Transport Of Nonuniform Bedload In The Changjiang Estuary And Its Numerical Simulation

In this paper, a brief review of the basic theory and the computational methods for bedload transport have been made with the emphasis on the great influence of bedload on the bed-forming in rivers. A new way is proposed by using numerical simulation to study the nonuniform bedload transport in the Changjiang Estuary.Firstly, a 1-D flow numerical model based on the Preissmann scheme, a 1-D rivernet flow numerical model based on the Direct Solution Method and a 2-D tidal model based on the implicit scheme of finite difference method are programmed.The Object Oriented Programming (OOP) is introduced into the coding of 1-D rivernet model by using Fortran 90 for the first time. Both a database and a program frame are constructed, which could be extended and reused in the future. Some aspects of the arithmetic such as the simplification of rivernet, the arrangement of coefficients of equations, the treatment of internal boundary conditions and .the solution of sparse matrix are also improved.The 1-D flow model is used to simulate the full-year hydraulic process in the Changjiang Estuary in 1998 with a desired validation by alternating the river roughness in the flood season. Using the 1-D rivernet model calculates the water discharge ratio of the two branches and also duplicates the back-flow in the north branch during flood tide. The 2-D model with a fine grid is applied to the field of flow in the Changjiang Estuary and the results are well in agreement with the measured flow. All the applications of the three models obtain inter-confirmed results so that the credible hydraulic conditions could be provided for the further calculation of the bedload transport.Both an amended Meyer-Peter formula and a stochastic-mechanistic formula for nonuniform bedload are firstly used to calculate the hourly, monthly and yearly bedload flux in the Changjiang Estuary respectively. The results show that bedload of about 6,000,000 tons is transported through the Xuliujing section into the river mouth in 1998, which accounts for 1.2% of the total sediment flux. Then, the influence of several hydraulic factors on the bedload transport is discussed. It is concluded that the amount of bedload transport is dominated by the runoff discharge in the Datong Station, and partially influenced by the range of tide.1-D and 2-D model for non-uniform bedload transport are applied to compute the amount of bedload in the south branch of the Changjiang Estuary. It can be found that the peak value of bedload transport in a tidal cycle, computed by 2-D model, is obviously larger than that computed by 1-D model, which is induced by the disequilibrium of bedload transport in cross-river direction. The relationship between the bedload transport and the immigration of sandbars is also analyzed theoretically.