A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering, School of Engineering and Technology

Thesis (Ph.D.) - Asian Institute of Technology, 1984

This study formulates laterally homogeneous, two-dimensional, hydrodynamic and sediment transport models which describe the flow and fine cohesive sediment patterns in well-mixed and partially mixed estuaries. They are intended to serve as tools homogeneous tidal estuaries. careful treatment of the free surface fluctuations, the irregular bed boundaries, the near bed velocity profile and the mass balance of fine cohesive sediments have generated quite general models which can be applied to the inviscid fluids, sediment without cohesion, and soluble material. On application of a transformation technique, the models are solved by an implicit alternating direction method. The models are verified by application to simple problems with analytical solutions. The stability criteria of the sediment transport model are developed and expressed in terms of the fluid velocities, the settling velocity and the sediment diffusion coefficients. In the process, stability criteria for both models and the effects of such physical parameters as the weighting factor, the grid spaces and the time interval on their accuracy are investigated. For the hydrodynamic model, the maximum value of the time interval which gives a stable condition is about twice the value given by the Courant - Friedrichs - Lewy criterion. The hydrodynamic and sediment transport models agree we ll with the analytical solutions for the weighting factor = 1.0 and = 0.5, respectively, when the grid spaces and the time interval satisfy the stability criteria. Tests show that the weighting factor, the grid space in x-direction and the time interval affect strongly the stability and accuracy of the models, while the grid space in z-direction has only an effect on their accuracy. The models are applied to the flow field and the distribution of fine cohesive sediment in the lower part of the Chao Phraya Estuary. The results are compared with previous recorded data. It is shown that during the dry season the settling velocity is constant and equal to 0.0002 m/s, while during the wet season it is a function of sediment concentration. Suitable values and forms for the settling velocity, the diffusion coefficients and other parameters are found. It is also indicated that the discharge of fresh water and the tide amplitude control the magnitude of the water level, the velocity, and the amount of deposition and erosion of sediment, but have little influence on their patterns. The settling velocity, the sediment diffusion coefficient in z-direction and the critical shear stresses for erosion and deposition have a significant effect on the distribution and the deposition of sediment, while the sediment diffusion coefficient in x-direction and the forms of suspended sediment distribution profile have a small effect. A more detailed study of some of the model parameters and the processes which govern the distribution and deposition of fine cohesive sediment, such as flocculation , erosion and deposition, are recommended.