment concentration of flood tide stage (2.64g/L) was more than that of ebb tide stage (2.25g/L). During tidal cycles in summer, the average concentration of suspended sediment was 0.8g/L, and the average suspended sediment concentrations of flood tide stage (0.79g/L) was slightly less than that of ebb tide stage (0.8g/L).Suspended sediment particles were measured by LS 13 320 Laser Diffraction Particle Size Analyzer. In the near-bottom salt marshes, the overall average diameter of suspended sediment particles was slender, which was about 15.2μm. The average particle size of suspended sediment of flood tide (16.6μm) was rougher than that of ebb tide (13.8μm). In the distribution of grain size, the percentage of fine sand and clay was the greatest, which occupied about 69.9% of the total volume.(3) It was showed in the regression analysis that the near-bottom shear stress (or turbulent energy) in salt marshes was positively correlated with the level velocity, and the correlation coefficient R~2 ranged from 0.57 to 0.83. No direct correlation between the near-bottom suspended sediment concentration and flow velocity in salt marshes was found. However, the near-bottom suspended sediment concentration had a peak phase and the level velocity was small in the high water level stage of flood tide, which showed that upper suspended sediments settled obviously. There was a positive correlation between near-bottom suspended sediment particle size and the level velocity in salt marsh. The correlation coefficients R2 of two seasons were respectively 0.77 and 0.66. The average particle size changes of suspended sediment in the tidal cycle were similar to the velocity changes: There was a "bimodal" feature in spring tide and a "single-peak" feature in neap tide. In the distribution of grain size, the coarse silt had a great impact on water velocity.(4) According to compared data of the spring and summer, the height and density of saltmarsh vegetation on observation site in spring were significantly less than that of saltmarsh vegetation in summer. The values of average flow rate, shear stress, suspended sediment concentrations and particle size of near-bottom salt marsh in spring were all higher than those of the average values in summer tidal cycle. The results showed that with the growth of vegetation, salt marsh vegetation slowing flow, dissipating energy, and suspended sediment intercepting would gradually be strengthened.