Surface Sediment Transport Trends, Grain Size Characters And Its Environmental Recorded By The Core Sediment In The Tidal Flat Around Yuantuojiao Point, Estuary Of The Northern Branch Of The Changjiang River

The Yuantuojiao Point located at the southern tip of Jiangsu Province coast, belonging to the north branch of the north wing of the Changjiang River with rapid accretion. Its unique tidal depositional environments and complex dynamic process determined on the coastal environment in the region was very sensitive to changes in response. Runoff, wave, tidal, littoral current and storm surges were major factors in the vicinity of tidal landforms dynamic changes in this region. Sediment recorded the environment change information. Surface samples and core samples were collected from tidal flat around Yuantuojiao point. Based on experimental analysis of grain size, radionuclide137Cs dating, corresponding with the field investigation and remote sensing image analysis. The sedimentary processes and environmental evolution of tidal flat around Yuantuojiao point were discussed as follows aspects:firstly, the surface sediment transport trends and grain size characters; secondly, the modern deposition rate; thirdly, the response to sediment hydrodynamic environment form development of tidal flat.The result of analyzing of surface sediment grain size parameters shows that the sediments are mainly composed of fine sand, which is well sorted with positive skewnesses. Due to the impact of human activities, such as large-scale reclamation, hydrodynamic force of tidal flat experienced obvious change in this region. In front of the dike, the hydrological dynamics is intensified and the fine sediments is hard to deposit in front of the dike. Tidal flat type gradually has been changed from muddy flat into sandy beach. The plane distribution of sediment grain size parameters reflects the sedimentary processes has a good corresponding relation with sandbar-grooved terrain The results of sediment grain size analysis show that the tidal flat sediments are mainly composed of sandy silt and silt, indicating that the area is mainly fine-grained sediment. The grain size of YTJ-5core sediments from the bottom to the up was more and more thin, belonging to a typical tidal deposition. Sediments contained lots of plant roots in the upper part (0-70cm), and the deposition rate was strengthened obviously, that was related to the large-scale introduction of Spartina alterniflora which can promote sediments deposit and protect the coast. At the depth of100-160cm of YY Core sediments, there are some coaser silt layers, which may be related to the storm surge, indicating that storm surge is an important power which can change the tidal flat sedimentary process in a short period of time. The grain size of YY core sediments from the bottom to the up was thinner first then thicker. Under the depth of116cm, the grain size of core sediments became more and more thin with the tidal continuously deposit, that was related to the natural silting up of the tidal flat. Up the depth of116cm, sediments were coarser, that was related to the large-scale reclamation in the region. Obviously changes in the core grain size records were often associated with the environment changes induced by storm surge and human reclamation.The137Cs dating results of the YY core revealed that the sedimentation rates at the Yuantuojiao Point changed rapidly with the silting up of the tidal flats since1960’s. The rapid deposition caused probably by human activities such as large-scale reclaimation and the introduction of Spartina altemiflora. Between1975-1986, the deposition rate of YY core sediments was strengthened obviously, even reached4.73cm/a, indicating that the sedimentation rate changes in the areas near to Yuantuojiao Point were mainly controlled by the local sedimental environment.Sediment transport influences the formation and development of tidal flat physiognomy and sedimentary structure. There developed a series of sand spits in the near shore around Yuantuojiao Point, extending from the northwest to southeast direction, which almost parallel to the coast. By extracting the image data of the coastal spit, author find that sediment transport by alongshore current is an important factor which can strongly influence the topography of the coastal tidal flats near to Yuantuojiao Point.