Analysis Of Reclamation In The North Branch And Its Effects On Hydrodynamic Environment And Morphological Change For Nearly30Years

The shorelines are changed greatly by the reclamation in the coast and estuary, which has an impact on the flow fields near the projects and the sediment transportation, followed by the changes of long-term stable erosion and deposition of the seabed. The morphologic adjustment of the seabed adapt to the flow and sediment fields. Reclamations without scientific and proper planning have negative effects on the surrounding sea areas. Therefore, it is important to conduct the feasibility studies on the use of sea areas in the initial stage of the projects.The North Branch is the first tidal inlet into the sea of the Yangtze Estuary. In recent50years, the reclamations amount to an area of406km2, which covers40%of Shanghai. The reclamations have changed the shorelines of North branch. The runoff has been weakened, while the tidal current has been enhanced, which initiate sea bore and sea water intrusion into the South Branch. Therefore, reclamations become the main control factor in the change of hydrodynamic environment and the evolution of channel in the North Branch. Nowadays, with the increase of human activities, it is of important theoretical significance to study the change of hydrodynamic environment and suspended sediment concentration, and trend of evolution of channel bed with the case of reclamations in North Branch.Base on the hydrodynamic and morphologic observations, and the basic equations and the theory of two-dimensional vertical flow, suspended sediment movement and geomorphology evolution, the paper mainly models the2-D vertical average tidal currents, suspended sediment transportation and morphologic evolution in the Yangtze Estuary and Hangzhou Bay with Mike21_FM. The model is versified by the observation datum. With the validation of the model, three stimulation schemes are set of the North Branch for the study, including (1) the topography and shoreline in1981;(2) the topography and shoreline in2004;(3) the topography in2004and the new shoreline after the narrow project in2004. The paper study the change of hydrodynamic environment, suspended sediment and the evolution of channel bed in the North Branch after the long-term reclamation from1981to2004, and the narrow project in2004. Firstly, the tidal current model stimulates the hydrodynamic environment, including average tidal sea level, higher or lower sea level, ebb and flood flow velocity and tidal volume in the three different stimulation schemes. And the changes of the hydrodynamic environment mentioned above are also analyzed in the case of the long-term reclamation from1981to2004, and the narrow project in2004. Secondly, based on the suspended sediment transportation model, the sediment dynamic environment is modeled, with the results of the changes of suspended sediment concentration and the evolution of channel bed in the three different stimulation schemes. Therefore, the paper analyzes the changes of suspended sediment concentration and the evolution of channel bed in the North Branch after the long-term reclamation from1981to2004, and the narrow project in2004. The results provide the foundations for the scientific reclamation of the tidal flat in the estuary and the management of the North Branch.The stimulated results show that:the long-term and large-scale reclamation from1981to2004makes that,(1) Most of the average tide level of the north branch are reduced differently; Average high tidal level rose in most river channels, except Daxing Harbor. On the other hand, the low tidal level dropped at different extents in the North Branch.(2) Because of the narrowing of the channel width, the flood tidal volume of the Chongtou profile in the entrance of the North Branch was decreased by46.2%, and68.1%for the ebb tidal volume. The tidal volume of the Lianxing Harbor profile in the exit of the North Branch was decreased by13.7%, and22.3%for the ebb tidal volume.(3) As for the hydrodynamics of the flood and ebb tidal currents, the flood tidal current velocity was increased and the ebb tidal current velocity is decreased by the reclamations during1981～2004, which fundamentally change the dynamic relationship between the flood and ebb tidal currents.(4) The suspended sediment concentration (SSC) was increased in most river channels during1981～2004, except Lianxing Harbor. While, there was no change but slightly enhance that the SSC of flood tide was higher than that of the ebb tide.(5) The characters of erosion and deposition in the North Branch were almost fifty-fifty in1981, while, it turned to deposit on the whole in2004. The flood tidal force was enhanced during1981-2004because of the reclamation, in addition, the SSC in flood period was increased. With the decrease of ebb tidal force, the sediment carried by the flood tidal current from the open sea tended to deposit in the North Branch during the ebb tide, moreover, the increase of SSC during the ebb tide was relatively small compared with that of flood tide. Consequently, the imbalance between input and output of sediment resulted the overall deposition of the North Branch, on the other hand, the changes of flood and ebb tidal current and SSC is the dynamic mechanism of the evolution in North Branch.After the narrow project,(1) the average tidal water level basically kept the same along the river bank. Average high tidal level dropped at different extents along the river channel. Average low tidal level showed slight floating, while, the amplitude of the variation was not obvious.(2) The flood tidal volume of the Chongtou profile was decreased by9.5%, and4.3%for the ebb tidal volume. The flood tidal volume of the Lianxing Harbor profile was decreased by30.6%, and30.2%for the ebb tidal volume.(3) As for the hydrodynamics of the flood and ebb tidal currents, the flood tidal current velocity was decreased and the ebb tidal current velocity is increased after the narrow project. There was no basic change that the flood tide took the priority in the channel of North Branch. While, after the project, the flood tidal current velocity was decreased, and the ebb tidal current velocity was increased, which narrowed down the difference between the flood and ebb tidal forces.(4) The suspended sediment concentration (SSC) kept at a high level in the North Branch after the narrow project, while, the SSC during the flood tide was still larger than that of the ebb tide. As for the whole river channel, the SSC was decreased at different extents.(5) The erosion and deposition condition of the North Branch after the narrow project kept the same as that in2004, while, the intensity of deposition in part of the channel was decreased and the scour range extended. The dynamic mechanism how narrow project resulted to the evolution of erosion and deposition was that, after the narrow project, the flood tide force was weakened in the channel of the North Branch, and the ebb tide force was enhanced. As for the SSC, the SSC during the flood tide was decreased, while it was increased during the ebb tide. Therefore, the sediment input of the North Branch was decreased. On the other hand, the output was increased, which declined the deposition of the North Branch.